Observational Constraints on Surface Characteristics of
Comet Nuclei
Humberto Campins ([email protected] u)
Lunar and Planetary Laboratory, University of Arizona
Yanga Fernandez
University of Hawai'i
Abstract.
Direct observations of the nuclear surfaces of comets have b een dicult; however
a growing number of studies are overcoming observational challenges and yielding
new information on cometary surfaces. In this review, we fo cus on recent determi-
nations of the alb edos, re ectances, and thermal inertias of comet nuclei. There is
not much diversity in the geometric alb edo of the comet nuclei observed so far (a
range of 0.025 to 0.06). There is a greater diversity of alb edos among the Centaurs,
and the sample of prop erly observed TNOs (2) is still to o small. Based on their
alb edos and Tisserand invariants, Fernandez et al. (2001) estimate that ab out 5%
of the near-Earth asteroids have a cometary origin, and place an upp er limit of
10%. The agreement between this estimate and two other indep endent metho ds
provide the strongest constraint to date on the fraction of ob jects that comets
contribute to the p opulation of near-Earth asteroids. There is a diversity of visible
colors among comets, extinct comet candidates, Centaurs and TNOs. Comet nuclei
are clearly not as red as the reddest Centaurs and TNOs. What Jewitt (2002) calls
ultra-red matter seems to be absent from the surfaces of comet nuclei. Rotationally
resolved observations of b oth colors and alb edos are needed to disentangle the e ects
of rotational variability from other intrinsic qualities. New constraints on thermal
inertia of comets are consistent with previous indep endent estimates. The thermal
inertia estimates for Centaurs 2060 Chiron and 8405 Asb olus are signi cantly lower
than predicted by thermal mo dels, and also lower than the few upp er limits or
constraints known for active, ordinary nuclei.
Keywords: Comets, Nuclei, Surfaces
1. Intro duction
The nucleus is where cometary activity originates. However, direct ob-
servations of the nuclear surfaces of comets have been dicult. This
diculty is due to the gas and dust coma generally present when
comets are close to the Sun, and due to the faintness of comet nuclei
when at large helio centric distances. A growing number of studies are
overcoming these observational challenges and yielding new information
on cometary surfaces. In this review, we fo cus on recent determinations
of the alb edos, re ectances, and thermal inertias of comet nuclei. We
also compare these surface characteristics to those of related p opula-
c
2002 Kluwer Academic Publishers. Printed in the Netherlands.
finalversion.tex; 14/07/2002; 20:40; p.1
2 H. Campins and Y. Fernandez
tions such as extinct comet candidates, Centaurs, near-Earth asteroids
(NEAs), transneptunian ob jects (TNOs) and Tro jan asteroids. The size
distribution of cometary nuclei is discussed elsewhere by Fernandez et
al (1999). and by Weismann and Lowry (2001).
Successful observations of comet surfaces have used observational
techniques that fall into three categories, (a) observations of comets at
large helio centric distances, (b) observations of comets near Earth and
(c) imaging by visiting spacecraft. Observations of distant comets aim
to identify the nucleus in the absence of a coma; however, many comets
remain active at large distances. For example, photometry of comet
Encke throughout its orbit reveals a p eculiar behavior, with an actual
increase in Encke's intrinsic brightness near aphelion (e.g., Meech et al.
2001, Licandro et al. 2001, Sekanina 1991, Barker et al. 1981). Hence,
it is often necessary to estimate and subtract a remnant coma, which
is dicult to characterize due to the low spatial resolution. One of the
main uncertainties asso ciated with observations of distant comet nuclei
is the p ossibility that an unresolved coma can go undetected. Neverthe-
less, recent studies have rep orted apparently successful observations of
comet nuclei at visible (e.g., Jewitt 2002) as well as mid-infrared wave-
lengths (Fernandez et al. 2002). These studies allow estimates of the
nucler size, assuming an alb edo in the case of visible wavelengths only,
and measuring the alb edo, in the case of simultaneous mid-infrared and
visible detections.
At smaller geo centric distances, the increased spatial resolution al-
lows a better characterization and subtraction of a coma. This tech-
nique was initially applied to low activity comets Neujmin 1, Arend-
Rigaux and Temp el 2 (Campins et al. 1987, Millis et al. 1988, A'Hearn
et al. 1989). More active comets with very close approaches to Earth
have yielded useful but more limited information ab out their nuclei. For
example, estimates of the sizes of comets IRAS-Araki-Alco ck, Sugano-
Saigusa-Fujikawa and Hyakutake resulted from mid infrared observa-
tions and radar observations near closest approach (Hanner et al. 1985,
Hanner et al. 1987, Harmon et al. 1989, Harmon et al. 1997). The
Hubble Space Telescop e has brought many more comets within the
reach of the coma subtraction technique. The sizes, and approximate
shap es of some 15 comet nuclei have b een estimated so far based on
HST imaging, in a few cases the nuclear colors have also been extracted
(Lamy et al. 1998, 1999, 2001).
Finally, imaging of the resolved nuclei of comets Halley and Bor-
relly have b een obtained by visiting spacecreaft. Results from the 1986
encounters with comet Halley have been summarized in a number
of publications, including Huebner (1990) and references therein. On
September 22, 2001, NASA's Deep Space 1 (DS1) spacecraft encoun-
finalversion.tex; 14/07/2002; 20:40; p.2
Surface Characteristics of Comet Nuclei 3
tered comet Borrely. Initial results on Borrelly's nuclear characteristics
have b een rep orted (e.g., Britt et al. 2001, Buratti et al. 2001) and
additional details are exp ected as the science teams have more time to
analyze the observations. Figure 1, is the DS1 image of comet Borrelly
at closest approach. One of the most notable features of the surface of
comet Borrelly is the low value of the alb edo (average geometric alb edo
2.2%, which is somewhat dep endent on the phase curve assumed; Bu-
ratti et al. 2001) and its variability across the surface (at least a factor of
two). When considering alb edo values obtained from ground based ob-
servations, it is imp ortant to keep in mind the range of values observed
in comet Borrelly. The ground based values are averaged over the side
of the comet nucleus that faces Earth. Rep orts of rotational variability
of color and sp ectral shap e in a comet and a Centaur, suggest that
even with the coarse sampling achievable from Earth based telescop es,
rotational variability of the alb edo of comet nuclei could be detectable
photometrically (see section 3).
