Investigation of surface homogeneity of (3200) Phaethon
H.-J. Leea,b, M.-J. Kimb1, D.-H. Kima,b, H.-K. Moonb, Y.-J. Choib,c, C.-H. Kima, B.-C. Leeb, F. Yoshidad, D.-G.
Rohb and H. Seob,e
a
Chungbuk National University, 1 Chungdae-ro, Seowon-Gu, Cheongju, Chungbuk, 28644, Korea
b
Korea Astronomy and Space Science Institute, 776 Daedeukdae-ro, Yuseong-gu, Daejeon, 34055, Korea
c
d
University of Science and Technology, 217, Gajeong-ro, Yuseong-gu, Daejeon 34113, Korea
Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba,
275-0016, Japan
e
Intelligence in Space, 96 Gajeongbuk-ro, Yuseong-gu, Daejeon, 34111, Korea
ABSTRACT
Time-series multi-band photometry and spectrometry were performed in Nov.-Dec. 2017 to investigate the
homogeneity of the surface of asteroid (3200) Phaethon. We found that Phaethon is a B-type asteroid, in agreement
with previous studies, and that it shows no evidence for rotational color variation. The sub-solar latitude during
our observation period was approximately 55 °S, which corresponded to the southern hemisphere of Phaethon.
Thus, we found that the southern hemisphere of Phaethon has a homogeneous surface. We compared our spectra
with existing spectral data to examine the latitudinal surface properties of Phaethon. The result showed that it
doesn’t have a latitudinal color variation. To explain this observation, we investigated the solar-radiation heating
effect on Phaethon, and the result suggested that Phaethon underwent a uniform thermal metamorphism regardless
of latitude, which was consistent with our observations. Based on this result, we discuss the homogeneity of the
surface of Phaethon.
Keywords: Minor planets, asteroids, 3200 Phaethon (1983 TB), photometric, spectrometric, surface properties
1
Corresponding author: skarma@kasi.re.kr
1. Introduction
(3200) Phaethon (1983 TB) (hereafter “Phaethon”) was first discovered by the Infrared Astronomy Satellite
(IRAS) in October 1983 (Green & Kowal 1983). Classified as an Apollo-type Near-Earth Asteroid (NEA),
Phaethon is an object that approaches very close to the Sun with a perihelion distance of 0.14 AU. In particular,
Phaethon is regarded as a parent body of the Geminids meteor stream (Whipple 1983; Fox et al. 1984; Green et
al. 1985; Gustafson 1989; Williams & Wu 1993). The parent body of a meteor shower emits dust and a part of the
dust reaches to the Earth. Therefore Phaethon is a body providing extraterrestrial materials to the Earth. The
activity of Phaethon is very unique and appears to be sporadic. Its cometary activity has not been observed in
previous observation data (Urakawa et al. 2002; Hsieh & Jewitt 2005; Kraemer et al. 2005; Wiegert et al. 2008),
and has been only detected by the STEREO (Solar Terrestrial Relations Observatory) spacecraft data (Jewitt & Li
2010; Li & Jewitt 2013). Therefore, Phaethon is a rather unique object that is considered to be a comet-asteroid
transition object (Licandro et al. 2007). Due to these properties, Phaethon was selected as the target of the
DESTINY+ mission of JAXA/ISAS.
Phaethon is a B-type asteroid (Green et al. 1985; Binzel et al. 2001, 2004; Bus & Binzel 2002; DeMeo et al.
