Yanga R. Fernandez

Water ice and organics on the surface of the asteroid 24 Themis

It has been suggested that Earth’s current supply of water was delivered by asteroids, some time after the collision that produced the Moon (which would have vaporized any of the pre-existing water). So far, no measurements of water ice on asteroids have been made, but its presence has been inferred from the comet-like activity of several small asteroids, including two members of the Themis dynamical family. Here we report infrared spectra of the asteroid 24 Themis which show that ice and organic compounds are not only present on its surface but also prevalent. Infrared spectral differences between it and other asteroids make 24 Themis unique so far, and our identification of ice and organics agrees with independent results that rule out other compounds as possible sources of the observed spectral structure. The widespread presence of surface ice on 24 Themis is somewhat unexpected because of the relatively short lifetime of exposed ice at this distance (∼3.2 au) from the Sun. Nevertheless, there are several plausible sources, such as a subsurface reservoir that brings water to the surface through ‘impact gardening’ and/or sublimation.

The Strange Case of 133P/Elst-Pizarro: A Comet among the Asteroids*

We present a new investigation of the comet-asteroid transition object 133P/(7968) Elst-Pizarro. We find mean optical colors (BV =0 .69� 0.02, VR =0 .42� 0.03, RI =0 .27� 0.03) and a phase-darkening coefficient (� =0 .044� 0.007 mag deg � 1 ) that are comparable both to other comet nuclei and to C-type asteroids. As in 1996, when this objects comet-like activity was first noted, data from 2002 show a long, narrow dust trail in the projected orbit of the object. Observations over several months reveal changes in the structure and brightness of this trail, showing that it is actively generated over long periods of time. Finson-Probstein modeling is used to constrain the parameters of the dust trail. We find optically dominant dust particle sizes of ad � 10 � mr eleased with low ejection velocities (vg � 1.5 m s � 1 ) over a period of activity lasting at least 5 months in 2002. The double- peaked light curve of the nucleus indicates an aspherical shape (axis ratio a/b � 1.45 � 0.07) and rapid rotation (periodProt = 3.471 � 0.001 hr). The practical identification of 133P/Elst-Pizarro as a comet (i.e., a mass-losing body) is not in doubt, but the origin of the mass loss is unclear. The 1996 trail has been previously explained as debris released by a chance impact, but our discovery of recurrent activity renders this interpretation implausible. We consider two hypotheses for the activity in 133P/Elst-Pizarro. The ejection of particles is naturally explained if the object is a barely active Jupiter-family comet that has evolved into an asteroid-like orbit, perhaps under the prolonged action of nongravitational forces due to asymmetric mass loss. In this case, the orbital similarity to the Themis family must be considered coincidental. Alternatively, 133P/Elst-Pizarro could be a true member of the Themis family on which buried ice has been recently excavated by impact.

THE ALBEDO DISTRIBUTION OF JOVIAN TROJAN ASTEROIDS

We present radiometrically derived V-band geometric albedos and effective radii for 32 Jovian Trojan asteroids, using near-simultaneous mid-infrared and visible observations. We sampled the large end of the group’s size distribution, down to a radius of 25 km, using 14 objects in the L4 swarm and 18 in the L5 swarm. We find that the albedo distribution is much narrower than previously derived from IRAS measurements. The Trojans, for the most part, have very similar albedos. The actual mean and standard deviation of the distribution depend on the average Trojan beaming parameter � . The ‘‘ standard ’’ value of 0.756, which was used for the IRAS analysis, yields a mean albedo of 0.056 � 0.003 and a standard deviation of 0.009. However, a value of � = 0.94, which we found represented our data better, yields 0.041 � 0.002 and a standard deviation of just 0.007. The thermal behavior of the Trojans seems to follow the ‘‘ slow rotator ’’ model, and the thermal inertia itself can be no greater than about half the Moon’s value. The Kolmogorov-Smirnov test was used to compare the Trojans’ albedo distribution with that of cometary nuclei, dead-comet candidates, and outer solar system objects. We find that the Trojan distribution is similar only to the cometary ones, and only if the Trojans’ � � 1. Observations of the binary (617) Patroclus reveal that its albedo is rather typical among the distribution. We have also discovered that (4709) Ennomos has an extremely elevated albedo, about 0.15. This object may have a very unusual thermal behavior or have recently suffered a large impact that excavated the surface down to a layer of highly reflective, pristine ice.

