# Investigating the Relationship between (3200) Phaethon and (155140) 2005 UD
through Telescopic and Laboratory Studies

Theodore Kareta Lunar and Planetary Laboratory
University of Arizona
Tucson, AZ, USA  Vishnu Reddy  Lunar and Planetary Laboratory
University of Arizona
Tucson, AZ, USA  Neil Pearson  Planetary Science Institute
Tucson, AZ, USA  Juan A. Sanchez  Planetary Science Institute
Tucson, AZ, USA  Walter M. Harris  Lunar and Planetary Laboratory
University of Arizona
Tucson, AZ, USA

(Received April 14, 2021; Revised July 16, 2021; Accepted August 5, 2021)

###### Abstract

The relationship between the Near-Earth Objects (3200) Phaethon and (155140)
2005 UD is unclear. While both are parents to Meteor Showers, (the Geminids
and Daytime Sextantids, respectively), have similar visible-wavelength
reflectance spectra and orbits, dynamical investigations have failed to find
any likely method to link the two objects in the recent past. Here we present
the first near-infrared reflectance spectrum of 2005 UD, which shows it to be
consistently linear and red-sloped unlike Phaethon’s very blue and concave
spectrum. Searching for a process that could alter some common starting
material to both of these end states, we hypothesized that the two objects had
been heated to different extents, motivated by their near-Sun orbits, the
composition of Geminid meteoroids, and previous models of Phaethon’s surface.
We thus set about building a new laboratory apparatus to acquire reflectance
spectra of meteoritic samples after heating to higher temperatures than
available in the literature to test this hypothesis and were loaned a sample
of the CI Chondrite Orgueil from the Vatican Meteorite Collection for testing.
We find that while Phaethon’s spectrum shares many similarities with different
CI Chondrites, 2005 UD’s does not. We thus conclude that the most likely
relationship between the two objects is that their similar properties are only
by coincidence as opposed to a parent-fragment scenario, though the ultimate
test will be when JAXA’s DESTINY+ mission visits one or both of the objects
later this decade. We also discuss possible paths forward to understanding
Phaethon’s properties from dynamical and compositional grounds.

asteroids, comets, near-earth objects, meteor showers, laboratory astrophysics

††journal: and Accepted in the Planetary Science Journal††facilities: IRTF
(SpeX, MORIS)††software: NumPy (Harris et al., 2020), SciPy (Virtanen et al.,
2020), AstroPy (Astropy Collaboration et al., 2013, 2018)

##  1 Introduction

###  1.1 Overview

The Geminid Meteor Shower is one of the best-studied and easily observed
meteor showers, but until 1983 no parent comet had been identified.
Furthermore, the orbit of the Geminids is not that of a Long-Period or
Jupiter-Family comet, with a period of only 1.4 years and a perihelion
distance inside the orbit of Mercury at $q=0.140$ AU. Near-Earth object (NEO)
1983 TB, later named (3200) Phaethon, was discovered in a Geminid-like orbit
and was quickly deemed a plausible parent for the meteor shower (Whipple,
1983). While Phaethon was originally presumed to be a dead comet (see, e.g.,
Belton & A’Hearn 1999), with the Geminids thought to have been produced
through conventional cometary activity in the past, the story has become much
less clear over the past two decades. While studies of the Geminids themselves
have mostly concluded that they are fundamentally cometary in nature, studies
of Phaethon have been converging on the idea that it is an (active) asteroid
from the Main Belt as opposed to stochastically captured from the outer Solar
System (a review of both of these topics is presented in the next subsection).
Phaethon is thus a key object to understand on the now-recognized continuum
between comets and asteroids. The NEO (155140) 2005 UD (Kasuga et al., 2005;
Jewitt & Hsieh, 2006) is compelling for the same reasons, as it is the
inactive parent of the Daytime Sextantids – as well as proposed fragment of
Phaethon itself. Investigations into the modern properties and relationship
between members of the ’Phaethon Geminid Complex’ (PGC) are thus
investigations into how meteor showers form as well as the properties of low-
albedo NEOs.

###  1.2 (3200) Phaethon

Phaethon’s properties are much better studied than 2005 UD’s due to its
earlier discovery date and larger size and thus sets the framework for which
these related bodies are compared and understood. The surface of Phaethon is
strongly blue at visible and near-infrared wavelengths ($\sim 0.35-2.5$
$\mu{m}$, see, e.g. Tholen 1984; Licandro et al. 2007), unlike the typical
strongly red reflectance spectrum of traditional cometary nuclei and D-type
asteroids. Phaethon’s orbit, while highly eccentric to the point where its
perihelion is lower than Mercury’s at $q=0.14AU$, is generally accepted to be
hard to produce from a typical Jupiter Family Comet orbit (Bottke et al.,
2002) and probably more likely to come from the Main Belt of asteroids (de
León et al., 2010), possibly from (2) Pallas’s collisonal family. However,
there are serious issues with producing Phaethon’s reflectance spectrum and
albedo from an originally Pallas-like composition (see the discussion of
Kareta et al. 2018). Furthermore, the radar-derived size of Phaethon of more
than $6km$ across at the equator (Taylor et al., 2019) is inconsistent with
much previous thermal modeling of the object, supporting a lower albedo nearer
to $\sim 8\%$ like that of Kareta et al. 2018 as opposed to the higher $\sim
12-16\%$ values of studies like that of Hanuš et al. 2016 or Masiero et al.
2019 for reasons not yet well understood, though many of these authors have
speculated that Phaethon may have odd thermal properties in light of the
discrepancy. While Phaethon’s visible-wavelength albedo might be typical for
B-type asteroids (Alí-Lagoa et al., 2013), its radar albedo might be more
consistent with comets than B-types (Taylor et al., 2019). Previous models of
Phaethon’s surface largely involved hydrated materials (Licandro et al.,
2007), but the surface of Phaethon appears to be totally dehydrated (Takir et
al., 2020). All of these non-converging lines of inquiry about Phaethon’s
surface are further complicated by the revelation that Phaethon is not totally
inactive, but actually develops a small dust tail near perihelion (Jewitt &
Li, 2010; Jewitt et al., 2013). This dust tail is most likely caused by
intense solar radiation pressure sweeping small ($\sim 1\mu{m}$) dust grains
off the surface, not sublimation of volatiles or another process, though
Phaethon’s short rotation period ($\sim 3.604$ hours, Hanuš et al. 2016) may
assist the process (Nakano & Hirabayashi, 2020). This is assumed to be a
process that could happen on any sufficiently sungrazing object, though it has
yet to be observed conclusively on other objects (see a discussion in Knight
et al. 2016).

###  1.3 The Geminids

As opposed to the strange properties of their mostly-inactive parent body, the
study of the orbital distribution and physical properties of the Geminids
themselves has been a cleaner story. The age of the Geminids appears to be
among the youngest of the large meteor showers, with a maximum age of a few
thousand years (Ryabova, 1999, 2016). The spatial distribution of meteoroids,
especially its asymmetry about the peak, probably requires a large change in
parent body orbit during stream ejection, such as by non-gravitational forces
(Lebedinets, 1985; Ryabova, 2016). The meteors themselves appear to be denser
than typical cometary meteors (Halliday, 1988). They have a wide variety of
sodium abundances (Kasuga et al., 2005; Borovička, 2010; Abe et al., 2020),
probably due to intense solar heating (Kasuga, 2009), but have an overall
composition consistent with cometary material (Borovička, 2010; Abe et al.,
2020). The higher-than-cometary densities could also be explained as a
heating-related effect, with the weaker grains being preferentially removed
from the stream with time by thermal stresses (Čapek & Vokrouhlický, 2012).
Abe et al. 2020 found that smaller meteoroids were more depleted in sodium,
consistent with this idea.

###  1.4 The Phaethon-Geminid-Complex and Paper Overview

A comprehensive explanation of the ’Phaethon-Geminid-Complex’ (PGC) needs to
explain the modern asteroid-like properties of Phaethon (surface reflectivity,
orbit) as well as the comet-like properties of the Geminid meteors themselves
(composition, orbital distribution) as described in the previous section. One
avenue for further study is characterizing other small bodies thought to be
dynamically associated with Phaethon and the Geminids. (155140) 2005 UD is a
$\sim 1.6$ km NEO with a blue reflectance spectrum (Jewitt & Hsieh, 2006;
Devogèle et al., 2020) thought to be dynamically linked to Phaethon and the
Daytime Sextantids, as well as the Geminids less directly (Kasuga et al.,
2005). This is a second NEO with similar (visible) reflectance properties, a
very low perihelion ($q=0.16AU$), and associated with its own meteor shower,
and thus is expected to be a highly valuable target for comparison with
Phaethon.