2009) and assumed to be a fragment of the large main-belt asteroid, (2) Pallas (de León et al. 2010a). F/B-type
asteroids are commonly linked to dehydrated CI/CM chondrites (Hiroi et al. 1993; Hiroi et al. 1996). The spectral
features of CI/CM chondrites are chemically altered from hydrated C-type to F/B-type by thermal metamorphism
(Tholen 1984; Bell et al. 1989; Hiroi et al. 1993; Hiroi et al. 1996). In other words, a B-type asteroids are
considered to have undergone surface alteration by thermal metamorphism. However, we note that other surface
properties, such as the average grain size of the surface regolith (Binzel et al. 2015, Vernazza et al. 2016), may
also make carbonaceous asteroids to have negative spectral slopes. Ohtsuka et al. (2009) assumed that the heating
source that caused the thermal metamorphism on Phaethon’s surface is solar-radiation and they investigated its
effects. Ohtsuka et al. (2009) calculated its surface temperature caused by solar radiation using the pole solution
of Krygly et al. (2002) and the two extreme thermal models: the Near-Earth Asteroid Thermal Model (NEATM;
Harris 1998) and the Fast Rotation Model (FRM; Lebofsky & Spencer 1989). They revealed that Phaethon can
have a sufficient temperature to dehydrate and decompose surface materials due to solar radiation and that the
solar-radiation heating effect is latitude-dependent. They estimated that the northern hemisphere, in particular the
arctic region north of 45 °N, must have undergone much more dehydration and decomposition than the southern
hemisphere. Furthermore, they attempted to verify their hypothesis by collecting previously observed spectra of
Phaethon, but could not confirm their hypothesis with existing dataset.
As mentioned above, Phaethon is an object that is assumed to have undergone changes on its surface by
collisional/thermal evolutions since its formation. In particular, since Phaethon’s parent body is assumed to have
split into Phaethon and (155140) 2005 UD (hereafter 2005 UD) that have very similar orbit with Phaethon’s one,
approximately ~100 kyr ago (Hanus et al. 2016), it is presumed that some traces from this event are left on the
surface. In fact, rotational surface color variations have been observed on 2005 UD (Kinoshita et al. 2007). Since
Phaethon and 2005 UD have similar orbits, they must have undergone similar space weathering. Therefore, if the
color variation of 2005 UD was caused by fragmentation with Phaethon and subsequent exposure of its fresh
surface, it is expected that rotational color variation should also be detected on Phaethon.
To test the above hypothesis, we performed multi-band photometry and visible-spectrometry in Nov. ~ Dec.
2017 to verify any change in its surface properties. In Section 2, we describe the observation and data reduction
methods. In Section 3, we discuss the surface properties of Phaethon based on our observations. Section 3.1
discusses the taxonomy type of Phaethon, and Section 3.2 discusses the surface homogeneity. Finally, Section 4
presents a summary and conclusions.
2. Observations and data reduction
2.1 Observations
In order to examine the surface homogeneity of Phaethon, we conducted a time-series multi-band photometric
observations at Mt. Lemon Optical Astronomy Observatory (LOAO) in Arizona, USA from November 11 to 13,
2017. We used the 1m telescope and the 4K x 4K e2v CCD. The pixel scale of the CCD camera was 0.8 arcsec/pix,
and the field of view was 28.1 arcmin x 28.1 arcmin. For investigating the suspected color variation according to
the rotation of Phaethon, we used Johnson-Cousins BVRI-filters and the exposure time for each image was 100 s.
The exposure time was determined to maintain the length of the trailed signal of trailed image of the target within
2” on average, considering its sky motion and apparent magnitude. The predicted seeing was smaller than the
seeing in the actual observation results. Therefore, this prediction is reasonable and the asteroid in our observation
were observed to be a point source.
At the same time, we carried out a visible-spectrometry to examine the changes in the spectra of Phaethon in
more detail. The observation was performed at Mt. Bohyunsan Optical Astronomy Observatory (BOAO) in Korea
using the 1.8m telescope, with the 4K x 4K e2v CCD and long-slit spectrograph on December 7, 2017. The width
of the slit used here was 300 𝜇𝑚, and the resolution was 2.4 Å/pix. The observation wavelength was 4,000 ~
7,000 Å, and the exposure time was 300 s. The detail of geometries and observational circumstances are shown
in Table 1.
Table 1. The geometries and observational circumstances. *
Time
Δ
R
α
Exp.
V
Sky Motion
SITE
Seeing
Sky
Observation
Tracking
condition
type
Mode
Time
(UTC)
[AU]
[AU]
[°]
[Mag.]
