Download Surface reflectance properties of distant Solar system bodies

1997MNRAS.290..186G
Mon. Not. R. Astron. Soc. 290,186-192 (1997)
Surface reflectance properties of distant Solar system bodies
s. F. Green,i
N. McBride, 1 D. P.
6 Ceallaigh,2 A. Fitzsimmons,2 I. P. Williams 3 and
M. J. Irwin4
Unit for Space Sciences and Astrophysics, Physics Laboratory, The University of Kent, Canterbury cn 7NR
APS Division, Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 1NN
3 Astronomical Unit, Queen Mary and Westfield College, Mile End Road, Landon E1 4NS
4 APM Unit, Royal Greenwich Observatory, Madingley Road, Cambridge CB3 OEZ
1
2
Accepted 1997 May 16. Received 1997 May 16; in original fonn 1996 September 18
ABSTRACT
The recent discoveries of over 40 new objects with orbits beyond 30 au represent the first
sampling of a reservoir of objects lying beyond Neptune, known as the Kuiper Belt (or
Edgeworth-Kuiper Belt), which may be a source of short-period comets and Centaurs (objects
whose orbits cross those of the giant planets). There are very few observations from which to
derive physical properties of these Kuiper Belt objects (KBOs) due to their faintness and the
concentration on discovery rather than follow-up, although optical and near-IR photometry of
the few brightest KBOs and Centaurs shows a diversity from neutral to extremely red colours.
We present new BVRI photometry of five KBOs (l994JQb 1995DC 2 , 1994JR b 1995DA2 and
one undesignated new KBO) and the Centaur object 1995DW2. With the current small sample,
we find no compelling evidence for a correlation of colours with orbital zones, and
consequently no clear mechanism to explain this diversity in terms of the irradiation mantle
model.
Key words: comets: general - minor planets, asteroids - Solar system: general.
1
INTRODUCTION
The recent discoveries of over 40 new objects with orbits beyond 30
au represent the first sampling of a reservoir of objects lying beyond
Neptune. This reservoir, known as the Kuiper Belt (or more
correctly the Edgeworth-Kuiper Belt) is one of the major discoveries of Solar system science in recent years. The existence of
Kuiper Belt objects (KBOs) was originally postulated from considerations of the mass distribution in the early Solar system (see
Edgeworth 1949; Kuiper 1951), but has since been strengthened by
observations of extended discs around main-sequence stars (e.g.
Aumann et al. 1984) and the need for a source of the short-period
(SP) comets. The spherically symmetric 60rt cloud (source of the
long-period comets) cannot produce the inclination distribution of
the predominantly prograde orbits of SP comets (Duncan, Quinn &
Tremaine 1988; Quinn, Tremaine & Duncan 1990), although
evolution from the inner edge of a disc of icy planetesimals orbiting
between ~40 and 100 au could provide this source, since dynamically chaotic regions exist inside 40 au (e.g. Morbidelli, Thomas &
Moons 1995; Malhotra 1996). The Centaurs (minor bodies in
planet-crossing orbits between Jupiter and Neptune), of which six
are currently known, are believed to represent the largest members
of a transitional group between the Kuiper Belt and SP comets. The
current detection rate implies a population of a few X 104 KBOs with
semi-major axes 30-:-50 au and diameters >100 kIn, with a few tens
of Centaurs of this size awaiting discovery. By a statistical analysis
of possible faint objects found on long-exposure frames taken with
the Hubble Space Telescope (HST), Cochran et al. (1995) found that
many of the possible objects identified were not just noise, but were
likely to be Halley-sized (i.e. ~ 10 kIn) KBOs (although no one
object could be confirmed as being 'real'). This suggests that there
must be > 2 x 108 objects with diameters> 10 kIn in the Kuiper Belt
with inclinations <120 and within ~40 au of the Sun.
There are very few observations from which to constrain the
physical properties of the trans-Neptunian objects, due to their
faintness and the concentration on discovery rather than follow-up.
However, the largest Centaurs (Chiron and Pholus) which presumably originated in the Kuiper Belt have low albedos and similar
sizes, but very different spectra. Chiron, which has recently exhibited cometary activity in its approach to perihelion, has neutral
colours similar to C-type asteroids and some cometary nuclei
(Hartmann et al. 1990). However, Pholus was found to have an
astonishing spectrum (Fink et al. 1992) which made it one of the
reddest objects in the Solar system (Buie & Bus 1992). Objects
1993HA2 (Luu 1994) and 1992QB 1 (Jewitt & Luu 1993) also
appear to have extreme red colours, whereas 1993FW is less red
(Luu & Jewitt 1993).