The specifics of the relationship between the two bodies are debated. For
example, Ryabova et al. 2019 makes dynamical arguments that the two objects
could not have been in close contact in the recent past given our
understanding of their orbits despite their similar-and-rare surface
properties. It is unclear whether or not 2005 UD (the so-called ’mini-
Phaethon’) is a fragment of its larger cousin and thus genetically related,
introduced to near-Earth space through some common process, or that the two
objects just coincidentally have similar orbits, surfaces, and associated
meteor showers. It is imperative to understand the relationship between these
objects not just to better understand the origin of their modern properties
and meteor showers, but also to make preparations and predictions for the
arrival of JAXA’s $DESTINY^{+}$ spacecraft at Phaethon in the mid-2020s (Arai
et al., 2018), which may be able to fly-by 2005 UD at a later date. There is
another object proposed to be in the ’Phaethon-Geminid-Complex’, 225416 (1999
YC) (Ohtsuka et al., 2008), but its orbit and physical properties are even
more different from Phaethon than UD’s (Kasuga & Jewitt, 2008), and has
received comparatively little characterization as a result.

In this paper, we present the first near-infrared reflectance spectra of
(155140) 2005 UD in Section 2 to better understand its modern properties and
compare it to its better studied cousin (3200) Phaethon. We also present new
laboratory reflectance measurements of the CI chondrite Orgueil heated to
Phaethon and 2005 UD relevant temperatures in Section 3 to better constrain
the thermal histories of and relationship between these bodies. In Section 4,
we discuss how these interrelated lines of evidence and inquiry change our
perception of their respective properties and histories.

##  2 Near-Infrared Observations of 2005 UD

Table 1:  UT DateaaAt observing slot start, see text for specific details. | UTC TimeaaAt observing slot start, see text for specific details. | $R_{H}$ | $\Delta$ | $\alpha$ | $m_{V}$ | Airmass | Humidity | Total Time
---|---|---|---|---|---|---|---|---
(YYYY-mm-dd) | (d) | (au) | (au) | $(^{\circ})$ |  |  |  | seconds
2018-09-19 | 14:39 | 0.98 | 0.27 | 86.6 | 17.5 |  $\sim$1.26 |  $\sim$26% | 800.0
2018-10-02 | 12:27 | 1.18 | 0.23 | 34.2 | 16.0 | 1.03-1.06 | 9-14% | 2800.0
2018-10-03 | 09:12 | 1.20 | 0.23 | 30.5 | 16.0 | 1.20-1.60 | 5-6% | 5600.0

We obtained the first near-infrared reflectance spectra of 2005 UD at the NASA
IRTF using the SpeX instrument (Rayner et al., 2003) on three dates in
September and October 2018, the details of which are presented in Table 1.
Guiding was accomplished with the MORIS camera (Gulbis et al., 2011) due to
the high apparent motion and faintness of the target. The observations
followed the procedure of Kareta et al. 2018, whereby observations of the
target were ’bookended’ by observations of a local G-type star for immediate
telluric correction and later slope-corrected further using a well-
characterized solar analog G2V star (SAO 93936) observed near-zenith. This
method has been used to effectively and reproducibly characterize faint Solar
System small bodies on many occasions (see, e.g., Sharkey et al. 2019.) The
reduction was performed partially within the ’spextool’ environment (Cushing
et al., 2004) and partially within custom-written scripts in Python (see
’Software’ after the Acknowledgements section).

Figure 1: Left: The observed near-infrared reflectance spectra of (155140)
2005 UD as observed on October 2 and 3 2018. All spectra are shown to be
primarily linear and weakly red-sloped. Right: The combined spectrum of 2005
UD from October 2 and 3 is compared against the average near-infrared
reflectance spectrum of its proposed parent body, (3200) Phaethon, from
(Kareta et al., 2018). Dates or object names are listed underneath each
spectrum, and all spectra were normalized such that $R(1.5\mu{m})=1.0$ and
offset vertically for clarity. The spectrum of Phaethon has had a long-
wavelength thermal emission component removed analytically. Phaethon and UD,
despite their widely reported similar visible surfaces, are shown to be highly
different at near-infrared wavelengths. A third spectrum taken September 19th
of the same year is of much worse quality and is described in the text.

The reflectance spectra obtained on October 2 and 3 2018 dates of 2005 UD are
presented and compared with the reflectance spectrum of Phaethon from Kareta
et al. 2018 in Figure 1. The September 19 data is much noisier compared to the
other two spectra, so it is only described here and omitted from Figure 1. The
reflectance spectrum of 2005 UD weakly red throughout the near-infrared ($\sim
0.7-2.5\mu{m}$ on all three dates, though we note that the use of MORIS as a
guider with SpeX can decrease the signal in shortest wavelength region
($<0.85\mu{m}$) due to low throughput near the visible/near-infrared dichroic
mirror (Personal Communication, S.J. Bus.) We also note that while the slit
was aligned to the parallactic angle, the object’s non-sidereal tracking rates
were so large that moderate short-wavelength slit losses may be possible. Both
of these issues would result in lost flux, and thus artificially lower
reflectance values in the $<0.85\mu{m}$ region if relevant. The lack of any
large change in slope at the shortest wavelengths indicates that neither
process had a significant effect on the data.

To check for consistency in spectral slopes between our three nights of
observations, we consider the region $0.85-2.4\mu{m}$ and calculate values for
$S^{\prime}$, where
$S^{\prime}_{\lambda}(\lambda_{1},\lambda_{2})=(dS/d\lambda)/S_{\lambda}$, of
$1.5\pm 0.2$, $0.7\pm 0.2$, and $0.5\pm 0.1\%/(0.1\mu{m})$ for September 19th,
October 2nd and 3rd respectively. The uncertainties are those from the formal
linear curve-fitting. The combined October 2 and 3 spectrum has $0.6\pm
0.1\%/(0.1\mu{m})$. For context, the Bus-Demeo average C-type has
$S^{\prime}=1.16\pm 0.07$ over the same range, so we find that 2005 UD is less
red than a C-type but still red in the near-infrared. We note that Marsset et
al. (2020) analyzed decades of similar observations and found a systematic
$4.2\%/\mu{m}$ uncertainty over this whole wavelength range ($0.8-2.4\mu{m}$),
larger than our uncertainties derived from our linear fits. However, those
same authors note that regular observations of standard stars (thus requiring
regular re-acquisition of the target on the slit) do mitigate much of this
systematic uncertainty. We also utilized standard star observations close in
airmass ($\Delta X<0.1$) which is the other dominant factor in this systematic
error. The rotation period of 2005 UD is 5.235 hours (Devogèle et al., 2020),
so our October 2 and 3 observations are separated by almost exactly $\sim 4.0$
rotation periods, and the September 19th and October 2nd observations are
separated by slightly more than $\sim 59$ rotation periods. These three
separate nights of observations do not show evidence for the previously
reported possible heterogeneous surface of 2005 UD (Kinoshita et al., 2007),
but that is perhaps unsurprising as the three sets of observations span the
same $\sim\frac{1}{4}$ of the surface. The observing geometry did change
significantly, so some difference might have been expected. The redder slope
of the September 19th observations is statistically different, but given that
the spectrum is likely phase-reddened (Sanchez et al., 2012), was taken at a
low enough heliocentric distance that the object likely has thermal emission
contaminating the longer wavelengths (see, e.g. Reddy et al. 2009; Delbó et
al. 2003; Kareta et al. 2018), and is much noisier due to the lower total
integration time on a dimmer target, we suspect the difference is not
resulting from heterogeneous surface properties. We again note that measuring
slope-differences at the level of subtlety discussed here is at or near the
limit of what can be provided by traditional observing schemes with this
instrument, so our actual knowledge of slope variations on 2005 UD is driven
by systematic uncertainties which are challenging to quantify in more detail
than we have described in this section.

##  3 Phaethon vs. 2005 UD

The revelation that (3200) Phaethon and (155140) 2005 UD might have very
different near-infrared reflectance spectra is very surprising given the ample
similarities of the two objects (very similar and uncommon visible
reflectivities, similar and uncommon orbits, both are meteor shower parent
bodies, etc), so two questions must be addressed: is the difference real and
if it is, what are plausible mechanisms to explain it?