1.1. Related Populations
Several p opulations of minor solar system bodies may be linked to
comet nuclei. Jupiter-family comets, Centaurs and TNOs are b elieved
to be closely related. More sp eci cally, the low inclination of Jupiter-
family comet orbits led Fernandez (1980) to prop ose that these ob jects
come from an ecliptic p opulation of icy ob jects in the transneptunian
region. Since then, a number of authors have studied how gravita-
tional interactions and collisions can bring TNOs (e.g., Duncan Quinn
and Tremaine 1988, Levison and Duncan 1997) and their fragments
(Farinella and Davis 1996) into orbits contained within those of the
giant planets (which is our de nition of a Centaur orbit). Interactions
with the giant planets can reduce the p erihelion distance of some of
these ob jects to the point where they are observed as active comets. In
addition, the Tro jan asteroids could be a source of some of the Jupiter-
family comets (e.g., Marzari et al. 1995). In turn, extinct or dormant
comets have been prop osed as one source of near-Earth asteroids (e.g.,
Bottke et al. 2002). Information ab out the surface comp osition of these
related p opulations is also increasing and helping us understand the
links between them.
Oort cloud comets are b elieved to have formed near the giant planets
4
and were gravitationally scattered into orbits with aphelia in the 10 to
5
10 AU range (e.g., Stern and Weissman 2001 and references therein).
As we discuss b elow, most observations of the nuclear surfaces made to
date are of Jupiter-family comets. Detailed observations of Oort cloud
finalversion.tex; 14/07/2002; 20:40; p.3
4 H. Campins and Y. Fernandez
Figure 1. Comet Borrelly is the rst Jupiter-family comet to be imaged by a visiting
spacecraft. This image was obtained 160 seconds b efore closest approach by NASA's
Deep Space 1 spacecraft. The image resolution was approximately 48 meters per
pixel. A variety of terrain and surface features are apparent. Smo oth rolling planes
containing brighter regions are present near the middle of the image and seem to
be the source of the dust jets observed in the coma. Darker and rougher terrain
is also observed and may represent older surface material. Alb edo variations of at
least a factor of two across surface have been identi ed (Britt et al. 2001, Buratti
et al. 2001). Stereo analysis shows that the smaller end of the nucleus (lower left)
is tipp ed toward the viewer (out of the frame). Sunlight is coming from the left of
the frame (image and caption information courtesy of NASA and the Jet Propulsion
Lab oratory).
finalversion.tex; 14/07/2002; 20:40; p.4
Surface Characteristics of Comet Nuclei 5
comet nuclei are an imp ortant missing element necessary to test current
views of the origin of comets.
2. Alb edo
2.1. Definitions
An essential element in making meaningful comparisons among comets
and with other minor b o dies is to insure the de nitions of the surface
characteristics are the same. In this section we discuss the de nitions
of alb edo and the main metho d for estimating it. Determination of the
alb edo for minor solar system ob jects is most commonly achieved from
a combination of mid-infrared and scattered light observations. Ideally,
these observations are obtained simultaneously to avoid errors intro-
duced by brightness variability, such as that pro duced by the rotation
of a non-spherical ob ject and/or an ob ject with variable surface alb edo.
This approach is commonly known as the radiometric metho d and also
yields the e ective radius of the ob ject. The radiometric metho d was
rst used by Allen (1970) to estimate the alb edo and radii of aster-
oids and is reviewed in detail by Leb ofsky and Sp encer (1989). Here,
we present a brief description. The measured ux density at visible
wavelengths and in the mid-infrared (also called thermal infrared)
vis
wavelength are a function of a number of parameters:
mir
F ( )
vis vis
2
R p ; (1) F ( ) =
vis vis
2 2
(r=1AU) 4
Z
mir
2
F ( ) = B (T (pq ; ; ); )d d cos R ; (2)
mir mir mir
2
4
Where F is the ux density from the Sun at 1 AU as a function
of wavelength; r and are the ob ject's helio centric and geo centric
distances, is the phase function in each wavelength regime; B is the
Planck function; is the infrared emissivity; is the infrared b eaming
factor; and T is the temp erature. The temp erature is a function of
the geometric alb edo p, surface planetographic co ordinates and
and the phase integral q. In most cases, the \standard thermal mo del"
(STM) for slow rotating ob jects is used to determine the temp erature
distribution and evaluate equations 1 and 2. Other mo dels, such as
the isothermal latitude mo del and the thermophysical mo del, must be
applied when the conditions of the STM are not met (Leb ofsky and
Sp encer 1989, Sp encer et al. 1989).
The Bond alb edo A, and the geometric alb edo are linked by the
phase integral, A = pq. When ob jects are observed near opp osition
finalversion.tex; 14/07/2002; 20:40; p.5
6 H. Campins and Y. Fernandez
and the alb edo is referred to a sp eci c wavelength (e.g., V or R), valid
comparisons can be made. However, care must be taken when compar-
ing alb edos since most published alb edos are geometric, mono chromatic
and for a sp eci c scattering angle (i.e., the corrections for phase e ects
in equations 1 and 2 are not always applied).
2.2. Trends
The number of comets with well-determined visible geometric alb e-
dos has recently jump ed from ve (Campins et al. 1995) to twelve
(Fernandez et al. 2002, and Buratti et al. 2001). The list of comets
and their geometric alb edos is given in Table I, which also includes
four Centaurs, two TNOs, and Pluto and its mo on Charon. Figure 2
(adapted from Fernandez et al. 2002) is a plot of the geometric alb edo
versus e ective radius (top), versus p erihelion distance (middle) and
versus color (b otton). One of the features evident in Table I and Figure
2 is that comet nuclei all have alb edos no larger than 6%. A trend with
p erihelion distance would have suggested that the alb edo is altered by
insolation, but no such trend is apparent. A trend with radius might
imply a connection with e ects that dep end on cross section (such as
impact rate) or surface gravity. Jewitt et al. (2001) found a correlation
with radius at the 3-sigma level; however, this correlation was solely due
to Charon. If we consider only the Jupiter-family comets, a slight trend
in the opp osite direction (decreasing alb edo with increasing radius)
may be suggested by the data. However, a rank correlation test yields
a correlation signi cant only at the 2.0-sigma level. The limited data
set do es not warrant a more detailed analysis at this time.