These extreme red optical and infrared colours (Davies, Sykes &
Cruikshank 1993) have been interpreted as due to the presence of
organic tholins which result from radiation bombardment of mixtures of methane and other organic molecules (Fink et al. 1992;
Mueller et al. 1992; Binzel1992). One possible scenario is that icy
© 1997 RAS
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..186G
Distant Solar system bodies
planetesimals containing simple volatiles (e.g. H 2 0, CO, CO2 ,
NH3 , CH4 , ~, N2 ) are irradiated by UV photons and cosmic
rays over 4.6 x 109 yr in the Kuiper Belt (or bort cloud) to produce
an 'irradiation mantle'. Collisions could also expose un-irradiated
material. Luu & Jewitt (1996) estimate ~ 10 impacts of km-sized
bodies on a 100-km KBO in the lifetime of the Solar system. This
may result in resurfacing by both fresh subsurface ice and material
from the impactor. The true colours of KBOs could therefore be an
indicator of the degree of collisional resurfacing and may be
correlated with location in the Kuiper Belt. Ideally, spectra of the
objects are needed, although their extreme faintness makes this
difficult. Luu & Jewitt (1996) obtained low-resolution optical
spectroscopy of 1993SC (one of the brightest objects), in the
wavelength range 400 to 800 nm. The relative reflectance spectrum
of 1993SC (object spectrum divided by the spectrum of a solar-type
star and normalized to 1 at A = 560 nm which provides a measure of
the intrinsic colour of the object) exhibited a positive gradient (i.e.
reddened object) intermediate in value between those of Chiron and
Pholus.
The overall effect of either of these two processes on the surfaces
of KBOs is unknown at present. Application of resurfacing models
requires the observation of colours of KBOs, of their probable
dynamical cousins Centaurs, and of inactive SP comets. In this
paper we present optical colours for bodies in all three populations.
2
OBSERVATIONS
2.1 Anglo-Australian Telescope
Observations with the 3.9-m Anglo-Australian Telescope, New
South Wales, were obtained on 1995 March 28, 29 and 30.
Images of 1024 x 1024 pixels were obtained using the TEK CCD
chip with Kitt Peak V, R and I filters used at prime focus, providing
an image scale of 0.39 arcsec per pixel. Readout noise was 4.8
electrons with a gain of 2.74 electrons per ADU. Exposures of
1000 s in R separated by at least an hour were used to identify the
targets before colour measurements were made. Exposures with the
I filter were limited to 500 s to prevent sky background saturation.
Guiding was at sidereal rate except for comet 109P/Swift- Tuttle
which was tracked. Flat-fields were obtained from median-filtered
twilight sky observations at the beginning and end of each night.
On March 28, seeing deteriorated from 2 to 3 arcsec during the
night, effectively preventing detection of any of the KBOs
attempted, with only the Centaurs 1995DW2 and 1993HA2 being
observed. Poor seeing also compromised observations on March 29,
although 109P/Swift-Tuttle was identified in a crowded field.
Several KBOs were not found, although 1994JS and 1994JV
should have been sufficiently bright for detection even in 3-arcsec
seeing. However, a pair of search frames adjacent to the predicted
position for 1994JS revealed a new object at 7 arcmin from the
expected 1994JS position which was confirmed the following night.
Although the short arc could be fitted by a main belt asteroid close
to its stationary point, a subsequent independent discovery at the
European Southern Observatory (Lagerkvist, Lagerros & Magnusson 1995) resulted in its identification as a KBO designated
1995FB 21 • 1994JR1 was detected but was too faint for colours to
be determined.
Improved seeing of 1 to 1.5 arcsec on March 30 allowed colours
to be determined for 1995DA2 and 1995DC2 , detection of 1995DB2
and 1994ES2 , and a search of 18 x 18 arcmin2 about the subsolar
point (although no new slow-moving objects were discovered from
this search), before patchy clouds prevented colour determination
187
for 1994JRI. During these observations calibration frames were
taken of fields RUl49, PGl047, PG1323 and PGl633 (Landolt
1992), providing colours in the Kron-Cousins photometric system.