As for whether or not the difference is real, there are multiple reasons that
we believe the results. First, both sets of observations (of UD and of
Phaethon) compared in Figure 1 were taken by the same instrument, by the same
observers, and reduced using the same packages and methods. Second, due to the
large non-sidereal rates which 2005 UD moved at during the portion of its 2018
apparition in which we observed it, we had to use different local telluric
stars each night (but the same master solar analog), which when combined with
the almost identical retrieved reflectance spectra on each night makes flukes
of calibration due to poor choice of calibration stars seem unlikely. Third,
though perhaps most important, the retrieved spectrum of Phaethon shown from
Kareta et al. 2018 is very consistent with many previous observations of the
object, and thus the pipeline by which both datasets has been reduced is
validated. The difference between the objects seems very real, though we again
remind the reader of the possible calibration issues in the 2005 UD data at
shorter wavelengths due to the dichroic and parallactic angle issues mentioned
previously.

There are then two kinds of scenarios to be considered when attempting to
explain these objects discrepant physical properties: either they are
genetically/dynamically related somehow, and some process has made their
surfaces appear different, or they are not related and their similarities are
either coincidental or simply by-products of some larger process in the inner
and middle Solar System.

If the two objects really are only coincidentally similar, then there should
be multiple pathways into Phaethon / Geminid - like orbits. While there are
serious issues to be resolved in understanding whether or not Phaethon is a
heated and devolatilized member of the Pallas Collisional Family (de León et
al., 2010; Kareta et al., 2018; MacLennan et al., 2020), 2005 UD’s spectrum is
a very poor match for Pallas or any member of the Pallas Collisional Family
(they are all strongly blue in the near-infared, many with Phaethon-like
concave up spectra), suggesting that those problems would be even more
challenging to resolve for it. Furthermore, the surface of the less studied
candidate member of the Phaethon-Geminid-Compolex 1999 YC (Ohtsuka et al.,
2008) is an even worse fit (Kasuga & Jewitt, 2008), so the problems continue
in that arena. Dynamical studies of how to implant objects into Phaethon-like
orbits from source populations beyond the Pallas family (e.g. other volatile-
rich asteroid families, the Outer Main Belt, Hildas, JFCs, etc.) could be
extremely useful in answering the plausibility of this scenario. The
significant inclination ($\sim 22^{\circ}$) of Phaethon and 2005 UD ($\sim
29^{\circ}$) limits the number of areas to search

If the two objects actually do share some sort of meaningful relationship,
then some divergent process should be able to create both kinds of surfaces.
We consider the five following processes:

  * •

Parent Body Heterogeneity: If the precursor object that disrupted into
Phaethon and 2005 UD had variable composition over its surface and throughout
its interior (such as if it had been fully or partially differentiated, as has
been argued for (24) Themis in Castillo-Rogez & Schmidt 2010), then the
daughter fragments could inherit some of these differences and appear
different from each other. Considering that Phaethon itself appears to have a
largely homogeneous surface (Kareta et al., 2018) and dominates the mass of
the two objects combined111If Phaethon has an average diameter of 5.7 km
(Taylor et al., 2019) and 2005 UD has an average diameter of $\sim$1.3 km
(Jewitt & Hsieh, 2006), then Phaethon has $\sim 99\%$ of the mass of the
combined system if their densities are similar., it seems unlikely that UD
would somehow inherit different properties by chance. Furthermore, if the
Geminids are a by-product or related to the breakup event, then they too
should be heterogeneous, which they do not appear to be outside of variable
Sodium depletion (Borovička, 2010) often-but-not-exclusively attributed to
their low perihelion distance.(In principle, we cannot rule out some stranger
scenario where the progenitor object was obviously differentiated or
heterogenous, but neither daughter object nor the Geminids inherited any
unambiguous heterogeneities by chance.)

  * •

Differential Space Weathering: Space weathering is a collection of processes
(energetic particle impact, UV irradiation) that acts to change the spectral
slope of an object. While in general the process is said to make the
reflectivity of a planetary surface redder and darker (Pieters et al., 1993)
(true for rocky surfaces like the Moon or S-type asteroids), there is evidence
that some volatile rich surfaces like those of CI or CM chondrites might
become bluer (Lantz et al., 2017). Furthermore, recent laboratory (Thompson et
al., 2020) and in-situ measurements at (101955) Bennu (DellaGiustina et al.,
2020) suggest that for carbonaceous surfaces, surfaces might initially become
bluer only to brighten and redden afterwards. It is important to note that
traditional space weathering products such as nano-phase iron seen in lunar
regolith have not been detected in abundance on CI or CM chondrites and it is
expected the same stimuli are resulting in fundamentally different processes
on carbonaceous objects, even though all of these effects are labelled ’space
weathering’. In general, space weathering apparently has a much larger effect
at visible wavelengths that tapers off into the near-infrared, so the very
similar visible reflectance spectra of Phaethon and 2005 UD (Jewitt & Hsieh,
2006) makes this scenario seem unlikely. Space weathering, thought to decrease
strongly in importance with increasing distance from the Sun, must play some
role in modifying the surfaces of these two objects. If Phaethon and 2005 UD
have different surface effective surface ages, this would allow them to be at
different parts of the blue-ing-then-reddening trends found in works like
Thompson et al. (2020) and DellaGiustina et al. (2020). However, the magnitude
of changes estimated from those and other works are not enough to account for
the observed difference between the two objects.111One issue complicating an
estimation of the role of Space Weathering on these objects is whether or not
they are active enough to outpace any potential weathering, e.g. if the
weathered material is quickly lost due to radiation pressure or the Solar Wind
or any of the other processes proposed for Phaethon.

  * •

Grain Size Effects: The distribution of grain sizes on a planetary surface can
change the way that light reflects off of it significantly (see, e.g., Clark &
Roush 1984). Moreover, more massive bodies should be able to retain even
smaller grains on their surfaces due to their stronger gravity – and Phaethon
is much more massive than 2005 UD. However, Phaethon also seems to be losing
the finest ($\sim 1\mu{m}$) grains off of its surface due to solar radiation
pressure (Jewitt & Li, 2010), so it is challenging to make a-priori estimate
of how the two objects might have different grain size distributions. A finer
point is that the texture of the larger grains can mimic the appearance of
smaller grains should it be of the right scale. That being said, while grain
size effects can be complex, there is no reason to expect that these effects
have little effect at visible wavelengths and a large effect in the near-
infrared. Furthermore, Devogèle et al. 2020 estimated that both objects have
grain sizes in the $\sim 1-10mm$ range, well outside of any transition in
scattering regime for the wavelengths considered. Again, in principle, this
may play some role in the overall difference of the two surface reflectance
spectra, especially if these similarly-sized larger grains have significantly
different textures, but it cannot be the dominant cause of the difference.

  * •

Phase Angle Effects / Phase Reddening: Observations of small bodies at higher
phase angles ($>30^{\circ}$) can appear artificially redder, and this effect
is most prominent at near-infrared wavelengths (Sanchez et al., 2012), making
it initially seem to be a better explanation than the other processes
discussed so far. The two highest quality observations of 2005 UD presented
here were at 30.5 and 34.2 ∘ phase, while the comparison observations of
Phaethon were at $\sim 22^{\circ}$. This difference of $8-14^{\circ}$ is
enough for only a very slight increase in slope, if any, and thus not enough
to explain the difference between the two objects. Sanchez et al. 2012 found
that observations at $\sim 30^{\circ}$ phase had a negligible slope change
from those at lower phase angles, from which we infer that any uncertainty in
the slope of our combined 2005 UD spectrum is likely dominated by factors
other than phase angle effects.

  * •

Differential Thermal Alteration: The individual materials that make up a
planetary surface can be altered by high temperatures, including physical
changes like the break down or exfoliation of boulders (recently studied up
close by OSIRIS-REx at the near-Earth asteroid (101955) Bennu, see Molaro et
al. 2020) and chemical changes brought on in individual minerals (Hiroi et
al., 1993, 1996b). The latter is thought to be one of the most important
processes that drives spectral diversity among carbonaceous asteroids,
particularly among near-Earth objects (Marchi et al., 2009). Phaethon and 2005
UD are both in highly eccentric orbits with very high but distinct perihelion
temperatures. If some material could be identified that can produce a
Phaethon-like spectrum when heated to a Phaethon-like temperature as well as a
UD-like spectrum at a UD-like temperature then their different spectral
behaviors could have a natural explanation that could then be studied on other
objects.