The range of alb edos observed indicates that there is a greater di-
versity among the Centaurs than among the comets. Activity, such as
that observed in 2060 Chiron, might lead to an overestimate of the
scattered light ux and of the alb edo; however, for 8405 Asb olus this
is not the case. It app ears that during the dynamical di usion from
the transneptunian region, through the Centaur region, into the inner
solar system, an ob ject do es not necessarily preserve its alb edo (or its
color, see Section 3). This e ect could be due to our biased sample, with
no small ob jects measured in the Centaur and TNO region. One the
other hand, this e ect may provide clues to the mechanisms of comet
activity, since the activity on Chiron do es not app ear to leave b ehind
the same dark, mantled surface we have observed in active comets
Halley and Borrelly. For example, an ob ject that becomes active in
the Centaur region may be exp osing pristine ice and/or covering the
surface with high alb edo icy grains. This scenario would suggest that
the high alb edo observed in Asb olus is indicative of recent activity
finalversion.tex; 14/07/2002; 20:40; p.6
Surface Characteristics of Comet Nuclei 7
Table I. Alb edos and Radii of Comets and Related Bo dies
Ob ject E . Radius Geom. Alb edo Notes
1P/Halley 5:2 0:1 0:04 0:01 a
2P/Encke 2:4 0:3 0:046 0:023 b
9P/Temp el 1 2:9 0:4 0:05 0:02 c
+0:25 +0:004
10P/Temp el 2 5:9 0:022 d
0:7 0:006
19P/Borrelly 2:5 0:1 0:022 0:003 e
22P/Kop 1:5 0:2 0:05 0:01 f
28P/Neujmin 1 10:0 0:5 0:025 0:008 g
49P/Arend-Rigaux 5:1 0:25 0:028 0:005 h
55P/Temp el-Tuttle 1:8 0:4 0:06 0:015 i
107P/Wilson-Harrington 2:0 0:25 0:05 0:01 j
C/1983 H1 IRAS-Araki-Alco ck 4:6 0:5 0:02 0:01 k
C/1995 O1 Hale-Bopp 30 10 0:04 0:03 l
95P/Chiron 80 10 0:15 0:03 m
(5145) Pholus 95 13 0:044 0:013 n
(8405) Asb olus 33 4 0:13 0:03 o
(10199) Chariklo 151 15 0:045 0:010 p
Pluto 1172 27 0:53 0:01 q
Charon 625 25 0:38 0:01 q
(15789) 1993 SC 164 30 0:022 0:010 r
(20000) Varuna 450 70 0:07 0:025 s
Errors are those quoted by the authors, except where noted. Notes: a: the
radius is the geometric mean of the three semiaxes rep orted by Keller et
al. (1987); the alb edo is that rep orted by Keller et al. (1987); the errors are
estimates of the errors in the mean; b: Fernandez et al. (2000); c: Fernandez et
al. (2003); d: A'Hearn et al. (1989); e: the radius is the geometric mean of the
three semiaxes rep orted by Boice et al, presentation at IAU Coll. 186, Tenerife,
2002); the alb edo is also that rep orted by Boice et al.; the errors are estimates
of the errors in the mean; f: Jorda et al. (2000); g: Campins et al. (1987); h:
Millis et al. (1988); i: average of results by Fernandez (1999) and Jorda et
al. (2000); j: Campins et al. (1995); k: the radius is the geometric mean of
the three semiaxes rep orted by Sekanina (1988); the alb edo is the mean value
derived by Sekanina (1988); the errors are approximate estimates; l: radius
from Fernandez (2002), alb edo a combination of that rep orted by Fernandez
(1999) and Jorda et al. (2000); m: combination of results by Campins et al.
(1994) and Fernandez et al. (2002); n: Davies et al. (1993); o: Fernandez et al.
(2002); p: Jewitt and Kalas (1998); q: from review by Tholen and Buie (1997);
Pluto's alb edo error is an estimate of the error in the mean; r: Thomas et al.
(2000); s: Jewitt et al. (2001).
finalversion.tex; 14/07/2002; 20:40; p.7
8 H. Campins and Y. Fernandez
Figure 2. The well-determined alb edos are plotted versus radii (top), versus peri-
helion distance (middle) and versus B-V, V-R and R-J colors (b ottom). We have
included active comets (squares), Centaurs (circles) and TNOs (diamonds). No ob-
vious overall trends are apparent with radius or p erihelion distance. A general trend
of redder ob jects being darker is apparent in the b ottom panel. However, a larger
sample will be necessary b efore there is high con dence in this conlcusion, b ecause
many of the ob jects in this plot are atypical (Figure adapted from Fernandez et al.
2002).
finalversion.tex; 14/07/2002; 20:40; p.8
Surface Characteristics of Comet Nuclei 9
(Fernandez et al. 2002). Recent activity in Asb olus would be consistent
with observations rep orted by Kern et al. (2000), where comp ositional
variation with rotational phase is interpreted as high alb edo, fresh ice
exp osed by a recent impact.
\Cometary" alb edo values, typically 4%, are assumed when estimat-
ing the sizes of most comet nuclei, Centaurs and TNOs from visible
wavelength observations alone. The results presented in this section
suggest that such an assumption may be reasonable for Jupiter-family
comets, with an uncertainty of ab out 50% based on the available sam-
ple. However, two of the four Centaurs prop erly observed show alb edos
ab out 3.5 larger than the average for comet nuclei. Only two TNOs
(1993 SC and Varuna) have had their alb edos measured, although these
are similar to cometary alb edos a greater sample is needed b efore valid
conclusions can be drawn. Pluto and Charon have geometric alb edos of
0:53 and 0:38, resp ectively (Table I); however, these two ob jects may
not be representative of the rest of the transneptunian p opulation due
to surface mo di cation by atmospheric e ects.
The alb edo is used by Fernandez et al. (2001) as an indicator of
p ossible cometary origin for a given asteroid in a comet-like orbit (i.e.,
an orbit with Tisserand invariant TJ < 3). Of the 10 such asteroids with
known alb edos, 9 have alb edos that are as low as those for active comet
nuclei. The similarity of both the orbital and physical characteristics
of these ob jects suggests that they are candidates for b eing extinct
comets. Moreover this alb edo distribution is much di erent from the
near-Earth asteroid p opulation as a whole: only 2 out of the 38 near-
Earth asteroids alb edos with TJ > 3 (collected from the literature by
Fernandez et al. (2001)) have comet-like alb edos. This strong correla-
tion between Tisserand invariant and alb edo suggests an evolutionary
connection between active comets and near-Earth asteroids with TJ <
3. Fernandez et al. (2001) estimate that ab out 5% of the near-Earth
asteroids have a cometary origin, and place an upp er limit of 10%.