["/min]
["]
Solar analog
[s]
Multi-band
2017-11-11.4
0.695
1.496
33.1
16.1
0.23
LOAO
3.2
100
Cirrus
photometry
Sidereal
-
Sidereal
-
Sidereal
-
Long-slit
Non-
HD 28099, HD
spectroscopy
sidereal
25680 and HD 29461
(BVRI)
Multi-band
2017-11-12.4
0.675
1.485
32.9
16
0.23
LOAO
2.7
100
Cirrus
photometry
(BVRI)
Multi-band
2017-11-13.4
0.654
1.473
32.8
15.9
0.24
LOAO
2.7
100
Cirrus
photometry
(BVRI)
2017-12-07.7
0.186
1.157
20.7
12.3
4.57
BOAO
2.9
300
Cirrus
*∆: geocentric distance. 𝑹: heliocentric distance. 𝜶: phase angle. V : apparent predicted magnitudes. Exp. Time : exposure time
2.2 Data reduction
2.2.1 Multi-band photometry
The IRAF/CCDRED package was used to pre-process the observed data. We corrected for BIAS, DARK, and
FLAT fielding during this process. The WCS solution of each image was determined by matching with the USNO
B1.0 catalog using SCAMP (Bertin 2006). Aperture photometry was performed using the IRAF/APPHOT
package. The aperture radius was set equal to the Full Width Half Maximum (FWHM) of the stellar profile in
order to have the maximum S/N ratio (Howell 1989). And the inner and outer sky annulus were set to five times
and six times the FWHM, respectively, considering the stellar density in the images. We carried out
standardization to obtain the color indices of Phaethon. The standardization was executed using the ensemble
normalization technique (Gilliland et al. 1988; Kim et al. 1999) with the Pan-STAARS Data Release 1 catalog
(PS DR1; Chambers et al. 2016). The magnitude of the SDSS filters of PS DR1 was convoluted to the JohnsonCousins filter magnitude using the transformation equations proposed by Tonry et al. (2012). The catalog stars
between 11 and 16 magnitude in BVRI were used for photometric calibration. The calibrated magnitude error,
which considers the zero magnitude error and the instrumental magnitude error, has a value of 0.02-0.05 mag. In
addition, the light curve amplitude of Phaethon is less than 0.15 magnitude along its rotation (Hanus et al. 2016;
Kim et al. 2018). Because it was rotated ~11-degrees between the time period when our observations were made
with the first and the last filter. In this case, the light variation of Phaethon can be assumed to be less than 0.01
magnitude during this period, when presuming a gradual change in its brightness. This is smaller than the
photometric error. Hence, the brightness variability was not be considered when calculating the color indices.
2.2.2 Visible spectrometry
For pre-processing of the observed spectroscopic data, BIAS, DARK, and FLAT fielding were corrected using
the same IRAF/CCDRED package as for photometry. For FLAT fielding, the observation images of the THL lamp
were used. In addition, the IRAF/SPECRED package was used for spectrum extraction, wavelength calibration,
flux calibration and Doppler shift correction, and a Fe-Ne lamp was used as the wavelength comparison light
source. To remove the solar spectrum from the observed spectrum of the asteroid, we additionally observed solaranalog stars HD 28099, HD 25680 and HD 29461 (Hardorp 1978; Hoffleit & Jaschek 1991). We used the solar-
analog stars observed at the same air mass as the asteroid to remove solar colors from the asteroid spectrum.
3. Results and discussions
3.1 Taxonomy type
The existing taxonomic type of Phaethon was verified using the color index obtained through standardization
of the multi-band photometry of Phaethon and the spectrum obtained through long-slit spectroscopy. First, we
determined the taxonomic type using the color index, for which we used the mean value of the observed color
indices. The color indices of Phaethon are shown in Table 2. Based on these indices, we estimated that the
taxonomy of Phaethon is B-type using the classification method suggested by Dandy et al. (2003) as shown in
Figs. 1 and 2. The taxonomy of Phaethon was additionally confirmed by the acquired spectra in Fig. 3 which show
to be typical for any B-type asteroid with a blue slope. It was a previously known (Green et al. 1985; Binzel et al.
2001, 2004; Bus & Binzel 2002; DeMeo et al. 2009) and our new observations are consistent with the existing
spectra.