Low Albedos Among Extinct Comet Candidates

We present radiometric effective radii and visual geometric albedos for six asteroids in comet-like orbits. Our sample has three of the four known retrograde asteroids (1999 LE31, 2000 DG8, and 2000 HE46) and three objects [(18916) 2000 OG44, 2000 PG3, and 2000 SB1] on prograde but highly elliptical orbits. These measurements more than double the number of known albedos for asteroids with a Tisserand invariant in the cometary regime. We find that all six of our objects, and nine of the 10 now known, have albedos that are as low as those of active cometary nuclei, which is consistent with their supposed evolutionary connection to that group. This albedo distribution is distinct from that of the whole near-Earth and unusual asteroid population, and the strong correlation between Tisserand invariant and albedo suggests that there is a significant cometary contribution to this asteroid population.

ALBEDOS OF ASTEROIDS IN COMET-LIKE ORBITS

We present the results of a mid-infrared survey of 26 asteroids in comet-like orbits, including six Damocloids and six near-Earth asteroids (NEAs). We define a ‘‘comet-like’’ orbit as one having a Tisserand invariant TJ under 3 (but only including objects that are NEAs or otherwise unusual). Visible-wavelength data were also obtained, so geometric albedos (in the Cousins R band) and effective radii are presented for 25 objects (plus one more with 3 � limits) as derived using the NEA Thermal Model. Nine of our objects were observed at two or more mid-infrared wavelengths, and in all cases the low-thermal inertia thermal model was found to be applicable, with various values of the beaming parameter. Our work more than quintuples the total number of observationally constrained albedos among TJ 3 NEAs. There are 10 NEAs among the 32 objects, and of those, 53% � 9% have comet-like albedos. With the current crop of NEAs, this implies that about 4% of all known NEAs are extinct comets. A comparison of the histogram of TJ < 3 asteroid albedos with that of active cometary nuclei shows that the former has a larger spread.

Thermal Properties of Centaurs Asbolus and Chiron

We have measured the mid-infrared thermal continua from two Centaurs, inactive (8405) Asbolus and active 95P = (2060) Chiron, and have constrained their geometric albedos, p, and effective radii, R, with the standard thermal model for slow rotators. These are the first such measurements of Asbolus; we find R = 33 ± 2 km and p = 0.12 ± 0.03. This albedo is higher than all of those confidently known for active cometary nuclei. The thermal inertia is comparable to or lower than those of main-belt asteroids, the Moon, and Chiron; lower than those of the icy Galilean satellites; and much lower than those of near-Earth asteroids. For Chiron, we find R = 74 ± 4 km and p = 0.17 ± 0.02. While this albedo is consistent with the established value, previous radiometry by others implied a larger radius. This discrepancy may be partially due to a varying infrared dust coma, but all data sets have too low signal to be sure. Four Centaur albedos (out of about 30 objects) are now known. They show a diversity greater than that of the active comets, to which they are evolutionarily linked.

Spitzer Space Telescope Observations of the Nucleus of Comet 103P/Hartley 2

We have used the Spitzer Space Telescope InfraRed Spectrograph (IRS) 22-μm peakup array to observe thermal emission from the nucleus and trail of comet 103P/Hartley 2, the target of NASA’s Deep Impact Extended Investigation (DIXI). The comet was observed on UT 2008 August 12 and 13, while 5.5 AU from the Sun. We obtained two 200 frame sets of photometric imaging over a 2.7 hr period. To within the errors of the measurement, we find no detection of any temporal variation between the two images. The comet showed extended emission beyond a point source in the form of a faint trail directed along the comet’s antivelocity vector. After modeling and removing the trail emission, a NEATM model for the nuclear emission with beaming parameter of 0.95 ± 0.20 indicates a small effective radius for the nucleus of 0.57 ± 0.08 km and low geometric albedo 0.028 ± 0.009 (1σ). With this nucleus size and a water production rate of 3 × 10^(28) molecules s^(-1) at perihelion, we estimate that ~100% of the surface area is actively emitting volatile material at perihelion. Reports of emission activity out to ~5 AU support our finding of a highly active nuclear surface. Compared to Deep Impact’s first target, comet 9P/Tempel 1, Hartley 2’s nucleus is one-fifth as wide (and about one-hundredth the mass) while producing a similar amount of outgassing at perihelion with about 13 times the active surface fraction. Unlike Tempel 1, comet Hartley 2 should be highly susceptible to jet driven spin-up torques, and so could be rotating at a much higher frequency. Since the amplitude of nongravitational forces are surprisingly similar for both comets, close to the ensemble average for ecliptic comets, we conclude that comet Hartley 2 must have a much more isotropic pattern of time-averaged outgassing from its nuclear surface. Barring a catastrophic breakup or major fragmentation event, the comet should be able to survive up to another 100 apparitions (~700 yr) at its current rate of mass loss.