2.2 Isaac Newton Telescope
Observations of 1993FW and 1994JQI were obtained with the
2.5-m Isaac Newton Telescope of the Observatorio del Roque de
Los Muchachos, La Palma, on the nights of 1995 March 31 and
April 1. Images of 1024 x 1024 pixels were obtained using the
TEK3 thinned CCD chip at the prime focus, giving an effective
image scale of 0.59 arc sec per pixel. Observations were made with
900-s exposures through broad-band Band R (Harris) filters during
which the telescope was auto-guided at the sidereal rate. The CCD
had a readout noise of 5-6 electrons and a gain of 0.76 electrons per
ADU. Each KBO was easily detected in the centre of the CCD
frame by visually blinking. Bias frames were recorded and flatfields of the evening twilight sky were obtained in both R and B
filters. Images of the standard fields SAlOl, SA107 and SA110
(Landolt 1992) were measured through both B and R filters on
both nights, allowing subsequent magnitudes in the Kitt Peak
system to be derived. The observing conditions were photometric
throughout the observing run, thus enabling absolute flux calibration. Unfortunately we could not obtain a value for the R-band
extinction for the second night due to difficulties in reading out
these particular standard field images. However, it was possible to
obtain a photometric calibration for the second night via the large
overlap with the first night's images. The nightly extinction values
obtained were larger than expected from theoretical calculations
(King 1985) due to the presence of Saharan dust above the
observatory.
2.3 Data reduction
The CCD images were processed in the usual manner. All the
images were bias-subtracted and divided by a flat-field using the
software package KAPPA (Currie 1992). A similar procedure was
then used to reduce the observations from each site, combining
aperture photometry using PHOTOM (Eaton 1989) and profile fitting
from DAOPHOT II (Stetson 1987). Instrumental magnitudes of the
Landolt standard stars were obtained using the aperture photometry
package PHOTOM. At this point, PHOTOM could have been used to
obtain the instrumental magnitudes of the KBOs, although on
investigation it was found that aperture photometry of these
extremely faint objects could result in large uncertainties due to
the non-uniformity of the sky from faint background objects.
Therefore the profile fitting from DAOPHOT II was used to obtain
the instrumental magnitudes of the KBOs [this was reasonable as
the motion of KBOs during each exposure was smaller than the
seeing, and hence the KBO point spread functions (PSFs) were
comparable to those offield stars]. However, the problem is further
complicated, as calibration using profile fitting alone would be
urrreliable due to the differences in PSF for exposures of a few
seconds on bright standards and ~ 1000 s on faint objects. Thus a
selection of 15-20 field stars in each KBO frame was used to
determine the PSF and provide instrumental magnitudes using both
aperture photometry and profile fitting. They could then be used to
transfer the standard star calibration to the KBOs' profile fitted
magnitudes.
This procedure was not possible for 109P/Swift-Tuttle because
of the trailed field star images, and so just aperture photometry
using PHOTOM was employed on three frames taken in V, R and l.
© 1997 RAS, MNRAS 290, 186-192
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..186G
188
S. F. Green et ai.
Table 1. Aspect data and magnitudes. The dates are for mid-frame, and are light-time-corrected. The slow-moving object (SMOl) was
discovered on the same frame as 1993FW.
Object
1995DW2
1993HA2
I 994JR1
109P/Swift
-Tuttle
1995FB21
1995DA2
1995Dy
1995DB2
1995ES2
1993FW
SMOI
1994JQI
Date
r
..i
[AU]
Exp
[sec]
Magnitude
[AU]
ex
[deg]
Filter
[UT]
19950328.5817
.5861
.6291
.6577
.6675
.6745
19950328.6214
.6830
.6901
.6972
.7919
19950329.7180
.7741
19950330.7437
.7601
19950329.4112
.4180
.4253
.4805
.4898
.4973
19950329.7412
.7814
19950330.7491
.7898
19950330.3992
.4652
.4917
.5018
.5086
19950330.3603
.5252
.5648
.5747
.5815
19950330.4127
19950330.4388
19950401.0245
.0841
.1373
19950401.0245
.0841
.1373
19950401.1229
.1765
18.892
17.896
0.2
R
R
R
100
300
300
1000
500
500
500
500
500
500
500
1000
1000
1000
1000
500
500
500
500
500'
500
1000
1000
600
300
1000
1000
1000
500
500
1000
1000
1000
500
500
1000
1000
900
900
900
900
900
900
900
900
21.69:<::: 0.12
21.85:<::: 0.10
21.65:<::: 0.08
21.96:<::: 0.06
21.08:<::: 0.08
21.07:<::: 0.08
19.88:<::: 0.02
V
12.509
11.848
R
3.5
R
V
34.786
34.247
1.4
8.531
8.009
5.9
R
R
R
R
V
R
R
R
R
V
42.426
41.782
1.0
34.000
33.533
1.5
R
R
R
R
R
R
V
I
45.208
44.406
R
R
0.8
V
I
I
40.575
45.805
42.094
40.090
44.943
41.095
1.2
0.6
0.0
43.131
42.190
0.5
Three additional R frames were available, but the object passed
through the outer wings of a reasonably bright star image in these.