This result is nicely consistent with two other indep endent estimates
of the p ercentage of cometary ob jects among near-Earth asteroids by
Whiteley (2001) and Bottke et al. (2002). Whiteley (2001) nds that
only a small p ercentage of near-Earth asteroids have the 8-color sp ectra
consistent with a primitive surface. Bottke et al. (2002) use dynamical
mo deling to constrain the source regions of the observed near-Earth
asteroid p opulation, and they predict that 6% 4% of near-Earth as-
teroids come from the Jupiter-family comet region (TJ values between 2
and 3). This remarkable agreement between three indep endent metho ds
is the strongest constraint to date on the fraction of ob jects that comets
contribute to the p opulation of near-Earth asteroids. This result also
finalversion.tex; 14/07/2002; 20:40; p.9
10 H. Campins and Y. Fernandez
has implications on the exp ected fraction of meteorites that can come
from comets (e.g., Campins and Swindle 1998).
Our understanding of the alb edo distribution among all of these
groups will improve once the Space Infrared Telescop e Facility (SIRTF)
is launched and commences observations in 2003. The mid-infrared
instrument MIPS will not only let us measure the radii and alb edos
of many Centaurs and TNOs that are beyond the reach of ground-
based facilities, but also of comet nuclei several AU from the Sun, thus
mitigating the problem of contaminating coma.
3. Colors and Rotational Variability
3.1. Colors
There are di erent ways to quantify colors, the two most common are
color ratios and re ectivity gradients. Color ratios are expressed as the
di erence between magnitudes at standard bandpasses, for example
B-V, V-R, V-J, etc. The continuum sp ectrum of an ob ject can be
parametrized using the normalized re ectivity gradient, which is usu-
0 0 00
ally denoted with S (in %/1000A), and de ned as S = dS/d/S (e.g.,
Jewitt 2002). Where S is the re ectivity (ob ject ux density divided
00
by the ux density of the sun at the same wavelenght ) and S is
the mean value of the re ectivity in the wavelength range over which
0
dS/d is computed. The gradient S is used to express the p ercentage
change in the strength of the continuum per 1000A. Broadband color
ratios can be converted to normalized re ectivity gradients using the
following relation (Luu and Jewitt 1990):
0
2 + S
m m = (m m ) + 2:5log (3)
V R V R
solar
0
2 S
In which (m m ) is the color of the ob ject, (m m ) is the
V R V R
solar
color of the Sun.
The most recent observations and a compilation of published colors
for comet nuclei and related ob jects are given by Jewitt (2002). Table
II presents the colors for comet nuclei and Table III compares the mean
colors with those of extinct comet candidates, D-typ e and Tro jan as-
teroids, Centaurs and TNOs (Tables II and III are from Jewitt 2002).
Figure 3 (also from Jewitt 2002) presents histograms of the normal-
ized re ectivity gradients for the comet nuclei and four of the related
p opulations in Table III.
Table III and Figure 3 show that there is a signi cant di erence
between the colors of comet nuclei (and extinct comet candidates) and
finalversion.tex; 14/07/2002; 20:40; p.10
Surface Characteristics of Comet Nuclei 11 1989 1988 1988 1988 1999 1996 2001 . . h h h 1989 1990 . al al al Keller et et 1999 1998 1988 Meec Luu Meec Meec et . . . Jewitt al al al and 2002 2002 2002 and and and and 2002 2002 2002 et et et b erlin and 1993 1993 1993 y y primary Lam Lam Jewitt Millis Jewitt Delaho dde Jewitt Jewitt Jewitt Jewitt Luu Jewitt Luu Jewitt Jewitt Cham Reference Thomas Luu Bo ehnhardt Luu Jewitt theses. the c 10) 07) 03) 02) 01) 03) 03) 02) 01) paren : : : : : : : : : that 06 01 05 05 05) 04 03 04 02 02 10 02 R : : : : : : : : : : : : 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 in 0 m not 47 47 58 50 32 31 43 48 47 : : : : : : : : : V 38 47 45 45 43 54 53 50 44 31 42 58 : : : : : : : : : : : : (0 (0 0 0 0 0 0 0 0 (0 (0 0 (0 0 0 (0 (0 (0 0 (0 0 m indicates b ers R 0) um 5) : 2) V : 0 : 6 n 5) 5) 5) 5) 0) 2 : : : 3 : : : : 8) 0 . 2 0 4 4 4 3 2 0 1) 3) 4) 2) 3 10 7 3 2 S 5 3 5 : 2 : from 8 : 1 0 5 5 4 5 13 5 : : : : : : : 5 3 0 ed 10 (16 10 (10 (2 (16 12 (8 (13 (8 ( (6 20 ( 10 (7 6 11 (21 S a ed. compute deriv y b 2 to Nuclei are ; 1 emplo 5500-6500 VR 5500-6500 VR VR VR 3800-6300 VR VR VR VR 4000-6000 VR 4400-7200 VR 3800-6300 3800-6300 VR 4400-8100 4400-7200 VR used theses lters. os Cometary system range of 1 1 1 paren used 2 2 t in al-Mrk lter w e e e Colors k k k emp el emp el b ers elength v II. a W Num Johnson 45P/HMP 6P/d'Arrest 46P/Wirtanen 49P/Arend-Rigaux 2P/Enc 10P/T 21P/GZ 95P/Chiron 107P/WH 10P/T 26P/GS 28P/Neujmin 28P/Neujmin 28P/Neujmin 49P/Arend-Rigaux 95P/Chiron 95P/Chiron Ob ject 1P/Halley 2P/Enc 143P/Ko 2P/Enc able b measuremen c a
T
finalversion.tex; 14/07/2002; 20:40; p.11
12 H. Campins and Y. Fernandez 1994 . 1990 2001 al et Luu Luu 2002 2002 and and d Reference Jewitt Jewitt Fitzsimmons Jewitt Note Jewitt c n 12 12 19 32 9 28 median measure- 02 02 01 01 07 01 R : : : : : : 0 0 0 0 0 0 N m and of um 2001. V 44 45 45 46 54 61 : : : : : : . m 0 0 0 0 0 0 al means et 7 maxim : 0 8 5 9 : : : : 1 2 2 0 0 5 the um, 9 : 2 3 8 6 0 on : : : : eixinho 7 8 S 8 9 17 22 P minim b and errors m the 0 , 6 10 S 9 9 10 25 2001 ely . b al group. standard et max 0 the 18 21 S 13 25 39 40 the resp ectiv in group. b are h are, Barucci a 0 y m eac min b ties S 0 ob jects S -6 -13 3 3 1 2 Colors of and within 0 ber 0 S max uncertain Mean compiled S um of , Comets n III. yp es taurs ts. 0 min Listed ro jans Colors S The alues Group Dead Nuclei D-T T Cen KBOs able a men b d c v
T
finalversion.tex; 14/07/2002; 20:40; p.12
Surface Characteristics of Comet Nuclei 13
12 10 KBOs 8 6
Number 4 2 0 5 4 Centaurs 3 2 Number 1 0 8 7 Nuclei 6 5 4
Number 3 2 1 0 8 7 Dead Comets 6 5 4
Number 3 2 1 0 30 25 Trojans 20 15
Number 10 5 0 -20 -10 0 10 20 30 40 50
Reflectivity Gradient, S' [%/1000 Å]
0
Figure 3. Histograms of the re ectivity gradient, S (%/1000A), for TNOs, Centaurs,
comet nuclei, extinct comet candidates (dead comets in Jewitt's nomenclature), and
Tro jan asteroids (from Jewitt 2002)
finalversion.tex; 14/07/2002; 20:40; p.13
14 H. Campins and Y. Fernandez
the colors of Centaurs and TNOs. Jewitt (2002) presents two statistical
tests that indicate, at the 99.97% level of con dence, that the colors of
comet nuclei and TNOs are not drawn from a single parent p opulation.