Table 2. Color indices of Phaethon
B–V
V- R
R–I
Ref.
-
0.34
-
Skiff et al. (1996)
0.59 ± 0.01
0.35 ± 0.01
0.32 ± 0.01
Dundon (2005)
0.61 ± 0.01
0.34 ± 0.03
0.27 ± 0.04
Kasuga et al. (2008)
0.64 ± 0.02
0.34 ± 0.02
0.31 ± 0.03
This work
Figure 1. V-R vs B-V two-color diagram. The characters indicate the taxonomic types classified by Dandy et al.
(2003) and the blue dot indicates Phaethon from our observation.
Figure 2. V-R vs V-I two-color diagram. The characters and dot have the same meanings as in Fig. 1.
Figure 3. The spectra of Phaeton taken on Dec. 07 2017. The gray area corresponds to the range of B-type
asteroid spectra (Bus & Binzel, 2002).
3.2 Surface homogeneity of Phaethon
Our photometry and spectroscopy were also used in the investigation of the surface homogeneity by converting
the photometric data to reflectivity using the color index of the Sun proposed by Ramirez et al. (2012). We used
the normalized spectral gradient of reflectivity (S ′ ; Luu & Jewitt 1990) to check whether there are any change in
reflectivity. S ′ can be determined through the following equation:
S ′ (𝜆1 , 𝜆2 ) =
𝑑𝑆 ⁄𝑑𝜆
𝑆𝜆𝑐
(1)
where 𝑑𝑆⁄𝑑𝜆 is the change rate of the reflectivity in the range 𝜆1 < 𝜆 < 𝜆2 , and 𝑆𝜆𝑐 is the reflectivity at 𝜆𝑐 .
𝜆𝑐 denotes a wavelength between 𝜆1 and 𝜆2 . To compare our BVRI photometry with our spectra, we used 4,420
Å, 5,400 Å and 6,470 Å, which are the effective wavelengths of the B, V and R filters, for 𝜆1 , 𝜆2 and 𝜆𝑐 ,
respectively. B-type asteroids don’t show variation in the spectral gradient as function of phase angle (Lantz et al.,
2018). Hence, we didn’t correct the phase reddening effect. Then, the apparent geometry of Phaethon at each
observation date was determined and defined as sub-earth points using the pole solution and shape model obtained
by Kim et al. (2018).
3.2.1 Longitude-dependent surface homogeneity
We examined the longitudinal color variation using our photometry and spectroscopy during the observation
period. The sub-earth latitude during the observation period was ~55°S; thus, our measured colors correspond to
the surface of Phaethon’s southern hemisphere. Fig. 4 shows the spectral gradient for the longitude, obtained from
the observed data. Fig. 4 reveals that there is no longitude-dependent color variation. This suggests that the
southern hemisphere of Phaethon has homogeneous surface properties.
As mentioned in Section 1, if the color variation detected on 2005 UD was caused by fragmentation with
Phaethon, a color variation should appear on Phaethon’s surface as well. However, no color variation was detected
in the observation data for the southern hemisphere. Hence, time-series multi-band photometry and spectra
observations are also required during the apparition in which Phaethon’s northern hemisphere can be observed.
Figure 4. Spectral gradient 𝐒 ′ according to the longitude of Phaeton. The dots indicate 𝐒 ′ determined on the
observation data, and the dashed lines represent the mean values of 𝐒 ′ . The errors in the slope 𝐒 ′ for
spectroscopy were estimated considering the noise in the spectra.
3.2.1 Latitude-dependent surface homogeneity
We collected existing visible spectral data that include un-published data observed by IRTF/SpeX (Rayner et
al., 2003) acquired during various apparitions to examine the latitudinal surface homogeneity of Phaethon. As for
our observation data, we determined S ′ from the collected spectra and examined the latitude-dependent color
variations. The collected spectra, our own spectral dataset and the geometry at the time of observation are listed
in Table 3. In addition, Fig 5. displays Phaethon’s geometrical aspects at the time of spectral observation. Fig. 6
does not show significant latitude-dependent color change in the southern hemisphere. At the same time, it appears
that slope S ′ may drop to below average if more of the northern hemisphere is visible. Hence, we may conjecture
that the northern hemisphere might have experienced more thermal metamorphism than the southern hemisphere.