However, by measuring the object on all three frames, and also
measuring the sky contribution at the positions that the object
would/did occupy, we obtained separate object and sky contributions for each frame. The R magnitudes derived with this method
compare favourably (Table 1) with the R magnitude obtained when
the object was away from the bright star.
Subsequent inspection of the 1993FW frames revealed a slowmoving object with similar near-opposition motion, called here
SM01. Unfortunately, attempts to recover this object about 1 month
later failed, and it remains unrecovered.
R
R
R
B
R
R
B
R
B
R
3
Comment
Blended with star
Blended with star
Blended with star
19.82:<::: 0.03
22.19:<::: 0.04
22.18:<::: 0.04
22.30:<::: 0.Q7
23.05:<::: 0.05
20.80:<::: 0.10
20.85:<::: 0.10
20.87:<::: 0.10
20.83:<::: 0.03
21.39:<::: 0.04
20.06:<::: 0.05
23.29:<::: 0.09
23.38:<::: 0.10
23.18:<::: 0.Q7
23.22:<::: 0.10
23.06:<::: 0.06
23.10:<::: 0.06
23.65:<::: 0.08
22.67:<::: 0.13
22.53:<::: 0.11
23.09:<::: 0.06
23.62:<::: 0.Q7
24.39:<::: 0.13
22.91 :<::: 0.11
23.17:<::: 0.13
24.07:<::: 0.12
24.07:<::: 0.13
22.44:<::: 0.17
24.12:<::: 0.24
22.30:<::: 0.16
22.07:<::: 0.11
24.00:<::: 0.19
22.21 :<::: 0.15
24.18:<::: 0.25
22.71:<::: 0.14
Near star
Near star
Near star
Discovery
frames
RESULTS
Aspect data and magnitudes are listed in Table 1. Quoted 1cr
uncertainties are based on statistical errors from photon counting
combined with calibration transfer. The statistical uncertainties
are generally larger than might be expected from extrapolating
the performance of the CCD cameras on these telescopes quoted
in the user manuals, probably due to contamination by faint
background objects. In addition, any unknown rotational light
curve effects may contribute to temporal changes in brightness,
and these have not as yet been fully investigated. Williams et al.
(1995) suggested the possibility that 1993SC might have a
© 1997 RAS, MNRAS 290, 186-192
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..186G
Distant Solar system bodies
189
Table 2. Optical colours of outer Solar system objects.
Object
Group
q
e
[AU]
1992QB!
1993FW
1994JQ!
1995DC2
1993SC
1994JR!
1995DA2
SM01
KBO
KBO
KBO
KBO
KBO
KBO
KBO
KBO?
CEN
95P/Chiron
5145 Pho1us
CEN
B-V
a
[AU]
[deg]
40.0
41.6
43.0
40.1
32.1
34.7
32.9
0.07
0.05
0.03
0.09
0.19
0.13
0.09
44.2
43.8
44.1
44.0
39.8
39.8
36.3
2.2
7.7
3.7
2.3
5.1
3.8
6.6
8.5
0.38
13.7
6.9
B-R
V-R
R-J
Diameter
[km]
0.6:t0.1a
0.4:t0.1b
1.0:t0.2a
0.77:t0.16
0.54:t0.14c
0.75:t0.09
0.55:t0.11
0.58:t0.16
0.43:t0.14c
283
286
220
224
0.38:t0.03 e !
0.39:t0.03 e2
0.34:t0.03 e3
0.34:t0.03 e4
0.34:t0.03 e !
0.32:t0.04e2
0.28:t0.03 e3
168:t2of
0.76h !
0.68h2
189:t26i
1.47:t0.28
0.50:t0.16
Albedo
214
145
1.88:t0.21
8.7
0.58
20.4
24.7
0.70:t0.02d
1.35h2
0.75 h !
0.66h2
0.81O:t0.00&'
0.81 :to.02k
0.87:t0.05 k
0.78:t0.ozi
0.84:t0.03/
~0.7m
1993HA2
1995DW2
1PIHalley
2PlEncke
10PfTempe12
CEN
CEN
COM
COM
COM
11.8
18.9
0.59
0.33
1.48
0.52
0.25
0.97
0.85
0.52
24.8
25.0
17.9
2.2
3.1
15.6
4.1
162
11.9
12.0
0.73:t0.03 n
0.81:t0.0]!'