0
Jewitt also de nes ultra-red matter as having S > 25%/1000A, where
0
the latter is the median S for the TNOs. This ultra-red matter is
found only among Centaurs and TNOs, and is missing from the comet
nuclei and extinct comet candidates. Given that these four p opulations
are thought to be related by a dynamical evolutionary pro cess, this
di erence in color is surprising. Jewitt (2002) considers several expla-
nations for this di erence, which include (a) observational errors (coma
contamination and phase reddening), (b) dynamical error (i.e., a source
for Jupiter-family comets other than the transneptunian region), (c) a
restricted (yet unobserved) transneptunian source, (d) thermal insta-
bility of the ultra-red matter, (e) a size-color gradient among TNOs,
and (f ) resurfacing by sublimation. In addition, collisional resurfacing
had already b een discussed by Luu and Jewitt (1996) and Jewitt and
Luu (2001). Jewitt (2002) favors resurfacing by sublimation; however,
thermal instability, a size-color gradient, and collisional resurfacing (see
b elow) may also contribute to the observed color di erences. A p ossible
test of the size-color gradient hyp othesis would come from colors of
Centaurs down to sizes in the range of Jupiter-family comet nuclei.
Centaurs with sizes in the desired range would be more accessible
to color observations than TNOs of similar sizes, and in principle, a
TNO color-size gradient should be preserved in the Centaur p opula-
tion. Currently, there are only three known Centaurs with an absolute
visible magnitude (H) fainter than 12, which assuming a 4% geometric
alb edo means radii less than 10 km. Hence, testing the presence of any
size-color relation among the Centaur p opulation will require a larger
sample.
A set of recent results on colors among TNOs may also be relevant
to the color di erences discussed ab ove. Several authors have rep orted
a correlation between color and dynamics among TNOs. More sp ecif-
ically, Tegler and Romanishin (2000) rep orted that TNOs with low
eccentricity and low inclination orbits and p erihelion distances beyond
40 AU (i.e., classical TNOs) all exhibit very red surface colors. As of late
2001, Tegler and Romanishin (2001) rep orted that all 19 classical TNOs
in their sample were very red. Similar results have been rep orted by
Stern (2001), Trujillo et al. (2001), and by Doressoundiram et al. (2001
and 2002). In other words, there is a signi cant correlation between
colors and mean collision sp eed. Dynamically cold TNOs are all red,
while TNOs with high eccentricity and/or inclination orbits (plutinos
and scattered disk ob jects) show a range of colors. Although a causal
relationship between collisions and TNO colors is not well established,
finalversion.tex; 14/07/2002; 20:40; p.14
Surface Characteristics of Comet Nuclei 15
this dichotomy suggests that collisional pro cessing may be an imp ortant
factor in the resurfacing of these ob jects.
Farinella and Davis (1996) suggest that Jupiter-family comets are
likely to be collisional fragments of larger TNOs. Hence, the collisional
pro cess that created the smaller fragments, which we now observe as
Jupiter-family comet nuclei, could also be resp onsible for the color
di erences discussed by Jewitt (2002). However, one must keep in mind
that considerable mass loss and surface pro cessing must have o ccurred
as a result of the activity and splitting in comets. In fact, splitting is
an imp ortant mass loss pro cess in many comets (Bo ehnhardt, these
pro ceedings). Hence, a simple connection between the surface colors of
comet nuclei and their collisional history in the transneptunian region
is unlikely to be the whole story.
Another piece of the puzzle is the p ossible correlation of colors and
alb edo rep orted by Fernandez et al. (2002). The b ottom panel of Figure
2 is a plot of the geometric alb edo versus B-V, V-R and V-J colors
of comet nuclei, Centaurs and TNOs. The number of ob jects is very
limited, but a general trend of redder ob jects having a lower alb edo
is apparent. This would be consistent with the idea that cosmic ray
exp osure pro duces surface darkening and reddening.
Clearly, there is more complexity among TNOs and in their rela-
tionship with Centaurs and Jupiter-family comets than exp ected, and
the full explanation of the color diversity remains unknown.
3.2. Rotational Variability
Comp ositional variations across the surfaces of asteroids are well do c-
umented, with the most prominent case b eing that of 4 Vesta (e.g.,
Co chran and Vilas 1998). Sp ectral variability has also b een rep orted in
the nucleus of comet Temp el 2 (A'Hearn et al. 1989) and in Centaur
8405 Asb olus (Kern et al. 2000). Although there are no reasons to
doubt either of these results, we must point out that they have not
b een con rmed. In the case of comet Temp el 2, the two maxima in
the rotational lightcurve were clearly bluer than the minima, and one
maximum was marginally bluer than the other. In Asb olus, a dramatic
sp ectral change in the 1-2 m region was detected using observations
obtained over a 1.7 hour p erio d with the NICMOS instrument of the
Hubble Space Telescop e. One rotational phase in Asb olus had a feature-
less neutral sp ectrum (in agreement with a ground based sp ectrum),
while the other was very red with an absorption consistent with H O
2
ice. As mentioned earlier, Kern et al. (2000) interpret this sp ectral
change as a relatively recent impact crater that exp oses fresh ice on
one side of Asb olus. The variability in these two ob jects illustrates the
finalversion.tex; 14/07/2002; 20:40; p.15
16 H. Campins and Y. Fernandez
need for rotationally resolved studies. It is imp ortant to establish how
much of the observed sp ectral diversity among comets, Centaurs and
TNOs is due to intrinsic variability.