However, we should be cautious because the deviation is within the error bar.
Table 3. The geometries and spectral gradient 𝑺′ from archival and our spectral datasets.*
Time (UTC)
∆ [AU]
𝑹 [AU]
𝛼 [°]
𝝓𝒔𝒆 [°]
1994-11-15
0.810
1.710
20.0
-35.2
2004-12-10
0.638
1.574
17.5
-8.8
2004-12-20
0.614
1.469
29.6
0.3
2014-11-28
0.841
1.807
9.4
-14.2
2014-11-29
0.834
1.798
9.5
-14.2
2014-12-01
0.819
1.782
10.2
-9.6
2017-10-16
1.235
1.754
33.6
-53.0
2017-12-07
0.185
1.167
20.8
-55.1
𝑺′ [%/103 Å]
Ref.
-1.70 ± 0.15
SMASS web
-1.64 ± 0.15
SMASS web
-1.64 ± 0.15
SMASS web
-1.63 ± 0.10
This work
-1.63 ± 0.15
Binzel et al. (2004)
-1.73 ± 0.10
de León et al. (2010b)
-1.64 ± 0.15
SMASS web
-1.64 ± 0.15
SMASS web
*∆: geocentric distance. 𝑹: heliocentric distance. 𝛼: phase angle. 𝝓𝒔𝒆 : the sub-earth latitude. SMASS web :
http://smass.mit.edu/smass.html
Figure 5. The aspects of Phaethon at the time of spectral observation. The Symbol 𝝓𝒔𝒆 is the sub-earth
latitude.
Figure 6. Spectral gradient 𝐒 ′ according to the latitude of Phaeton. The blue dots indicate 𝐒 ′ determined from
the observation data, and the dashed lines represent the mean values of 𝐒 ′ .
In order to explain why latitude-dependent color variation does not appear, we assumed that the surface of
Phaethon were experienced decomposition from hydrated C-type to F/B-type by thermal metamorphism as it
passes close to the Sun. Hence, we re-calculated the solar-radiation heating effect, which was previously calculated
by Ohtsuka et al. (2009), using the solution and shape model of Kim et al. (2018). For the ephemeris calculation
the JPL Horizons service was used, and the asteroid sub-solar point temperature was determined based on the
energy balance equation on an asteroid’s surface,
𝑆⊙ (1−𝐴) 1/4
𝑇𝑆𝑆 = [
𝑟 2 𝜂𝜀𝜎
]
(2)
where A is the bond albedo, A ≈ (0.290 + 0.684G) 𝑝𝑉 , and we used 𝐺 = 0.15 ± 0.03 and
𝑝𝑉 = 0.12 ± 0.01 (Hanus et al. 2016). 𝑆⊙ denotes the solar constant, solar flux at 1 𝐴𝑈 , for which
~1366 𝑊𝑚−2 was used. And 𝜀 is the infrared emissivity and was assumed to be 0.9. In addition, 𝜂 is the
beaming parameter at the solar phase angle of 𝛼 = 0°, for which the mean value for a B-type asteroid of 1.0 ± 0.1
was used (Alí-Lagoa et al. 2013).
The local surface temperature was calculated using the following equation:
𝑇𝑙𝑜𝑐 = 𝑇𝑆𝑆 cos 𝜃𝑖 1/4
(3)
where 𝜃𝑖 is the angular distance between the sub-solar point and the local point. The temperature of the night
side was assumed to be 0 K.
To explain the surface properties of Phaethon in term of local surface temperature, we used the thermal
metamorphism condition of CI/CM chondrites that was proposed by Nakamura (2005). He distinguished that the
thermal metamorphism conditions of CI/CM chondrites occur in four stages according to the surface temperature.
Stage I (heating < 250 °C; 523 K): This is the lowest heating stage, which is a layered sensitive matrix,
and the hydrous minerals of saponite, serpentine and tochilinite exist with no considerable heating
effect.