To assess the plausibility of this last scenario, we performed a blind search
for high-quality spectral match throughout all of the RELAB database (Pieters,
1983; Pieters & Hiroi, 2004) after making an initial cut to only include
spectra whose visible albedos were less than $15\%$ at $0.55\mu{m}$ to
restrict the candidates to those with low albedo while still being agnostic to
the discrepant estimates of Phaethon’s albedo (Hanuš et al., 2016; Kareta et
al., 2018; Masiero et al., 2019). As with previous studies of Phaethon’s
surface (e.g. Licandro et al. 2007), the best matches of both objects were
heated CI and CM chondrites – both those heated in the laboratory and those
found already showing evidence of thermal metamorphism. The in-air spectra of
the CI chondrite Ivuna after heating to 873 K and 973 K were among the best
fits for Phaethon and 2005 UD respectively, with and without forcing the
visible spectrum to be blue-sloped. Both in-air laboratory spectra have linear
blue slopes at visible wavelengths, while the 873 K spectrum becomes linear
and red near $\sim 0.9-1\mu{m}$ and the 973 K spectrum continues as blue and
linear in the near-infrared without any very obvious slope break.222We note
for any readers interested in utilizing or inspecting these Ivuna spectra that
they are labelled in RELAB in degrees Celsius, not Kelvin, e.g. 600 ∘C and 700
∘C, and seem to be from Hiroi et al. 1996a. This trend with heating, whereby
the visible reflectance remains blue but the near-infrared reflectance goes
from linear and red to blue-sloped is the right kind of trend to explain these
objects. The matches were imperfect in key ways, however, the 873 K spectrum
was slightly too red for 2005 UD and the 973 K spectrum was not sufficiently
blue to match that of Phaethon, and it did not have the same slight convex-
upwards curvature seen throughout the same wavelength range. Assuming an
albedo like that of Kareta et al. 2018’s for Phaethon for both objects
($p_{v}=0.08\pm 0.01$) and a standard emissivity value ($\epsilon=0.9$), we
would expect their surfaces to be heated to approximately these temperatures.
Phaethon’s surface-averaged temperature $T_{ave}\sim 969K$ and sub-solar
temperature $T_{SS}\sim 1085K$ at perihelion are similar and slightly higher
than the 973 K laboratory data respectively, while 2005 UD’s temperatures
($T_{ave}\sim 895K$, $T_{SS}\sim 1002K$) at its perihelion are similar and
higher than the 873 K laboratory data as well. The explanation that 2005 UD is
simply a less thermally metamorphosed Phaethon is an exciting prospect, but
the hypothesis is largely based on a limited number of laboratory measurements
that were conducted in-air and not under a space-like vacuum. We deemed it
necessary to collect more laboratory data of heated CI chondrites to confirm
the spectral trends seen and try to better understand how well they could
constrain the thermal histories – and thus relationship – between these two
objects.

##  4 Laboratory Reflectance Spectroscopy

###  4.1 Methodology

In order to test the hypothesis that Phaethon and 2005 UD are made of the same
material but heated to different degrees, new laboratory measurements of the
reflectance of CI Chondrites at higher temperatures than available in the
literature from RELAB had to be obtained, and ideally in a vacuum as opposed
to in-air. In particular, the heating experiments should be designed in such a
way to both mimic the grain sizes thought to be relevant for Phaethon and UD
($\sim 1-30mm$, see Devogèle et al. 2020) and the vacuum of space. To achieve
this goal, we constructed a vacuum heating chamber (affectionately referred to
as the ”Bar-B-Cube”) in the Reddy Spectroscopy Lab at the University of
Arizona. After initial sample preparation, which might vary based on the
sample and topic being investigated (e.g., grinding and size sorting with a
mortar and pestle), the sample is placed inside an No. 200 Nickel sample cup
(chosen based on Nickel’s high melting point and ease with which it can be
machined) and lowered into the chamber from above. The sample cup is placed on
the center of the heating element and the sapphire window on the top of the
chamber is tightened. The vacuum pump is then turned on and the chamber is
slowly lowered to $\sim 10^{-6}$ Torr or lower. The vacuum chamber was able to
reach pressures as low as $\sim 10^{-8}$ Torr if untouched for several hours.
Highly volatile rich samples, such as CI or CM chondrites, require this
process to be quite slow as to avoid sudden outgassing disrupting the surface
of the sample once the pressure inside approaches the triple point of water at
room temperature ($\sim 7$ Torr). Reflectance spectra of the sample within the
chamber are obtained by first calibrating on a nearby spectral standard
through an identical sapphire window and then moving the optical fibers to
observe the sample. The optical fibers are held in a standard orientation for
both the calibrations and the science observations using a 3D printed part
which keeps the two fibers $30^{\circ}$ apart. The fibers are held in a near-
vertical orientation otherwise in both configurations to avoid differential
flexure which might introduce subtle artifacts. This procedure was
successfully tested on a variety of meteoritical and artificial samples to
verify that the retrieved spectra were consistent with those measured through
standard in-air procedures, though the spectra are often somewhat noisier. We
note that we used a lamp optimized for the near-infrared, so the reported
spectra are only shown beyond $0.5\mu{m}$. While the retrieved spectra are
consistent in spectral behavior with their in-air counterparts after
calibrations, the absolute albedo (vertical scale) is likely not after heating
for volatile-rich samples. As volatiles leave the samples, the surface
subsides somewhat in a way that cannot be measured well enough to correct for
the change. As a result, the reflecting surface is further away from the
optical fibers and thus appears artificially darker to the spectrometer.

The heating of a particular sample is done in increments of $50-150K$ to both
attain a fine temperature resolution to record spectral changes over and also
not to cause the sample to lose too many volatiles at once and risk exploding
out of the cup into the vacuum. The general procedure is to raise the
temperature of the heating element to a desired temperature, wait until the
temperature stabilizes at the set value ($~{}5-15$ minutes depending on how
high the temperature is set), and then keep the temperature at that level for
1 hour or until outgassing of the sample has been decreasing steadily for 15
minutes, whichever is longer. For the volatile-rich samples tested, this was
almost always 1 hour with the exception of temperatures near $\sim 950-1000$
Kelvin, where outgassing from the sample was much more significant. (This is
also the highest temperature range reached for the previous study of Ivuna
Hiroi et al. 1996a.) Once this time limit is reached, the sample is allowed to
cool radiatively back to near-ambient temperatures ($\sim 300$ K), which could
take up to 2 hours from the highest temperatures the samples were heated to.
Blackbody emission clearly contaminated the retrieved spectra at any higher
temperature than room temperature, and all reflectance spectra suspected of
having a blackbody component were discarded when possible. A full heating,
cooling, and observation cycle took approximately $\sim 3$ hours on average.
The maximum temperature reliably maintained by the heating element with a
filled sample cup was $\sim 1350$ K, or approximately $\sim 375$ K higher than
the samples of Ivuna we compared our data to from RELAB.

###  4.2 Heating CI Orgueil

This study largely focuses around our heating of a sample of the CI Chondrite
Orgueil, specifically the sample 1070B from the Vatican Observatory’s
Meteorite Collection. The previous CI chondrite that was heated (Ivuna, heated
up to 973 K in Hiroi et al. 1996a) has somewhat different reflective
properties than Orgeuil, and our sample preparation routine was not identical
to theirs, so differences are to be expected. In particular, Ivuna is somewhat
bluer at near-infared wavelengths than Orgueil (at least before heating) and
Hiroi et al. (1996a) heated their samples as unground chips and then ground
them down into a semi-fine powder ($<125\mu{m}$.) Their spectra were also
obtained in-air, as opposed to a near-vacuum more analogous to the conditions
on planetary surfaces. Cloutis et al. 2011 found that even slight changes in
particular mineral abundances, particularly magnetite, can cause comparatively
large slope changes in CI chondrites. For comparison with that study and to
meet our goal of matching the estimated grain size for these bodies as best as
possible considering the limited amount of sample of Orgueil provided ($\sim
0.5$ grams), we divided our sample roughly in two into an unground $\sim 1mm$
subsample and a $<45\mu{m}$ subsample, which largely stuck together in larger
clumps after being sorted into the sample cup. While the orbital history of
the CI Ivuna parent meteoroid is not known, the orbital history of the Orgueil
strongly suggests that the parent was on a Jupiter Family Comet or Halley Type
Comet orbit (Gounelle et al., 2006). The nature of the CI chondrites in the
context of their parent bodies is discussed further later. In order for a
particular laboratory spectrum of Orgueil to be considered good evidence that
a CI-like meteorite could fit the spectrum of the small body in question, the
deviation from the fit should therefore be within the range of variability
expected from changes in grain size or mineral content (within reason). The
reflectance spectra of large mm-sized pieces of the CI Chondrite Orgueil
heated from room temperature up to $\sim 1350$ K are shown in Figure 2.