4. Thermal Inertia
As discussed in section 2, the thermal emission for atmosphereless ob-
jects can be used to estimate their radius and alb edo. In addition, when
observations are made at more than one mid-infrared (thermal) wave-
length (e.g., 10 and 20 microns) the color temp erature of the surface
can be used to constrain another imp ortant parameter, which is the
thermal inertia. Knowing this prop erty can constrain existing thermal
mo dels of the interior of nuclei (Prialnik, these pro ceedings) and has
rep ercussions for understanding subsurface gas sublimation behavior
and internal volatile structure.
The ability of a surface to resp ond to insolation changes can be char-
acterized by the parameter , which combines rotation rate, thermal
inertia and surface temp erature (Sp encer et al. 1989).
p
!
=
3
T
ss
p
c, where is the thermal Where = thermal inertia (de ned as =
conductivity, is the ob ject's bulk density, and c is the heat capacity),
! = rotation rate, = emissivity, = Stephan-Boltzman constant, and
T = subsolar temp erature.
ss
If we know the temp erature and the rotation rate (and we assume
a value for the emissivity), we have only two unknowns and .
If the color temp erature is equal to that exp ected for a slow rota-
tor, by de nition must be smaller than or equal to 1. Hence, the
thermal inertia is constrained for those comet nuclei that have been
observed at two or more thermal wavelengths, have known rotation
rates, and have temp eratures consistent with the slow rotator mo del
or STM. These are Arend-Rigaux, Neujmin 1, Temp el 2, and IRAS-
1
Araki-Alco ck . The upp er limits for the whole body thermal inertia
2 4
(in J=m sK ) are 140 for Arend-Rigaux, 120 for Neujmin 1, 100 for
Temp el 2, and 520 for IRAS-Araki-Alco ck. These values are consistent
with published estimates for active areas of comet Halley's nucleus: 120
1
The mid infrared observations of comet Sugano-Saigusa-Fujikawa (Hanner et
al. 1987), do not provide a signi cant constraint on the thermal inertia because the
observed ux may not have b een dominated by ux from the nucleus and the color
temp erature is not repro duced well by the any of the thermal mo dels
finalversion.tex; 14/07/2002; 20:40; p.16
Surface Characteristics of Comet Nuclei 17
2 4 2 4
J=m sK (Weissman 1987) and 40-400 J=m sK (Julian et al. 2000).
Conversely, the recent estimates of the thermal inertia of the whole
2 4
body in Centaurs, 2060 Chiron ( 10 J=m sK , Groussin et al. 2000)
2 4
and 8405 Asb olus (< 10:5 J=m sK , Fernandez et al. 2002) are con-
siderably lower. The values for these two Centaurs are more than an
order of magnitude lower than those predicted for outer solar system
ob jects by thermal mo dels (Sp encer et al. 1989). By comparison, in
the same units the values are 10 for Ceres (Sp encer 1990), 50 for the
Mo on (Winter and Saari 1969), 45 to 70 for Europa (Sp encer et al
1999) 70 for Ganymede (Sp encer 1987), 170 for near-Earth asteroid
Eros (Harris and Davies 1999), and greater than 320 for near-Earth
asteroid and p ossible extinct comet 3200 Phaethon (Harris et al. 1998).
What makes the surfaces of these Centaurs have such p eculiar thermal
prop erties? Fernandez et al. (2002) suggest that a very p orous and/ or
rough surface would inhibit heat ow from the outermost surface layer,
resulting in a very low thermal inertia.
5. Summary
5.1. Albedo
There is not much diversity in the geometric alb edo of the twelve comet
nuclei prop erly observed so far (a range of 0.025 to 0.06). There is a
greater diversity of alb edos among the Centaurs, and the sample of
prop erly observed TNOs (2) is still to o small. Based on their alb edos
and Tisserand invariants, Fernandez et al. (2001) estimate that ab out
5% of the near-Earth asteroids have a cometary origin, and place an
upp er limit of 10%. The remarkable agreement between this estimate
and two other indep endent metho ds (Whiteley 2001, Bottke et al. 2002)
provide the strongest constraint to date on the fraction of ob jects that
comets contribute to the p opulation of near-Earth asteroids.
5.2. Colors
There is a diversity of visible colors among comets, extinct comet candi-
dates, Centaurs and TNOs. Comet nuclei are clearly not as red as some
Centaurs and TNOs. What Jewitt (2002) calls ultra-red matter seems
to be absent from the surfaces of comet nuclei. Several mechanisms
to account for this color di erence have been prop osed, but the full
explanation of the color diversity remains unknown. The dynamically
coldest TNOs are all red, while other TNOs have a range of colors.
Although a relationship between collisions and TNO colors is not well
established, this dichotomy suggests that collisional pro cessing may be
finalversion.tex; 14/07/2002; 20:40; p.17
18 H. Campins and Y. Fernandez
an imp ortant factor in the resurfacing of these ob jects. There is more
complexity among TNOs and in their relationship with Centaurs and
Jupiter-family comets than exp ected, and the full explanation of the
color diversity remains unknown. Rotational variability of the sp ectra
has b een rep orted for one comet (Temp el 2) and one Centaur (8405
Asb olus). It is imp ortant to establish how much of the observed alb edo
and sp ectral diversity among comets, Centaurs and TNOs is due to
intrinsic (rotational) variability.
5.3. Thermal Inertia
New constraints on thermal inertia of comets are consistent with previ-
ous indep endent estimates. The thermal inertia estimates for Centaurs
2060 Chiron and 8405 Asb olus are signi cantly lower than exp ected.
What makes the surfaces of these Centaurs have such p eculiar thermal
prop erties? Possibly very porous and/or rough surfaces.
6. Acknowledgements
We thank Carl Hergenrother and Rob ert Whiteley for useful comments.
This work was supp orted by NASA and NSF grants to H. Campins and
D. Jewitt.
References
A'Hearn, M.F., H. Campins, D.G. Schleicher, and R.L. Millis: 1989, 'The Nucleus
of Comet P/Temp el 2', Astrophysical Journal 347, pp. 1155-1166
Allen, D.A.: 1970, 'The Infrared Diameter of Vesta', Nature 227, pp. 158-159
Barker, E.S., A.L. Co chran, and R.M. Rybski: Mo dern Observational Techniques
for Comets, Pasedena, CA, USA, 1981.