Stage II (approximately 250–500 °C; 523-773 K): The serpentine is amorphous, but the olivine is
recrystallized. No tochilinite exists, and newly created troilite with low crystallinity exists.
Stage III (approximately 500–750 °C; 773-1023 K): This exists between low crystallinity, finegrained olivine amorphous matrix phases, and newly created troilite with a low crystallinity exists.
Stage IV (approximately > 750 °C; 1023-1273 K): The matrix olivine is perfectly anhydrous. The
matrix olivine has become well-crystalline, and the newly created troilite is intergrown.
We examined the newly calculated local surface temperatures by dividing them into six regions based on the
latitude. The range of each region is shown in Table 4. Fig. 7 shows the surface temperature variations for different
regions along the orbit. This figure also confirms that Phaethon's temperature reaches the highest at 15 ° N and it
tends to decrease as it moves away from the 15 ° N region. Nevertheless, the latitude temperature distribution of
Phaethon suggests that it may have almost compositionally homogeneous surface. At the same time, the 15 ° N
region might have partially experienced more regional thermal metamorphism. Our speculation seems to be
consistent with our observations. The result of Ohtsuka et al. (2009) conflicts with our simulation. In particular,
the surface temperature of its south pole does not reach Stage III and IV in their FRM model. This difference is
caused by the different pole orientation they used. The pole orientation is important parameter to determine the
incident flux of solar radiation on surface which heavily depends on latitude. Their calculation was based on pole
orientation solved by Krugly et al. (2002). However, the pole orientation of Krugly et al. (2002) shows
disagreements with subsequent studies (see, e.g. Table 3 of Kim et al. 2018), and our test was calculated employing
Kim et al. (2018)’s pole solution. Hence, the uniform color of Phaethon in latitude determined from available
spectral data is consistent with the result of our analysis of solar-radiation heating effect. However, spectral data
currently available for Phaethon does not fully cover its surface in latitude, and especially miss the northern
hemisphere. Therefore, follow-up observations for other apparitions with a different viewing geometry in which
the other hemisphere can be observed is required to confirm the homogeneity of the surface.
Table 4. Latitude region ranges
Latitude
Range
NP
60° ~90°
45°N
30° ~ 60°
15°N
0° ~ 30°
15°S
-30° ~ 0°
45°S
-60° ~ -30°
SP
-90° ~ -60°
Figure 7. The maximum surface temperature, Tloc, according to the mean anomaly, M, of each region. The
dashed lines show the boundary points of each stage as presented in Nakamura (2005).
4. Summary and Conclusions
We conducted time-series observations using 4-band photometry and long-slit spectroscopy to verify the
taxonomy and homogeneity of Phaethon’s surface. The results showed that it is a B-type asteroid, which is
consistent with the results of previous studies, and that its southern hemisphere has a homogeneous surface. We
also collected archival data and compared them with our dataset which revealed that that there was no surface
inhomogeneity in latitude direction. We re-calculated the solar-radiation heating effect for Phaethon using the
solution of Kim et al. (2018). We calculated its surface temperature using the energy balance equation on an
asteroid’s surface, and the result confirmed that the solar-radiation heating effect on Phaethon’s mineralogy
appears similar at every latitude, implying a latitudinally uniform surface for this asteroid. However, our
observation data are still insufficient to cover the entire surface. Therefore, additional time-series multi-band
photometry and spectral observations are required to completely understand the surface nature of Phaethon.
Acknowledgements
This research is supported by the Korea Astronomy and Space Science Institute (KASI). Part of the data
utilized in this publication was obtained and made available by the MIT-UH-IRTF Joint Campaign for NEO
Reconnaissance. The IRTF is operated by the University of Hawaii under Cooperative Agreement no. NCC 5538 with the National Aeronautics and Space Administration, Office of Space Science, Planetary Astronomy
Program. The MIT component of this work is supported by NASA grant 09-NEOO009-0001 and by the
National Science Foundation under Grants Nos. 0506716 and 0907766.
Appendix A. Supplementary data
Supplementary data related to this article can be found at
https://data.mendeley.com/datasets/k3b7s7d6dt/draft?a=f351eeee-7923-492c-a507-7004da1ca5f9.
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