Figure 2: The retrieved reflectance spectra of the CI Chondrite Orgeuil after
heating to successively higher temperatures are shown, with room temperature
to 923 K on the left side and 973 to 1350 K on the right side. Temperatures
are listed underneath each spectrum at $0.6\mu{m}$, and all spectra were
normalized such that $R(0.6\mu{m})=1.0$ and offset vertically for clarity.

The spectrum of the large grains ($\sim$ mm-sized) of CI chondrite Orgueil is
shown to change significantly as it is heated to successively higher
temperatures, but the trends observed are not identical to those demonstrated
for the CI chondrite Ivuna in (Hiroi et al., 1996a). At visible wavelengths,
the slope of the reflectance spectra become increasingly linear with
increasing heating (a decrease in near-UV absorption strength, as previously
noted for Ivuna as well) and has variations in absolute slope while still
being blue (negative) until 1123-1173 K, where the slope becomes red and
linear. At near-infrared wavelengths, the slope is always overall blue
(negative) though curvature (concave-up) is apparent at a handful of
temperatures, most prominently between 973 - 1173 K, or approximately the
range of temperatures expected for Phaethon’s surface at the present day and
in the recent past near perihelion. Figure 3 shows a comparison of our Orgueil
and Hiroi et al. (1996a)’s Ivuna both heated to 973 Kelvin compared with the
reflectance spectrum of Phaethon. Figure 4 shows a comparison of the derived
spectral slopes for the whole dataset as a function of temperature at a
variety of wavelength ranges again compared with Phaethon and 2005 UD. The
relationship between the visible and near-infrared slopes, and specifically
how they change in the $0.8-1.0\mu{m}$ region and the curvature at longer
wavelengths could indeed be a useful way to identify thermal metamorphism in
Orgueil-like bodies, similar to that shown in the in-air Ivuna data of Hiroi
et al. 1996a and suggested in other contexts by Vernazza et al. 2017; Marsset
et al. 2016. The reflectance maximum seen near $\sim 0.6\mu{m}$ decreases in
prominence with increasing heating, but does not appear to totally disappear
as reported for the Ivuna samples in Hiroi et al. 1996a. We see no evidence
for any curvature in the $\sim 1.3\mu{m}$ region as was seen in Ivuna, and in
fact the slope changes due to heating appear less obvious there. We note that
the higher SNR in the room-temperature in-vacuum spectrum appears to be
primarily due to the fact that the sample shrunk in size afterwards due to
loss of volatiles. In other words, the room-temperature spectrum shown had the
sample surface be physically closer to the fiber that was going to the
spectrometer, resulting in higher throughput.

Part of the difference between our work on Orgueil and previous work on Ivuna
is almost certainly the usage of finer grain sized powders in (Hiroi et al.,
1996a), which generally would result in redder spectra, but the overall trends
are different in a way that hints at more processes in play. This is perhaps
to be expected, as carbonaceous chondrites are often highly heterogenous in
composition at small scales (Bischoff et al., 2006). Another critical
difference is that our data were obtained in-vacuum, and thus in an
environment more similar to that of Phaethon and UD, as opposed to the in-air
spectra presented of Ivuna.

While the spectra of Orgueil heated to Phaethon-like temperatures show many
similarities with Phaethon’s reflectance spectrum (with some key differences
discussed later), the fact that none of the retrieved spectra show red or
neutral slopes in the near-infrared seem to conflict with any positive match
with 2005 UD. Even if the standard is broadened to ’a near-infrared slope that
is redder than the visible slope of the same sample’, there seem to be no
retrieved spectra that meet the criterion.

##  5 Further Discussion

###  5.1 Spectral Comparisons

In this study, we have obtained the first near-infrared reflectance spectra of
the proposed fragment body of (3200) Phaethon (155140) 2005 UD and found the
two objects to be strikingly different. Phaethon’s reflectance spectrum is
concave-up and overall blue-sloped throughout the near-infrared, while UD’s is
approximately linear and very weakly red-sloped. One process that we
hypothesized could explain their discrepant spectral slopes and curvatures
would be thermal metamorphism due to their highly eccentric but distinct near-
Sun orbits, meaning that they were made from the same material originally but
had subsequently been altered to different extents. A comparison of our newly
obtained data of an increasingly heated sample of CI Orgueil and similar data
from Hiroi et al. (1996a) of CI Ivuna data to (3200) Phaethon’s properties is
shown in Figure 3.

Figure 3: The reflectance spectrum of (3200) Phaethon (black) compared with
our sample of the CI Orgueil heated to 973 Kelvin (left, maroon) and Hiroi et
al. 1996a’s CI Ivuna heated to the same temperature (right, orange), all
normalized such that $R(1.5\mu{m})=1.0$. Neither are perfect fits to the
overall spectrum, but both show qualitative similarities that might indicate
some material similarity given the lack of known samples of CI chondrites on
the ground.

Through visual inspection alone of Figures 2 and 3, it can be seen that none
of the heated samples appear similar to 2005 UD at near-infrared wavelengths,
while those in the temperature range expected for Phaethon (roughly
$1000-1200$ K) are indeed approximately linear and blue-sloped at visible
wavelengths while also being blue and concave-up at near-infrared wavelengths
in good qualitative agreement. Phaethon shows no near-UV decrease in
reflectance above $0.35-0.4\mu{m}$, unlike the data for Orgueil shown here. If
the near-UV drop-off in our data is real (and not driven by, perhaps, very low
light levels and a light source optimized for the near-IR), then we conclude
that Phaethon’s spectrum is better described as Ivuna-like a short wavelengths
and Orgueil-like at longer wavelengths. One other key aspect to note is where
the VIS/NIR slope change occurs. In our data on Orgueil, the slope change
occurs somewhere between $0.8-1.0\mu{m}$ but can’t be discerned more clearly
due to its proximity to a changeover in detector usage in our spectrometer
near those wavelengths. In the Ivuna dataset of Hiroi et al. 1996a, the slope
change occurs nearer to $0.6-0.7\mu{m}$. In Phaethon and 2005 UD, the location
of this spectral slope change is near to $\sim 0.8\mu{m}$ but, again, is often
the area where to separate datasets (one from a visible-wavelength instrument
and one from a near-infrared instrument) are stitched together. Again, the
Orgueil dataset seems ’closer’ than Ivuna does to matching Phaethon, but the
location of the spectral slope change needs to be tied down more tightly with
future work. Neither Orgueil nor Ivuna fits 2005 UD well, but a combination
thereof seems to share many similarities with Phaethon. The rarity of CI
Chondrites is likely a contributing factor to the existence of a more
”perfect” match at this date.

In Figure 4, we show a comparison of the derived slopes S’ (Luu & Jewitt,
1990) of the laboratory data at three wavelength ranges ($0.60-0.74\mu{m}$,
$1.1-1.60\mu{m}$, and $1.8-2.2\mu{m}$) compared against the slopes of Phaethon
and 2005 UD calculated over the same wavelengths. These ranges were chosen to
try to encapsulate the changing curvature of the spectrum while avoiding the
noisier parts of the laboratory data where there was a change in detectors.
Where the laboratory data intersects the colored areas corresponding to each
NEO is where their slopes agree (within errors) at those wavelengths. In
agreement with visual inspection, Phaethon’s spectrum appears most similar to
the Orgueil samples heated to $973-1073$ Kelvin. 2005 UD’s apparent argreement
in slope at longer wavelengths is not indicative of an actual spectral match,
but instead due to the increasing curvature of the sample of Orgueil
throughout the near-infrared. The $1.1-1.60\mu{m}$ slope shows much less
variability than the other two regions, with slopes near $S^{\prime}\sim-1.25$
to $-1.50$ for most temperatures. The $0.6-0.74\mu{m}$ region generally blues
from $\sim 623K$ to $\sim 1073K$ before sharply becoming more red, and a
similar trend is shown for the $1.8-2.2\mu{m}$, though the bluest slope is
achieved at $\sim 973K$. The fact that these changes are centered around the
$973-1073K$ region is unsurprising, as many materials common in CI chondrites
(namely phyllosilicates) begin to break down at these temperatures into other
substances. Despite the fact that the laboratory data appears to intersect the
spectral slopes of Phaethon for all three spectral regions within the range of
temperatures expected for this body, we argue that this is indicative of a
similarity between the sample and the NEO as opposed to a direct spectral
match considering the imperfections in the comparison detailed in the previous
paragraph as well as Figure 3.

