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Project Progress

Carbon Dioxide, Carbon Monoxide, Water and Comet Activity

Recent comet missions have shown the importance of CO₂ to understanding how volatiles were distributed in the early solar system. We have developed a model that can be used to characterize the brightness behavior of an active comet as the activity develops and declines on its orbit around the sun. With sufficient constraints about the physical characteristics of the comet and of measurements of gas production, we are now able to make assessments about those comets whose activity is controlled by CO₂ rather than water. This model was utilized to characterize the Rosetta mission target, 67P/Churyumov-Gerasimenko prior to the spacecraft arrival in early 2014. The model confirmed that the onset of activity from water-ice sublimation occurred at a distance from the sun of 4.2 AU inbound, and suggested that only a small fraction of the nucleus surface is active (Fig. 1). We participated in coordinating a major observing campaign to watch the development of activity in the dynamically new comet C/2012 S1 (ISON), a sungrazing comet which was had the potential to become extremely bright at perihelion. In contrast to 67P, a similar model examining the inbound behavior of this comet showed that the inbound light curve was likely controlled by CO₂-ice sublimation, and that there was a contribution from a long, slow CO outburst near 6 AU, both of which contributed to the creation of a halo of large slow-moving icy grains, much like that seen around the EPOXI mission target, 103P/Hartley 2. Our models showed that contrary to the early suggestions of Jan Oort (1950), the activity at large distances was not likely to be just the result of the loss of a radiation processed surface layer of materials, and that CO₂-driven activity may play a key role in the activity of comets.

We performed a detailed analysis of comet 49P/Arend-Rigaux, combining observations from 3 apparitions with dust-dynamical models and ice sublimation models. Data from March 2012, 160 days after perihelion, showed that the comet had a prominent jet (Fig. 2). Our models show that the jet duration was short, and was populated by relatively small grains moving at low velocities. Using this as constraints, we were able to model the bulk of the light curve for this comet as being due to water-ice sublimation, except near perihelion where a combination of CO₂ and H₂O-ice sublimation was required (Fig. 3). In an apparition earlier than our observations, there was apparently another outburst, which was coincident with a Spitzer observation of CO₂, which strengthens our conclusion that CO₂ still exists in this comet and is an important driver for activity. We have a large set of data for other comets that we are modeling in this context, and combining with measured gas production rates, so we can start to look at the distribution of these volatiles in the context of formation location.

Cometary Volatiles

We are collaborating with the Goddard team to investigate parent volatiles in comets using high resolution near-IR spectroscopy, and are using this to both characterize the behavior of specific comets, and to work toward a taxonomy of comet chemistry for both minor organic volatile species and for the major drivers of activity. The UHNAI work is focusing in particular on looking at the correlation of CO in comets with dynamical class and orbital properties. We have studied comet C/2009 P1 (Garradd), which showed no change in the relative abundances for trace species pre- and post-perihelion, and comet C/2012 F6 (Lemmon), which in contrast had indications of heterogeneity in the nucleus. Recent observations of comet 29P/Schwasswachmann-Wachmann 1 in one of its frequent outburst states showed strong lines of CO at a heliocentric distance of 6.21 AU, and are working on the analysis of this data.

Our team also coordinated an observing campaign for comet C/2011 L4 (Pan STARRS). This dynamically new comet became very bright (visual mag -1) when it reached perihelion in 2013 March, which allowed for characterization of the comet activity with a variety of experiments. We conducted a multiwavelength observing campaign on C/2011 L4 from the optical to the submilimeter using Gemini-North, NASA Infrared Telescope Facility (IRTF), James Clerk Maxwell Telescope (JCMT) and UH2.2m atop Mauna Kea, Hawaii. We investigated the chemical composition as well as the dust properties of C/2011 L4 and compared them with those of other comets. We detected a strong absorption band of water ice at 2.0µm in the in the coma with moderate resolution spectra. We modeled the lack of the 1.5µm band by the presence of submicronsized fine ice grains (Fig. 4). No gas (CN, HCN or CO) emission was observed preperihelion, although an upper limit was derived for CO production of 10²⁷ mol/sec. Our dust observations suggest a very high dust to gas ratio, making the comet unusually dustrich.