0.94:t0.03 Q
26P/Gicobini-Zinner
28PINeujmin 1
COM
COM
1.03
1.55
0.71
0.78
3.5
6.9
14.2
0.84:t0.04"
49PIArend-Rigaux
COM
1.44
0.60
3.5
109P/Swifi-Tuttle
COM
0.96
0.96
26.3
113
0.31:t0.1O
0.44:t0.03 n
0.47:t0.0]!'
0.56:t0.03 Q
0.53:t0.03'
0.49:t0.02'
0.47:t0.03"
0.50:t0.04'
0.47:t0.01'
0.56:t0.10
0.48:t0.07 n
0.56:t0.1O .
0.77:t0.1O
0.51:t0.02'
~O.1:Y
0.14~g:~g
«l.044:t 0.0 13i
82
80
9.3:t0.3°
4.4-9.8P
~0.04°
1O.6~?:!r
0.03:t0.01r
~20"
0.02-0.03"
9.6:t0.8 v
9.0~?:6w
24
0.054:t0.01 v
0.06:t0.03 w
Colours are derived from observations reported here, and diameters assuming an albedo of 0.04, except: a Jewitt & Luu (1993); b Luu & Jewitt (1993); c Davies et
al. (1996); d Lebofsky et al. (1984); e Hartmann et al. (1990) [(1) 1988 February 20; (2) 1988 February 21; (3) 1988 February 22; (4) 1988 September 9];
f Altenhoff & Stumpff (1995); g Campins et al. (1994); h Mueller et al. (1992) [(1) 1992 January 9; (2) 1993 January 23]; i Davies et al. (1993); j Buie & Bus
(1992); k Fink et al. (1992); deduced here from their reflectance spectrum; I Binzel (1992); deduced here from his reflectance spectrum; mLuu (1994); nThomas &
Keller (1989); deduced here from their reflectivity gradients; ° Keller et al. (1994); P Luu & Jewitt (1990); deduced here from their reflectivity gradient; qJewitt &
Luu (1989); deduced here from their reflectance spectrum; r A'Hearn et al. (1989); , Jewitt & Meech (1988); , Luu (1993); deduced here from her reflectivity
gradient; U Campins et al. (1987); deduced here from their reflectance spectrum; v Tokunaga & Hanner (1985); albedo at 1.25 fLm; W Birkett et al. (1987); albedo
at 1.25 fLm.
O.5-mag light curve, but Davies, McBride & Green (1997) find no
light curve behaviour greater than 0.2 mag (i.e. within the uncertainties in the photometry) deduced from 33 frames taken over
five nights. Significant light curve behaviour, however, cannot be
ruled out for other objects. We assume here that any variations
due to varying phase angle are less than the formal errors in our
photometry.
The colours listed in Table 2 have been derived from the data with
the shortest timebase. Where multiple exposures in the I filter were
performed consecutively, the results have been combined. Quoted
errors are simply the combined errors from each magnitude,
although the unknown effects of background contamination and
light curves may result in rather larger errors.
Table 2 also lists derived diameters assuming an albedo of 0.04
unless otherwise stated. The only objects for which radiometric data
are available are Pholus, with a low geometric albedo (:50.08,
Howell et al. 1992; :50.044, Davies et al. 1993), and Chiron, which
has a surprisingly high albedo of -0.12 (Lebofsky et al. 1984;
Campins et al. 1994; Altenhoff & Stumpf 1995). Cometary nuclei
close to the Sun also have low albedos in the range 0.03-0.05 (see
the review by A'Hearn 1988 and references therein). Since the
calculated diameter is proportional to the inverse square root of the
albedo and we assume a most likely value near the bottom of the
possible range (0.03 to -0.2), true diameters might be much
smaller than the values derived. In this case light curve amplitudes
might also be expected to be larger due to the lower gravity,
allowing greater deviations from spheroidal shapes. In addition,
Centaurs or KBOs may have significant albedo variegation such
as that detected on Pluto (Binzel 1988; and with the HST: Stem,
Buie & Trafton 1997). It is clear that conclusive data can only be
obtained by repeated observation to identify the magnitude of
such effects, as has been performed for 1993SC (Davies et al.
1997), and careful removal of potential background sources.
Davies et al. (1997) comment on the potential hazards in photometry of such faint moving objects.