Figure 4: The derived spectral slopes (black, in the $S^{\prime}$ notation of
Luu & Jewitt (1990)) at visible ($0.60-0.74\mu{m}$) and near-infrared
($1.1-1.60\mu{m}$ and $1.8-2.2\mu{m}$) of our sample of CI/Orgueil as a
function of maximum temperature reached. The spectral slopes are compared with
those of Phaethon (in blue, using the spectrum from Kareta et al. (2018)) and
2005 UD (in orange-red). Figure 5:  A comparison of the new laboratory data of
CI/Orgueil from this study (left) with the RELAB samples of CI/Ivuna (right,
Hiroi et al. 1996a). The spectra shown are subsets of both datasets centered
around the temperatures most relevant for the study of 2005 UD and Phaethon.
The spectra were all normalized at $0.6\mu{m}$ and subsequently offset
vertically to facilitate comparison with Figure 2. The process by which each
of these sets of spectra were produced and reduced into their current forms is
not identical, see the text for details.

In other words, our new laboratory data and pre-existing laboratory data
support the hypothesis that Phaethon’s surface resembles that of a CI
Chondrite-like material that has been heated significantly due to its orbit.
However, the same cannot be said for 2005 UD. While a red slope in the NIR and
a blue-slope in the visible can be achieved with the Ivuna data of Hiroi et
al. 1996a, the change in slope is at altogether the wrong location, and no
red-sloped spectrum is ever seen in our Orgueil dataset. In summary, we
conclude that a heated CI Chondrite origin for Phaethon is compatible with the
laboratory data, while the same does not appear to be true for 2005 UD given
the currently available spectra.

###  5.2 The Relationship Between Phaethon and 2005 UD

In Section 2, we argued that parent body heterogeneity, differential space
weathering, grain size effects, and phase reddening are each unlikely to be
able to explain the different spectra of Phaethon and 2005 UD. If the objects
were related, then we argued that their spectra could be resultant from
differential heating of their surfaces due to their different orbits, but our
laboratory data does not appear to support that hypothesis.

While it is possible there may be some carbonaceous material (either not yet
recognized or not available among terrestrial sample collections) for which
this hypothetical scenario could have more traction, the fact that CI
Chondrites like Orgueil and Ivuna can reproduce all the key features of
Phaethon’s visible and near-infrared spectrum while not reproducing 2005 UD’s
spectrum suggests that the difference might be more than evolutionary and
instead related to different origins for the two bodies.

While the orbital relationship between the two bodies is indeed debated (see,
e.g., Ryabova et al. 2019), the objects are similar in so many ways that a
shared origin is often assumed in spite of contrary orbital evidence. Blue-
colored objects are fundamentally rare in the inner Solar System (perhaps
$\sim$ 1 in 23 objects, see Bus & Binzel 2002), and inactive meteor shower
parent bodies even more so. The other best-studied inactive parent body,
(196256) 2003 EH1, has a more common organic-rich red surface and a typical
cometary orbit (Kasuga & Jewitt, 2015; Kareta et al., 2021), both of which
lend themselves to more easy explanations than the situation of Phaethon, 2005
UD, and the Geminids. In the absence of other unknown processes operating on
these bodies now or in the recent past, we conclude that the least complicated
interpretation of the data is simply that the objects are only similar by
chance. Should they choose to do so, the $DESTINY^{+}$ team (Arai et al.,
2018) could thus visit two meteor shower parent bodies that have undergone
similar processes yet discrepant origins, thus facilitating a greater
understanding of near-Sun objects and meteor shower creation in general, as
opposed to studying one interconnected system in great detail. Of course,
$DESTINY^{+}$ could also find more compelling evidence for the relationship
between the two bodies than we have found here! Future characterization of
both bodies is an incredibly compelling task, both before and after they are
characterized in-situ.

###  5.3 The Origin of the Geminids?

While an unexpected by-product of trying to explain the difference in spectral
properties between Phaethon and 2005 UD, we have also provided more evidence
in this study that Phaethon’s surface is consistent with that of a heated CI
chondrite from the near-UV through to at least $2.5\mu{m}$. This is consistent
with the composition of the Geminid meteors themselves (Kasuga et al., 2005;
Borovička, 2010; Abe et al., 2020). Considering that the CI chondrites have
been suggested as derived from comets, could the story just be as simple as
”Phaethon really is a dormant comet, and the Geminids are just a normal meteor
shower”? As appealing as that prospect might be, the problem of Phaethon’s
modern non-cometary orbit remains, even if its other properties could likely
be squared with an outer Solar System origin. (There is also still the
question of how best to interpret the origins of the CI chondrites, as while
many of their properties are consistent with comets, the existence of
hydrothermally altered minerals is outside of the expected material properties
of comets significantly.) We consider two scenarios to explain the given
telescopic, meteoric, dynamical, and laboratory data.

The first scenario is that Phaethon’s progenitor region or parent in the Main
Belt was also made out of CI Chondrite like material, but instead of being
recently scattered in from the outer Solar System had been emplaced there
through a ’Grand Tack’-like scenario (Walsh et al., 2011) billions of years
ago. Considering the fact that outer Solar System material has to be in the
Main Belt, and a large fraction of the NEOs have to come from there (see, e.g.
Bottke et al. 2002; Morbidelli et al. 2002), there almost certainly have to be
members of the NEO population which do have CI-like properties. However, given
Phaethon’s non-zero inclination and high eccentricity, the number of plausible
parent bodies or families is not extremely numerous. The recent work by
MacLennan et al. 2020 shows that the high-inclination inner Main Belt Svea
family might be a better match than previous candidates like the family of (2)
Pallas (de León et al., 2010), so further characterization of objects in that
family is of interest.

Another scenario is motivated by dynamical studies of the Geminid meteor
themselves. First raised in Lebedinets 1985 and recently discussed further in
Ryabova 2016 is the idea that the Geminid forming event changed the orbit of
the parent body significantly. This particular aspect of the stream forming
event is thought to be necessary to explain particular aspects of its duration
and spatial extent in the modern day given its young age. If Phaethon’s orbit
did change significantly around the time of Geminid formation, then all
previous backwards orbital integrations of the body likely aren’t capturing
what really happened, and as a result different past dynamical scenarios are
possible. In addition to possibly opening up new possible source regions
(other parts of the Main Belt, the Jupiter Family Comets, etc.), it could also
change how we interpret the ’similar’ orbits of 2005 UD and Phaethon in the
modern day. If even a rough estimate of how much the Geminid parent orbit
would need to change were available, then a suite of dynamical integrations
could be investigated to see how much of our current understanding of
Phaethon’s orbital history is valid. Given the chaotic nature of the inner
Solar System and fairly small Minimum Orbital Intersection Distance (MOID)
with the Earth ($<0.02AU$, JPL Horizons), we suspect even a small change in
Phaethon’s orbit could result in a large change in its modeled dynamical
history, and thus a large change in our understanding of the history of the
object.

##  6 Summary

The relationship between (3200) Phaethon, the (semi-)inactive parent of the
Geminids meteor shower, and (155140) 2005 UD, the inactive parent of the
Daytime Quadrantids, is of great interest for several reasons. The bodies
appear to have similar visible reflectivities and orbits, neither object is
either active in a cometary way or looks obviously like a dormant comet, and
they are the primary and possible secondary targets of the upcoming JAXA
DESTINY+ mission scheduled for the late 2020s. In this work, we have
synthesized new telescopic observations and new laboratory data to attempt to
better understand the relationship between these two objects and their
individual properties.

  * •

We obtained the first near-infrared reflectance spectrum of 2005 UD utilizing
three nights of observations in September and October of 2018, each time
finding a primarily linear red-sloped spectrum. Phaethon’s spectrum is blue
and concave-upwards throughout the same wavelength range (even utilizing the
same instrument and reduction scripts), indicating that the difference between
the two objects is real despite their very similar properties at shorter
wavelengths.