Bo ehnhardt, 'Comet Splitting: observations and mo del scenarii', these pro ceedings
Britt, D.T., D.C. Boice, R.M. Nelson, L.A. So derblom, and N. Thomas: 2001, 'The
Geology of Comet 19P/Borrelly', Bul l. American Astron. Soc., DPS meeting 33,
28.03
Bottke, W.F., A. Morbidelli, R. Jedicke, J-M. Petit, H. F. Levison, P. Michel, and
T. S. Metcalfe: 2002, Icarus, in press.
Buratti, B.J., L.A. So derblom, D.T. Britt, M.D. Hicks, N. Thomas, J. Ob erst, R.H.
Brown, R.M. Nelson, J.A. Mosher, and J.K. Hillier: 2001, 'Photometry and Sur-
face Physical Prop erties of Comet 19P/Borrelly', Bul l. American Astron. Soc.,
DPS meeting 33, 28.04
Campins, H., M.F. A'Hearn, and L.-A. McFadden: 1987, 'The Bare Nucleus of Comet
Neujmin 1', Astronomical Journal 316, pp. 847-857
Campins, H., D.J. Osip, G.H. Rieke, and M.J. Rieke: 1995, 'Estimates of the Radius
and Alb edo of Comet-asteroid Transition Ob ject 4015 Wilson-Harrington Based
in Infrared Observations', Planet. Space Sci., 43, pp.733-736
finalversion.tex; 14/07/2002; 20:40; p.18
Surface Characteristics of Comet Nuclei 19
Campins, H., C. M. Telesco, D.J. Osip, G.H. Rieke, M.J. Rieke and B. Schulz:
1995, 'The color temp erature of (2060) Chiron: A warm and small nucleus',
Astronomical Journal, 108, pp.2318-2322
Campins, H. and T.D. Swindle: 1998, 'Exp ected Characteristics of Cometary
Meteorites', Met. and Planet. Sci. 33, pp. 1201-1211
Co chran A.L. and F. Vilas: 1998, 'The Changing Sp ectrum of Vesta: Rotationally
Resolved Sp ectroscopy of Pyroxene on the Surface', Icarus 134, pp. 207-212
Davies, J., J. Sp encer, M. Sykes, D. Tholen, and S. Green: 1993, '5145 Pholus',
Intrnational Astronomical Union Circular 5698
Doressoundiram, A., N. Peixinho, M.A. Barucci, S. Fornasier, and S. Blancho: 2001,
'Meudon Multicolor Survey of Outer Solar System Ob jects', Bul l. American
Astron. Soc., DPS meeting 33, 12.01
Doressoundiram, M.A. Barucci, J. Romon, and C. Veillet: 2002, 'Multicolor
Photometry of Trans-Neptunian Ob jects', Icarus, in press.
Duncan, M., T. Quinn, and S. Tremaine: 1988, 'The Origin of Short-p erio d Comets',
Astrophysical Journal Letters 328, pp. L69-73
Farinella, P. and D.R. Davis: 1996, 'Short-p erio d Comets: Primordial Bo dies or
Collisional Fragments?', Science 273, pp. 938-941
Fernandez, J.A.: 1980, 'On the Existence of a Comet Belt Beyond Neptune', Month.
Not. Roy. Astron. Soc. 192, pp. 481-491
Fernandez, Y.R.: 1999, 'Physical Prop erties of Cometary Nuclei' PhD Dissertation,
University of Hawaii
Fernandez, Y.R.: 2002, these pro ceedings.
Fernandez, Y.R., et al.: 2000, 'Physical Prop erties of the Nucleus of Comet
2P/Encke', Icarus 147, pp. 145-160
Fernandez, Y.R., D.C. Jewitt, and S.S. Sheppard: 2001, 'Low Alb edos Among
Extinct Comet Candidates', Astrophysical Journal 553, pp. L197-L200
Fernandez, Y.R., D.C. Jewitt, and S.S. Sheppard: 2002, 'Thermal Prop erties of
Centaurs Asb olus and Chiron, Astronomical Journal 123, pp. 1050-1055
Fernandez, Y.R., et al.: 2003, 'Thermal and Optical Investigation of 9P/Templel 1',
Submitted to Icarus.
Fernandez, J.A., G. Tancredi, H. Rickman, and J. Licandro et al., 'The p opulation,
magnitudes, and sizes of Jupiter family comets' Astron. Astrophys. 352, p.327-
340
Harris, A.W., J.K. Davies, and S.F. Green: 1998, 'Thermal Infrared Sp ectrophotom-
etry of the Near-Earth Asteroids 2100 Ra-Shalom and 1991 EE', Icarus 135, pp.
441-450
Harris, A.W. and J.K. Davies: 1999, 'Physical Characteristics of Near-Earth
Asteroids from Thermal Infrared Sp ectrophotometry', Icarus 142, pp. 464-475
Hanner, M.S., R.L. Newburn, H. Spinrad, and G.J. Veeder: 1987, 'Comet Sugano-
Saigusa-Fujikawa (1983V) - A Small, Puzzling Comet', Astronomical Journal 94,
pp. 1081-1087
Harmon, J.K., D.B. Campb ell, A.A. Hine, I.I. Shapiro, and B.G. Marsden: 1989,
'Radar Observations of Comet IRAS-Araki-Alco ck 1983d', Astrophysical Journal
338, pp. 1071-1093
Harmon, J.K., S.J. Ostro, L.A.M. Benner, K.D. Rosema, R.F. Jurgens, R. Win-
kler, D.K. Yeomans, D. Choate, R. Cormier, J.D. Giorgini, D.L. Mitchell, P.W.
Cho das, R. Rose, D. Kelley, M.A. Slade, and M.L. Thomas: 1997, 'Radar Detec-
tion of the Nucleus and Coma of Comet Hyakutake (C/1996 B2)', Science 278,
pp. 1921-1924
finalversion.tex; 14/07/2002; 20:40; p.19
20 H. Campins and Y. Fernandez
bibitem Huebner, W.: Physics and Chemistry of Comets. Springer-Verlag, New
York and Berlin, 1990.
Jewitt, D.C. and P. Kalas: 1998, 'Thermal Observations of Centaur 1997 CU26'
Astronomical Journal Letters 499, pp. L103-L106
Jewitt, D., H. Aussel, and A. Evans: 2001, 'The Size and Alb edo of the Kuip er-b elt
Ob ject (20000) Varuna, Nature 411, pp. 446-447
Jewitt, D.C. and J.X. Luu: 2001, 'Colors and Sp ectra of Kuip er Belt Ob jects',
Astronomical Journal 122, pp. 2099-2114
Jewitt, D.C.: 2002, 'From Kuip er Belt Ob ject to Cometary Nucleus: The Missing
Ultrared Matter, Astronomical Journal 123, pp. 1039-1049
Jorda, L., P. Lamy, O. Groussin, I. Toth, M. F. A'Hearn, and S. Peschke: 2001,
'ISOCAM Observations of Cometary Nuclei', ESA SP 455, p. 61
Keller, U. et al.: 1987, 'Comet P/Halley's nucleus and its activity', Astron.