Table 2 also contains all the measured optical colours to date,
including some derived from published spectroscopy. In most cases,
the data have been obtained in the Kron-Cousins system and we
have used appropriate wavelengths given by AB = 440 nm,
AV = 550 nm, AR = 650 nm and Al = 830 nm, and solar colours
given by (B-V)o = 0.67, (V-R)o = 0.36, (R-l)o = 0.35 (Meech,
Knopp & Farnham 1995). The quoted spectral indices S'
© 1997 RAS, MNRAS 290,186-192
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..186G
190
S. F. Green et al.
(100run)-1 are converted to relative reflectance X(A) using
X(A)
= 1 + S (A ~~v),
(1)
where Aand AV are in run, and colours using
mv - mA
4
= 2.5 log X(A) + (mv -
mA)o.
(2)
DISCUSSION
4.1 Asteroids and comets
Use of colorimetry usually represents the first attempt in classification of unresolved objects. In the case of asteroids, taxonomies
are based on spectrophotometry in the 0.3-1.1 f.Lm range augmented by radiometric or polarimetric albedos. Photometry in standard
broad-band or specially designed filters at diagnostic wavelengths
is used for fainter targets. Taxonomies of asteroids (see e.g. reviews
by Tholen & Barucci 1989; Gradie, Chapman & Tedesco 1989) are
related to mineralogic interpretation of spectral features which
provide insight into the formation conditions and processes as a
function of heliocentric distance. In the outer asteroid belt, objects
are characterized by low albedos and neutral colours associated
with carbonaceous minerals (C type). With increasing heliocentric
distance, D types (suggested carbon/organic-rich silicate composition), with low albedo and reddened spectra longward of 0.55 f.Lm
(some flat longward of 0.95 f.Lm), are prevalent.
Most comet photometry is dominated by gaseous emissions and
dust particle scattering from the coma, preventing direct photometry of the nucleus. The few nuclear spectra or colours available
(listed in Table 2) are obtained at large heliocentric distance where
there is no significant volatile emission, or from observations of
low-activity comets at small geocentric distances. These appear to
be similar to D-type asteroids (with similar low albedos), although
Neujmin 1 is considerably redder, as is Swift-Tuttle from data
reported here.
4.2 The Centaurs
There are currently seven known Centaur objects (2060 Chiron,
5145 Pholus, 1993HA2 , 1994TA, 1995DWz, 199500 and
1997CU26 ). After comet IPIHalley, Chiron is perhaps one of the
most studied comets of all. Discovered in 1977 (Kowal 1977), its
orbit ranges from 8.5 au (inside Saturn's orbit) to almost 19 au
(Uranus' orbit). Although originally classified as an asteroid, it has
since exhibited cometary behaviour in the form of secular brightness fluctuations as well as short-term impUlsive photometric
variations. Between mid-1987 and the end of 1988, Chiron began
to exhibit unequivocal cometary behaviour, brightening by 1.3 mag
by 1990 (Tholen, Hartmann & Cruikshank 1988; Hartmann et al.
1990). Meech & Belton (1990) detected a low surface brightness
coma around Chiron at a heliocentric distance of 11.8 au. In
addition to these long-term changes in brightness which occur
over a period of years, Chiron also exhibits short photometric
variations (-0.1-0.2 mag) which occur over periods as short as a
few hours (Luu & Jewitt 1990; Buratti & Dunbar 1991; Bus et al.
1991). Spectrally, Chiron is neutral in the near-infrared and nearneutral in the visible with no evidence for strong H2 0 absorption
bands (Lebofsky et al. 1984; Hartmann et al. 1990; Luu & Jewitt
1990). Although spectrally similar to C-type asteroids, it has an
unusually high albedo of 0.1-0.13, similar to 2 Pallas. The colours
listed in Table 2 are for times when the coma intensity contribution
is less than that of the nucleus, at heliocentric distances> 12 au.
5145 Pholus is the reddest Centaur observed to date. Its extreme
V-R colour of 0.810 ± 0.006 (Buie & Bus 1992) and maximum
albedo of 0.044 ± 0.013 (Davies et al. 1993) are believed to be due
to the presence of tholins on its surface as a result of long-term
exposure of organic molecules to energetic radiation. Deep CCD
imaging shows an object which is stellar in appearance with no
evidence of a coma (Hainaut & Smette 1992).
Fink et al. (1992) obtained a visible spectrum of Pholus which
displayed neither absorption nor emission features (although some
structure has been determined in the near-infrared: Davies, Sykes &
Cruikshank 1993; Luu, Jewitt & Ooutis 1994). They compared its
spectrum with those of several terrestrial and meteoritic spectra (see
fig. 3 of Fink et al. 1992). The only class of compounds that seemed to
match the steep spectrum ofPholus were the organic residues, tholins.