  * •

Either the two objects have similar properties by chance or some process or
effect has altered them from some common starting point. We make arguments
against parent body heterogeneity, differential space weathering, grain size
effects, and phase phase reddening as likely origins of the different spectra
in Section 2.2, but argue that differential thermal alteration could be a
plausible method. The two objects are in highly eccentric near-Sun orbits with
surface temperatures where many likely surface materials begin to thermally
alter and decompose drastically, and Phaethon’s surface (and the Geminids!)
has been previously been interpreted as showing signs of previous or ongoing
intense heating. CI Chondrites that have been heated to Phaethon/UD-like
temperatures seemed promising, but the existing laboratory data did not extend
to high enough temperatures to assess the likelihood of this scenario.

  * •

We heated a sample of the CI Chondrite Orgueil from the Vatican Meteorite
Collection to incrementally higher temperatures up to 1350 Kelvin with a novel
laboratory apparatus described in detail in Section 4. We measured the
reflectance spectra from $\sim 0.5\mu{m}$ to $\sim 2.3\mu{m}$ at each stage
after the sample had cooled back to ambient temperatures to see if our
hypothesis that a heated CI chondrite could match the features of both
Phaethon’s and UD’s spectrum. Our heating sequence of the CI Orgueil is
similar in some ways and different in others from the heating sequence of the
CI Ivuna of Hiroi et al. 1996a, again described in more detail in the text.
The spectrum of Phaethon shares many characteristics with both CI chondrites
(though neither are a perfect match), but UD is not a high quality match to
either object, which argues against the differential thermal alteration
hypothesis.

  * •

We thus propose that the two objects only have similar spectra by chance,
though similar processes likely have acted upon them both. Excitingly,
DESTINY+ might be able to visit two unrelated-but-similar meteor shower
parents, or it might find evidence for their relationship that isn’t easily
attainable from remote sensing. We also highlight several areas of future
inquiry to better understand Phaethon’s modern properties motivated by its
apparent similarity to multiple CI Chondrites and recent studies of the
Geminid meteors themselves.

The authors wish to recognize and acknowledge the very significant cultural
role and reverence that the summit of Mauna Kea has always had within the
indigenous Hawaiian community. We are most fortunate to have the opportunity
to conduct observations from this mountain. This work was supported by a NASA
Near-Earth Object Observations (NEOO) program grant NNXAL06G (PI: Reddy). We
are also thankful to the Vatican Observatory Meteorite Collection for being
willing to loan us sample 1070B of the CI chondrite Orgueil, especially for
these destructive analyses. This appendix provides labelled images and
descriptions of our laboratory apparatus such that the reader can better
understand its usage, both for a more thorough understanding of this work and
in case they one day pursue a similar experimental program.

##  Appendix A Photos of the Vacuum Heating Chamber and Samples

Figure 6: The vacuum heating chamber set up (”the Bar-B-Cube”) on a laboratory
bench at the University of Arizona with individual components labelled. A: the
vacuum pressure gauge, which shows the current pressure inside the chamber as
well as a graphical summary of recent pressure changes. B: the vacuum pump. C:
the light source (a common heating bulb) shining into the input of an optical
fiber which is connected to E. D: a residual gas analyzer mass spectrometer,
attached to diagnose leaks during the initial testing phase. E: the output of
the light emitting fiber as well as the input of the fiber that leads to the
VISNIR spectrometer held in a constant orientation with a 3-D printed black
ABS plastic holder. Figure 7: A: A close-up view from above the chamber
looking through the sapphire viewing window into the interior, showing a
filled sample cup ($D\sim 0.5"$ for scale) on top of the circular heating
element. ”Below” the picture is an identical sapphire window mounted at the
same height inside of which a spectral standard is mounted. The 3D printed
fiber holder fits on top of the window blocking stray external light, and
reducing internal reflections. B: Similar to the A, but showing the thermal
glow of the Nickel sample cup that was obviously visible at the highest
temperatures reached. The bright white-yellow crescent is the edge of the
underlying heating unit which is slightly larger in area than the footprint of
the sample cup.