Main Belt Comet Discovery, Observation and Characterization [Kleyna, Kaluna, Meech]

Main Belt Comets (MBS) are a relatively new (~1996) class of active objects on Main Belt asteroid orbits. Some MBCs are collision, but others have recurring activity driven by volatile sublimation. This second class is indicative of the existence of a new reservoir of water in the habitable zone, in bodies that were previously thought to have no volatiles. MBCs are difficult to detect, because their activity is observed as a faint tail or coma superimposed on asteroid images. Significant integration time is necessary to resolve the activity, making directed surveys observationally expensive. Accordingly, we have undertaken to search for MBCs in the Pan-STARRS1 (PS1) survey, a large-area automated survey located on Haleakalā on Maui. This search has involved constructing components of the solar system pipeline, and building a separate pipeline to analyze images for activity.

We constructed a key component of the processing pipeline, the postage stamp server in the Moving Object Processing System (MOPS). This component has enabled every Solar System discovery made so far, including MBCs 2006 VW139 and P/2012 T1. Currently, the targets are filtered and examined by hand by human operators, because PS1 has not delivered the level of asteroid repeatability, object identification, and orbit fitting originally specified. We have written an automated detection pipeline for MBCs. It has proved capable of detecting MBCs in test cases, but the normal dataflow of PS1 does not deliver data of the necessary quality. PS1 is now shifting to dedicated Solar System observations, which will improve the potential MBC data set. We also plan to modify the pipeline to use an external library of known asteroid orbits, rather than relying on PS1’s incomplete identifications. This will allow us to rerun the detector on several years of PS1 observations.

Our team has been very active in several collaborations to quickly respond to new main belt comet discoveries and characterize the nuclei and activity. Our team has investigated the activity for P/2010 R2 (La Sagra), P/2006 VW139, P/2012 T1 (Pan STARRS), P/2013 P5 (Pan STARRS), and has begun follow up the new discovered split main belt comet P/2013 R3 (Catalina-Pan STARRS). We have written a collisional debris simulator, and have used it show that P/2010 A2 is not a true MBC, but agrees more with the conical distribution characteristic of an impact. We have modeled dust production and searched for outgassing spectroscopically (producing only upper limits to the amount of gas). In addition for some targets we have constraints on the rotation period and mechanisms for activity. In particular, we have modeled/2006 VW139 to assess the possibility of rotational instability as a mechanism for activity, and found that it rotates too slowly for this to be effective. Detailed dust models for comet P/2013 P5 PanSTARRS, discovered in 2013 Aug., showed that the object had been producing dust since at least late 2013 January (Fig. 5). This MBC is relatively small (radius < 0.25-0.3 km) while the activity persisted longer than would be expected for a collisional event, it was previously believed that water ice should not be able to survive in the inner asteroid belt over the age of the solar system.

TNO Lightcurves and Binary Fractions [Sonnett, Meech]

Graduate student S. Sonnett completed her thesis entitled “Characterizing Neutral Trans-Neptunian Objects”. Trans-Neptunian Objects (TNOs) are small bodies with orbits beyond Neptune that are cold and small enough to have remained relatively well preserved since their formation. Roughly one third of TNOs have neutral surface colors that might indicate young surfaces from collisional processes, compositional differences or cometary outgassing. Through a brightness variation survey of 38 objects it was found that the amplitude distributions of red and neutral TNOs were similar suggesting similar collisional histories. Nine of these objects were selected for intense follow up to search for color variations over the surfaces, and no color variations were found. During the follow up light curve work, however, evidence was found for binary companions. The fraction of TNOs with binary companions as a function of separation distance can be used to distinguish between models of binary formation mechanism.

As part of a project related to this thesis, we began a collaboration with S. Desch (ASU) to search for lost members of the TNO Haumea collisional family. Haumea is the largest member of a collisional family in the Kuiper belt, and as such study of the family members provides a means to explore the potentially diverse composition of a body large enough to differentiate. Most family members were identified on the basis of their similar icy surface spectral properties. We have begun an observational program to search for darker non-icy family members that represent pieces from the undifferentiated rocky crust. This will allow us to test interior thermal models. Data were collected using the Magellan 6m telescope in Chile and are being analyzed.