Fink et al. (1992) compared tholins produced by UV irradiation and
those produced by a spark discharge. A combination of both of these
organics could adequately match the spectrum of Pholus.
1993HA2 has a V-R-0.7, approaching that ofPholus, although
the uncertainty in this value may be large. Davies, Tholen &
Ballentyne (1996) find a V-J intermediate in value between those
of Chiron and Pholus. 1995DW2 has similar colours to Chiron but
no activity has been reported.
4.3 Kuiper Belt objects
The known KBOs appear to be divided into two dynamical groups.
About half of them appear to have nearly circular, low-inclination
(and generally stable) orbits with perihelion distances 40-45 au.
The rest have perihelion distances of around 30-36 au. Marsden
(personal communication) speculated that these were in 2:3 resonance with Neptune (like Pluto), allowing several objects to be
recovered. Many of the individual object orbits are so poorly
determined that general solutions allow perihelia of around 30 au
(right at Neptune), yielding unstable orhits which could indicate
transitional objects.
Our data, combined with previously published colours as listed in
Table 2, are presented as a colour diagram in Fig. 1. The VRI colours
of the KBOs range from the extreme Pholus-like (V-R and/or
R-I> 0.7) red values for 1992QB 1, 1995DCz and 1994JR1, through
intermediate values of 1993SC and 1995DA2 , to the Chiron-like
(V-R-0.36, R-I-0.31) neutral colour of 1993FW. From inspection of Table 2, there appears to be no clear correlation of colours
with orbital semi-major axis. Such a correlation might be expected
if surface processes such as irradiation mantle creation are controlled by heliocentric distance, while eccentricity would affect the
impact velocity of collisions between KBOs and hence might affect
subsequent reflectance properties. The lack of such correlation in
our small sample implies either that these processes do not affect
surface colours on time-scales less than that for dynamical perturbations for these orbits (essentially stable for r > 42 au and highly
variable between 34 and 43 au), or that perhaps other mechanisms
are responsible for producing the observed range of colours.
4.4 Interrelations
Although all KBO, Centaur and cometary populations discussed
here are presumed to be composed of physically similar bodies, care
must be taken when looking at interrelations between them.
Because of observational selection effects and popUlation densities,
the sampled size distribution is different for comets, Centaurs and
KBOs. Therefore if the mechanisms producing the observed red
colours on primitive objects are dependent on size or mass of the
© 1997 RAS, MNRAS 290,186-192
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..186G
Distant Solar system bodies
191
1.0
1994JRl
(KBO)
0.8
1993HA2
(CEN)
Tempe12
~0.6
~ (COM)
G-Z(COM)
>-
~ Encke
0.4
Neujmin 1
(COM)
(COM)
• 1992QBl (KBO)
• 1995DC2 (KBO)
... 1993SC (KBO)
~
1993FW
(KBO)
•
1995DA2 (KBO)
Chiron (CEN)
Pholus (CEN)
1995DW2 (CEN)
® Halley (COM)
EB Swift-Tuttle (COM)
Arend-Rigaux (COM)
Sun
o
o
o
0.2
e
o
0.0
0.2
0.4
0.6
R- I
0.8
1.0
1.2
Figure 1. VRI colour-colour plot of distant Solar system bodies. Objects with no I magnitude are shown on the left with arrows. We assigned an error of
±O.15 mag to the approximate V-R colour for 1993HA2 reported by Luu (1994).
body, (e.g. retention of particles ejected by volatile outgassing or
collisional ejecta), then comparisons between the different populations may be misleading.
That said, it is clear from the data presented here that all three
populations share a wide range of colours, from neutral to extremely
red. The presence of low-albedo, red surfaces on outer Solar system
objects may be expected if their surfaces have not been altered since
formation. Their major constituents, volatile ices, when exposed to
high-energy photon irradiation and cosmic rays, are converted to
carbon-rich compounds and radicals to a depth of metres: an
'irradiation mantle' (e.g. Moore et al. 1983; Johnson 1991). This
mantle may be sufficiently cohesive to inhibit activity if an object is
perturbed closer to the Sun (Strazul1a et al. 1991). Short-period
comets, which have undergone many perihelion passages which
would have stripped away the irradiation mantle, will also have a
predominantly inactive surface (calculated active areas of SP
comets with known nuclear sizes are generally <10 per cent).