## References

  * Abe et al. (2020) Abe, S., Ogawa, T., Koji, M., & Arai, T. 2020, Planetary and Space Science, 105040, doi: https://doi.org/10.1016/j.pss.2020.105040
  * Alí-Lagoa et al. (2013) Alí-Lagoa, V., de León, J., Licandro, J., et al. 2013, A&A, 554, A71, doi: 10.1051/0004-6361/201220680
  * Arai et al. (2018) Arai, T., Kobayashi, M., Ishibashi, K., et al. 2018, in Lunar and Planetary Science Conference, Lunar and Planetary Science Conference, 2570
  * Astropy Collaboration et al. (2013) Astropy Collaboration, Robitaille, T. P., Tollerud, E. J., et al. 2013, Astronomy and Astrophysics, 558, A33, doi: 10.1051/0004-6361/201322068
  * Astropy Collaboration et al. (2018) Astropy Collaboration, Price-Whelan, A. M., Sipőcz, B. M., et al. 2018, The Astronomical Journal, 156, 123, doi: 10.3847/1538-3881/aabc4f
  * Belton & A’Hearn (1999) Belton, M. J. S., & A’Hearn, M. F. 1999, Advances in Space Research, 24, 1167, doi: 10.1016/S0273-1177(99)80211-7
  * Bischoff et al. (2006) Bischoff, A., Scott, E. R. D., Metzler, K., & Goodrich, C. A. 2006, Nature and Origins of Meteoritic Breccias, ed. D. S. Lauretta & H. Y. McSween, 679
  * Borovička (2010) Borovička, J. 2010, in Proceedings of the International Meteor Conference, 26th IMC, Bareges, France, 2007, 42–51
  * Bottke et al. (2002) Bottke, W. F., Morbidelli, A., Jedicke, R., et al. 2002, Icarus, 156, 399, doi: 10.1006/icar.2001.6788
  * Bus & Binzel (2002) Bus, S. J., & Binzel, R. P. 2002, Icarus, 158, 146, doi: 10.1006/icar.2002.6856
  * Castillo-Rogez & Schmidt (2010) Castillo-Rogez, J. C., & Schmidt, B. E. 2010, Geophys. Res. Lett., 37, L10202, doi: 10.1029/2009GL042353
  * Clark & Roush (1984) Clark, R. N., & Roush, T. L. 1984, J. Geophys. Res., 89, 6329, doi: 10.1029/JB089iB07p06329
  * Cloutis et al. (2011) Cloutis, E. A., Hiroi, T., Gaffey, M. J., Alexand er, C. M. O. D., & Mann, P. 2011, Icarus, 212, 180, doi: 10.1016/j.icarus.2010.12.009
  * Cushing et al. (2004) Cushing, M. C., Vacca, W. D., & Rayner, J. T. 2004, PASP, 116, 362, doi: 10.1086/382907
  * de León et al. (2010) de León, J., Campins, H., Tsiganis, K., Morbidelli, A., & Licandro, J. 2010, A&A, 513, A26, doi: 10.1051/0004-6361/200913609
  * Delbó et al. (2003) Delbó, M., Harris, A. W., Binzel, R. P., Pravec, P., & Davies, J. K. 2003, Icarus, 166, 116, doi: 10.1016/j.icarus.2003.07.002
  * DellaGiustina et al. (2020) DellaGiustina, D. N., Burke, K. N., Walsh, K. J., et al. 2020, Science, 370, eabc3660, doi: 10.1126/science.abc3660
  * Devogèle et al. (2020) Devogèle, M., MacLennan, E., Gustafsson, A., et al. 2020, The Planetary Science Journal, 1, 15, doi: 10.3847/PSJ/ab8e45
  * Gounelle et al. (2006) Gounelle, M., Spurný, P., & Bland, P. A. 2006, Meteoritics and Planetary Science, 41, 135, doi: 10.1111/j.1945-5100.2006.tb00198.x
  * Gulbis et al. (2011) Gulbis, A. A. S., Bus, S. J., Elliot, J. L., et al. 2011, PASP, 123, 461, doi: 10.1086/659636
  * Halliday (1988) Halliday, I. 1988, Icarus, 76, 279, doi: 10.1016/0019-1035(88)90073-5
  * Hanuš et al. (2016) Hanuš, J., Delbo’, M., Vokrouhlický, D., et al. 2016, A&A, 592, A34, doi: 10.1051/0004-6361/201628666
  * Harris et al. (2020) Harris, C. R., Millman, K. J., van der Walt, S. J., et al. 2020, Nature, 585, 357, doi: 10.1038/s41586-020-2649-2
  * Hiroi et al. (1993) Hiroi, T., Pieters, C. M., Zolensky, M. E., & Lipschutz, M. E. 1993, Science, 261, 1016, doi: 10.1126/science.261.5124.1016
  * Hiroi et al. (1996a) Hiroi, T., Pieters, C. M., Zolensky, M. E., & Prinz, M. 1996a, in Lunar and Planetary Science Conference, Vol. 27, Lunar and Planetary Science Conference, 551
  * Hiroi et al. (1996b) Hiroi, T., Zolensky, M. E., Pieters, C. M., & Lipschutz, M. E. 1996b, Meteoritics and Planetary Science, 31, 321, doi: 10.1111/j.1945-5100.1996.tb02068.x
  * Jewitt & Hsieh (2006) Jewitt, D., & Hsieh, H. 2006, AJ, 132, 1624, doi: 10.1086/507483
  * Jewitt & Li (2010) Jewitt, D., & Li, J. 2010, AJ, 140, 1519, doi: 10.1088/0004-6256/140/5/1519
  * Jewitt et al. (2013) Jewitt, D., Li, J., & Agarwal, J. 2013, ApJ, 771, L36, doi: 10.1088/2041-8205/771/2/L36
  * Kareta et al. (2021) Kareta, T., Hergenrother, C., Reddy, V., & Harris, W. M. 2021, The Planetary Science Journal, 2, 31, doi: 10.3847/PSJ/abd403
  * Kareta et al. (2018) Kareta, T., Reddy, V., Hergenrother, C., et al. 2018, AJ, 156, 287, doi: 10.3847/1538-3881/aaeb8a
  * Kasuga (2009) Kasuga, T. 2009, Earth Moon and Planets, 105, 321, doi: 10.1007/s11038-009-9311-1
  * Kasuga & Jewitt (2008) Kasuga, T., & Jewitt, D. 2008, AJ, 136, 881, doi: 10.1088/0004-6256/136/2/881
  * Kasuga & Jewitt (2015) —. 2015, AJ, 150, 152, doi: 10.1088/0004-6256/150/5/152
  * Kasuga et al. (2005) Kasuga, T., Watanabe, J., & Ebizuka, N. 2005, A&A, 438, L17, doi: 10.1051/0004-6361:200500142
  * Kinoshita et al. (2007) Kinoshita, D., Ohtsuka, K., Sekiguchi, T., et al. 2007, A&A, 466, 1153, doi: 10.1051/0004-6361:20066276
  * Knight et al. (2016) Knight, M. M., Fitzsimmons, A., Kelley, M. S. P., & Snodgrass, C. 2016, ApJ, 823, L6, doi: 10.3847/2041-8205/823/1/L6
  * Lantz et al. (2017) Lantz, C., Brunetto, R., Barucci, M. A., et al. 2017, Icarus, 285, 43, doi: 10.1016/j.icarus.2016.12.019
  * Lebedinets (1985) Lebedinets, V. N. 1985, Astronomicheskii Vestnik, 19, 152
  * Licandro et al. (2007) Licandro, J., Campins, H., Mothé-Diniz, T., Pinilla-Alonso, N., & de León, J. 2007, A&A, 461, 751, doi: 10.1051/0004-6361:20065833
  * Luu & Jewitt (1990) Luu, J., & Jewitt, D. 1990, Icarus, 86, 69, doi: 10.1016/0019-1035(90)90199-J
  * MacLennan et al. (2020) MacLennan, E. M., Toliou, A., & Granvik, M. 2020, arXiv e-prints, arXiv:2010.10633.  https://arxiv.org/abs/2010.10633
  * Marchi et al. (2009) Marchi, S., Delbo’, M., Morbidelli, A., Paolicchi, P., & Lazzarin, M. 2009, MNRAS, 400, 147, doi: 10.1111/j.1365-2966.2009.15459.x
  * Marsset et al. (2016) Marsset, M., Vernazza, P., Birlan, M., et al. 2016, A&A, 586, A15, doi: 10.1051/0004-6361/201526962
  * Marsset et al. (2020) Marsset, M., DeMeo, F. E., Binzel, R. P., et al. 2020, ApJS, 247, 73, doi: 10.3847/1538-4365/ab7b5f
  * Masiero et al. (2019) Masiero, J. R., Wright, E. L., & Mainzer, A. K. 2019, AJ, 158, 97, doi: 10.3847/1538-3881/ab31a6
  * Molaro et al. (2020) Molaro, J. L., Walsh, K. J., Jawin, E. R., et al. 2020, Nature Communications, 11, 2913, doi: 10.1038/s41467-020-16528-7
  * Morbidelli et al. (2002) Morbidelli, A., Bottke, W. F., J., Froeschlé, C., & Michel, P. 2002, Origin and Evolution of Near-Earth Objects, 409–422
  * Nakano & Hirabayashi (2020) Nakano, R., & Hirabayashi, M. 2020, ApJ, 892, L22, doi: 10.3847/2041-8213/ab7d36
  * Ohtsuka et al. (2008) Ohtsuka, K., Arakida, H., Ito, T., Yoshikawa, M., & Asher, D. J. 2008, Meteoritics and Planetary Science Supplement, 43, 5055
  * Pieters (1983) Pieters, C. M. 1983, J. Geophys. Res., 88, 9534, doi: 10.1029/JB088iB11p09534
  * Pieters et al. (1993) Pieters, C. M., Fischer, E. M., Rode, O., & Basu, A. 1993, J. Geophys. Res., 98, 20817, doi: 10.1029/93JE02467
  * Pieters & Hiroi (2004) Pieters, C. M., & Hiroi, T. 2004, in Lunar and Planetary Science Conference, ed. S. Mackwell & E. Stansbery, Lunar and Planetary Science Conference, 1720
  * Rayner et al. (2003) Rayner, J. T., Toomey, D. W., Onaka, P. M., et al. 2003, Publications of the Astronomical Society of the Pacific, 115, 362, doi: 10.1086/367745
  * Reddy et al. (2009) Reddy, V., Emery, J. P., Gaffey, M. J., et al. 2009, Meteoritics and Planetary Science, 44, 1917, doi: 10.1111/j.1945-5100.2009.tb02001.x
  * Ryabova (1999) Ryabova, G. O. 1999, Solar System Research, 33, 224
  * Ryabova (2016) —. 2016, MNRAS, 456, 78, doi: 10.1093/mnras/stv2626
  * Ryabova et al. (2019) Ryabova, G. O., Avdyushev, V. A., & Williams, I. P. 2019, MNRAS, 485, 3378, doi: 10.1093/mnras/stz658
  * Sanchez et al. (2012) Sanchez, J. A., Reddy, V., Nathues, A., et al. 2012, Icarus, 220, 36, doi: 10.1016/j.icarus.2012.04.008
  * Sharkey et al. (2019) Sharkey, B. N. L., Reddy, V., Sanchez, J. A., Izawa, M. R. M., & Emery, J. P. 2019, AJ, 158, 204, doi: 10.3847/1538-3881/ab46c0
  * Takir et al. (2020) Takir, D., Kareta, T., Emery, J. P., et al. 2020, Nature Communications, 11, 2050, doi: 10.1038/s41467-020-15637-7
  * Taylor et al. (2019) Taylor, P. A., Rivera-Valentín, E. G., Benner, L. A. M., et al. 2019, Planet. Space Sci., 167, 1, doi: 10.1016/j.pss.2019.01.009
  * Tholen (1984) Tholen, D. J. 1984, PhD thesis, University of Arizona, Tucson
  * Thompson et al. (2020) Thompson, M. S., Morris, R. V., Clemett, S. J., et al. 2020, Icarus, 346, 113775, doi: 10.1016/j.icarus.2020.113775
  * Čapek & Vokrouhlický (2012) Čapek, D., & Vokrouhlický, D. 2012, A&A, 539, A25, doi: 10.1051/0004-6361/201117697
  * Vernazza et al. (2017) Vernazza, P., Castillo-Rogez, J., Beck, P., et al. 2017, AJ, 153, 72, doi: 10.3847/1538-3881/153/2/72
  * Virtanen et al. (2020) Virtanen, P., Gommers, R., Oliphant, T. E., et al. 2020, Nature Methods, Vol. 17, pp. 261-272, 17, 261, doi: 10.1038/s41592-019-0686-2
  * Walsh et al. (2011) Walsh, K. J., Morbidelli, A., Raymond, S. N., O’Brien, D. P., & Mandell, A. M. 2011, Nature, 475, 206, doi: 10.1038/nature10201
  * Whipple (1983) Whipple, F. L. 1983, IAU Circ., 3881, 1