ALBEDOS OF MAIN-BELT COMETS 133P/ELST-PIZARRO AND 176P/LINEAR ∗

We present the determination of the geometric R-band albedos of two main-belt comet (MBC) nuclei based on data from the Spitzer Space Telescope and a number of ground-based optical facilities. For 133P/Elst-Pizarro, we find an albedo of pR = 0.05 ± 0.02 and an effective radius of re = 1.9 ± 0.3 km (estimated semiaxes of a ~ 2.3 km and b ~ 1.6 km). For 176P/LINEAR, we find an albedo of pR = 0.06 ± 0.02 and an effective radius of re = 2.0 ± 0.2 km (estimated semiaxes of a ~ 2.6 km and b ~ 1.5 km). In terms of albedo, 133P and 176P are similar to each other and are typical of other Themis family asteroids, C-class asteroids, and other comet nuclei. We find no indication that 133P and 176P are compositionally unique among other dynamically similar (but inactive) members of the Themis family, in agreement with previous assertions that the two objects most likely formed in situ. We also note that low albedo (pR < 0.075) remains a consistent feature of all cometary (i.e., icy) bodies, whether they originate in the inner solar system (the MBCs) or in the outer solar system (all other comets).

Thermal properties, sizes, and size distribution of Jupiter-family cometary nuclei

We present results from SEPPCoN, an on-going Survey of the Ensemble Physical Properties of Cometary Nuclei. In this report we discuss mid-infrared measurements of the thermal emission from 89 nuclei of Jupiter-family comets (JFCs). All data were obtained in 2006 and 2007 using imaging capabilities of the Spitzer Space Telescope. The comets were typically 4–5 AU from the Sun when observed and most showed only a point-source with little or no extended emission from dust. For those comets showing dust, we used image processing to photometrically extract the nuclei. For all 89 comets, we present new effective radii, and for 57 comets we present beaming parameters. Thus our survey provides the largest compilation of radiometrically-derived physical properties of nuclei to date. We have six main conclusions: (a) The average beaming parameter of the JFC population is 1.03 ± 0.11, consistent with unity; coupled with the large distance of the nuclei from the Sun, this indicates that most nuclei have Tempel 1-like thermal inertia. Only two of the 57 nuclei had outlying values (in a statistical sense) of infrared beaming. (b) The known JFC population is not complete even at 3 km radius, and even for comets that approach to ∼2 AU from the Sun and so ought to be more discoverable. Several recently-discovered comets in our survey have small perihelia and large (above ∼2 km) radii. (c) With our radii, we derive an independent estimate of the JFC nuclear cumulative size distribution (CSD), and we find that it has a power-law slope of around −1.9, with the exact value depending on the bounds in radius. (d) This power-law is close to that derived by others from visible-wavelength observations that assume a fixed geometric albedo, suggesting that there is no strong dependence of geometric albedo with radius. (e) The observed CSD shows a hint of structure with an excess of comets with radii 3–6 km. (f) Our CSD is consistent with the idea that the intrinsic size distribution of the JFC population is not a simple power-law and lacks many sub-kilometer objects.

The Volatile Composition of Comet 17P/Holmes after Its Extraordinary Outburst

The volatile abundances in comet 17P/Holmes were measured on three dates (UT 2007 October 27.6 and 31.3 and November 2.3) using high-dispersion (λ/Δ λ ~ 2.5 × 104) infrared spectroscopy with NIRSPEC at the W. M. Keck Observatory and CSHELL at the NASA Infrared Telescope Facility. Compared to other comets, the relative gas production rates in the coma show an enhancement of C2H6, HCN, and C2H2 with respect to H2O, by factors of ~2-3. CH3OH was also detected with an abundance relative to H2O that is similar to or perhaps slightly enhanced compared to the values observed in other comets. The apparent enrichment of some volatiles in the coma of 17P/Holmes does not necessarily imply an unusual composition for its nucleus because fractionation effects may be important at the relatively large heliocentric distance (Rh = 2.45 AU) at which our observations were performed. Rotational temperatures were determined for H2O, HCN, C2H6, and C2H2 in the coma on UT October 27.6 and found to be between 60 and 80 K. We used lines in both the ν5 and ν7 bands to obtain the best constraints yet achieved for the rotational temperature of C2H6. The spatial distributions of all measured volatiles in the coma are consistent with each other and suggest at most only a minor contribution from sublimating icy grains within our aperture. The overall gas production rate declined by approximately a factor of 7 between UT October 27.6 and November 2.3 with no significant change measured in the relative production rates of C2H6 and H2O during this time.