Indeed, there is dynamical and physical evidence for inactive
cometary nuclei observed as part of the planet-crossing asteroid
population. In this scenario, Pholus retains its irradiation mantle
while the cometary activity of Chiron has removed it. If this view is
correct then we require an explanation for
(i) the apparent diversity of colours in the KBO population,
(ii) the lack of activity of Pholus while at a similar heliocentric
distance to Chiron,
(iii) the difference in nuclear colours for Chiron compared with
SP comets.
The apparent diversity of colours on KBOs could result from real
differences in composition and/or surface scattering properties, or
from some process that has altered the surface of only part of the
popUlation.
One possibility is that the surfaces of cometary bodies (initially
with irradiation mantles) become neutral by the removal of the
surface layers due to outgassing; such a conclusion is certainly
consistent with the behaviour and colours of 2060 Chiron and 5145
Pholus. However, while the nucleus of 49P/Arend-Rigaux (Millis,
A'Hearn & Campins 1988) also has a neutral or bluish colour, other
currently active comets such as 1PIHalley (Thomas & Keller 1989),
PfNeujmin 1 (Campins, A'Hearn & McFadden 1987) and Ptremple
2 (Jewitt & Luu 1989) have reddish surfaces. In addition, any
mechanism for activity must be possible at 40 au, but not be
universal, even at 8 au. Thus outgassing alone probably cannot
explain the colour variation in KBOs. However, a possible consequence of activity on large bodies at large heliocentric distances is
selective resurfacing due to a proportion of ejected particles not
attaining escape velocity and returning to the surface (clearly
dependent on both the particle size distribution and composition,
and the size of the parent body). Recent HST observations of the
inner coma of Chiron (Meech et al. 1997) have been interpreted as
due to a bound population of particles on ballistic trajectories.
Alternatively, resurfacing may be a consequence of impact
processes at a range of sizes. If regolith 'gardening' of the cometary
surface due to (small) meteoroid impacts were an important process
for modifying colours, one would expect objects in similar orbits
to be equally exposed. However, individual impacts of larger
meteoroids could modify the surface (or part of the surface) by
exposure of, or resurfacing by, 'pristine' subsurface material. Such
random events could lead to a diversity of colours, without showing
a strong dependence on orbital parameters. Additional evidence for
this mechanism might be apparent from rotational light curve
variations due to colour/albedo variegation.
Another possibility is that KBOs originally possessed an intrinsic
spread in compositions, in which case the original bodies may have
had a range of colours, or have subsequently been processed by
different degrees (or at different rates). If this is the case, then the
observations presented in this paper imply that such primordial
variations were not a strong function of heliocentric distance,
© 1997 RAS, MNRAS 290,186-192
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..186G
192
S. F. Green et al.
assuming that the observed KBOs have survived in or near their
present orbits for the lifetime of the Solar system.
4.5 Conclusions
We have presented optical colours for five KBOs, one Centaur and
one comet. Combining these with previously published data for
these classes of bodies and cometary nuclei, we find that these
populations share a wide range of colours, from solar to extremely
red. While dynamical studies may point to an evolutionary path
from KBOs to Centaurs to comets, the cause of the range of colours
in populations supposedly composed of similar bodies remains
unclear. The small data set available shows that at present it is not
possible to differentiate observationally between the mechanisms
most likely to affect the surface colours, whether impact- or
outgassing-related. To do so in the future will require significant
enhancement in our knowledge of how these processes affect the
surface colours of primitive bodies. The optical/near-infrared, the
thermal infrared (from ISO) or spectra from a larger sample of
objects with a diversity of orbits and dynamical histories may
provide the only clues possible with current instrumentation.
ACKNOWLEDGMENTS
The Isaac Newton Telescope is operated on the Island of La Palma
by the Royal Greenwich Observatory in the Spanish Observatorio
del Roque de Los Muchachos of the Instituto de Astrofisica de
Canarias. The Anglo-Australian Telescope is operated by staff of
the Anglo-Australian Observatory on behalf of the UK Particle
Physics and Astronomy Research Council and the Australian
Department of Employment, Education and Training. Image processing and data reduction were performed using the Starlink
network and software. NM acknowledges the financial support of
the UK Particle Physics and Astronomy Research Council. DPOC
acknowledges support from the Gaeltacht Scholarship.
NOTE ADDED IN PROOF
We refer the reader to the paper by Jewitt & Luu (1996) which
appeared after this paper was .written. They have obtained a
complementary data set of KBO colours, which also shows the
apparent colour diversity reported here.
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© 1997 RAS, MNRAS 290,186-192
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System