Astrophys. 187, pp. 807-823
Kern, S.D., D.W. McCarthy, M.W. Buie, R.H. Brown, H. Campins, and M.
Rieke: 2000, 'Comp ositional Variation on the Surface of Centaur 8405 Asb olus',
Astrophysical Journal 542, pp. L155-L159
Lamy, P.L., I. Toth, L. Jorda, H.A. Weaver, and M. A'Hearn: 1998, 'The Nucleus
and Inner Coma of Comet 46P/Wirtanen', Astron. Astrophys. 335, pp. L25-L29
Lamy, P.L., I. Toth, M.F. A'Hearn, and H.A. Weaver: 1999, 'Hubble Space Telescop e
Observations of the Nucleus of Comet 45P/Honda-Mrkos-Pa jdusakova', Icarus
140, pp. 424-438
Lamy, P.L., I. Toth, H.A. Weaver, C.E. Delaho dde, L. Jorda, and M.F. A'Hearn:
2001, 'The Nucleus of 10 Short-Perio d Comets', Bul l. American Astron. Soc.,
DPS meeting 33, 31.01
Leb ofsky, L.A. and J.R. Sp encer: Asteroids II. The University of Arizona Press,
Tucson, AZ, USA, 1989.
Levison, H.F. and M.J. Duncan: 1997, 'From the Kuip er Belt to Jupiter-Family
Comets: The Spatial Distribution of Ecliptic Comets', Icarus 127, pp. 13-32
Licandro, J., G. , M. Lindgren, H. Rickman, and R.G. Hutton: 2000, 'CCD Pho-
tometry of Cometary Nuclei, I: Observations from 1990-1995', Icarus 147, pp.
161-179
Luu, J.X. and D.C. Jewitt: 1990, 'Cometary Activity in 2060 Chiron', Astronomical
Journal 100, pp. 913-932
Luu, J. and D. Jewitt: 1996, 'Color Diversity Among the Centaurs and Kuip er Belt
Ob jects, Astronomical Journal 112, pp. 2310-2318
Marzari, F., P. Farinella, and V. Vanzani: 1995, 'Are Tro jan Collisional Families a
Source for Short-p erio d Comets?', Astron. Astrophys. 299, pp. 267-276
Meech, K.J., Y. Fernandez, and J. Pittichova: 2001, 'Aphelion Activity of 2P/Encke',
Bul l. American Astron. Soc., DPS meeting 33, 20.06
Millis, R.L., M.F. A'Hearn, and H. Campins: 1988, 'An Investigation of the Nucleus
and Coma of Comet P/Arend-Rigaux', Astrophysical Journal 324, pp. 1194-1209
Prialnik, these pro ceedings
Sekanina, Z.: 1988, 'Nucleus of Comet IRAS-Araki-Alco ck (1983 VI I)', Astronomical
Journal 95, pp. 1876-1894
Sekanina, Z.: 1991, 'Encke, the Comet', Journ. Roy. Astron. Soc. Can. 85, pp.
324-376
Sp encer, J.R.: 1987, The Surfaces of Europa, Ganymede, and Cal listo: an Investiga-
tion Using Voyager IRIS Thermal Infrared Spectra, PhD Dissertation, University
of Arizona
finalversion.tex; 14/07/2002; 20:40; p.20
Surface Characteristics of Comet Nuclei 21
Sp encer, J.R., Leb ofsky, L.A., and M.V. Sykes: 1989, 'Systematic Biases in
Radiometric Diameter Determinations', Icarus 78, pp. 337-354
Sp encer, J.R.: 1990, 'A Rough-Surface Thermophysical Mo del for Airless Planets',
Icarus 83, pp. 27-38
Sp encer, J.R., L.K. Tamppari, T.Z. Martin, and L.D. Travis: 1999, 'Temp eratures
on Europa from Galileo PPR: Nighttime Thermal Anomalies', Science 284, pp.
1514-1516
Stern, S.A. and P.R. Weissman: 2001, 'Rapid Collisional Evolution of Comets During
the Formation of the Oort Cloud', Nature 409, pp. 589-591
Stern, S.A.: 2001, 'Evidence for A Statistical Correlation Between KBO Colors and
Mean Random Impact Sp eeds', Bul l. American Astron. Soc., DPS meeting 33,
08.02
Tegler, S.C. and W. Romanishin: 2000, 'Extremely Red Kuip er-b elt Ob jects in Near-
circular Orbits Beyond 40 AU', Nature 407, pp. 979-981
Tegler, S.C. and W. Romanishin: 2001, 'Optical Photometry of Classical Kuip er Belt
Ob jects With the Keck I Telescop e', Bul l. American Astron. Soc., DPS meeting
33, 08.01
Tholen, D. J., M. Buie: Pluto and Charon. The University of Arizona Press, Tucson,
AZ, USA, 1997.
Thomas, N, et al.: 2000, 'Observations of the Trans-Neptunian Ob jects 1993 SC and
1996 TL66 with the Infrared Space Observatory', Astrophysical Journal 534, pp.
446-455
Trujillo, C.A., M.E. Brown, and A.H. Bouchez: 2001, 'The Inclination Color Corre-
lation of the Kuip er Belt Ob jects', Bul l. American Astron. Soc., DPS meeting
33, 08.03
Weissman P. and Lowry S., 'The Size Distribution of Cometary Nuclei', Bul l.
American Astron. Soc., DPS meeting 33, 31.04
Weissman P. 1987, 'Post Perihelion Brightening of Comet P/Halley - Springtime for
Halley', Astron. Astrophys. 187, pp. 873-878
Whiteley R.J.: 2001, A Compositional and Dynamical Survey of the Near-Earth
Asteroids, PhD Dissertation, University of Hawaii
Winter, D.F. and J.M. Saari: 1969, 'A Particulate Thermophysical Mo del of the
Lunar Soil', Astrophysical Journal 156, pp. 1135-1151
Address for O prints: Lunar and Planetary Lab oratory
University of Arizona
Tucson, AZ 85721
finalversion.tex; 14/07/2002; 20:40; p.21
finalversion.tex; 14/07/2002; 20:40; p.22