Insights into Comet Activity [Meech, Hermalyn]

Team members completed work analyzing comet mission data from the Deep Impact, EPOXI and Stardust-NExT missions. The EPOXI encounter unexpectedly showed a nucleus surrounded by a debris field of large dust, likely dragged from the nucleus by the CO₂-driven jets. Analysis showed that the dust/ice particle sizes ranged from millimeter up to 20 cm and that most were within 10 km of the nucleus and moving at only a few meters per second. The distribution of light suggests a very steep size distribution.

In a related project we have been assessing the activity levels and dust properties in a large number of Jupiter family comets by combining data from the Spitzer space telescope with ground based observations, and have found that, as expected dust mantling of surface ices and/or seasonal heating of ices more volatile than water ice is common in Jupiter-family comets. Our team has also been characterizing the nucleus properties of both comets and Centaurs, the transition objects moving inward from the Kuiper belt to the inner solar system. The work on comet nucleus sizes combined thermal IR data from the Spitzer space telescope with ground based observations for 98 comets, and has resulted in the largest compilation of radiometrically derived physical parameters for comet nuclei to date. This data was used to derive an estimate of the Jupiter family comet cumulative size distribution with a power law slope of -1.9. We found that there was no strong dependence of albedo with radius. In work combining data from the WISE spacecraft in the thermal infrared with optical data obtained from the ground, we have found that on average Centaurs are low albedo (0.08±0.04) and have measured the cumulative size distribution power law (-1.7±0.3). The data also reveal a relation between albedo and color at the 3-sigma level. No relation between the observed sample’s sizes and albedos is found down to the 1-sigma level.

Our team continued to work on understanding the detailed thermophysical properties of the surface layers of comets through analysis of the Deep Impact mission and Stardust-NExT mission images of the impact crater into 9P/Tempel 1. Comparisons between preimpact images from the the Deep Impact probe and images from StardustNExT reveal a 50 mdiameter crater surrounded by a low rim ~200 m in diameter, a value consistent with the ejected mass derived from Earth- and spacebased observations. As a result, the DI crater visible today is consistent with either a larger transient crater, which collapsed, or a central crater of a nested crater resembling an inverted sombrero. The latter alternative would be expected from a layered target: a loose particulate surface about 1–2 m deep over a slightly more competent substrate. The data from the StardustNExT mission reveal that the nucleus density is very low (200-400 kg m-3) which suggests a highly porous nature and has implication for thermal conductivity and the preservation of accessible primordial volatiles near the surface.

Exploration of Trojan Asteroids [Yang]

With a total mass similar to the main asteroid belt, the Jovian Trojan asteroids are a major feature in the solar system. Work begun in the last reporting period to fit optical and near-IR spectra of Trojan asteroids has been published and shows that the surface materials can be explained by finegrained silicates (1-5 wt.) and highly absorbing material (e.g. carbon or iron, 2-10 wt.) suspended in a transparent matrix. The matrix is consistent with a deposit of salt on the surfaces of the large Trojans. We suggest that early in the Solar System history, short-lived radionuclides heated ice-rich Trojans and caused melting, internal circulation of water and dissolution of soluble materials. Briny water volcanism was facilitated by internal volatiles and a possibly global sill of frozen brine was formed beneath the cold primitive crust. The frozen brine layer was likely to be evacuated by impact erosions and evaporation of the exposed brines eventually left a lag deposit of salt. Over the Solar System’s history, fine dust from comets or impacts contaminated and colored these salty surfaces of the Trojans to produce the spectral properties observed today.

Tools for Planning Observations and Data Reduction (Kleyna, Meech)

Many of our observing projects are now done in collaboration with many colleagues, and as a consequence we have access to many telescopes in India, Chile, Spain, Arizona and in Hawaii. This has significantly increased the effort necessary to optimize the planning and preparation for the observing runs. Managing an observational program targeting a large set of Solar System bodies presents logistical challenges. The positions of targets on the sky are not static, and ephemerides and finder charts must be computed and processed by hand using incompatible tools. Several telescopes on different continents may be involved, targets change in brightness as they approach and recede from the Sun, and solar system bodies frequently pass in front of stars, rendering blocks of telescope time unusable for those objects. The process of matching blocks of available telescope time to a list of targets can take days if done by hand. Therefore we have developed an observational planning tool, QOMIT, to maintain a database of orbits and a list of telescope allocations, and automatically compute visibilities, star-crossings, and suitable observing slots. It attempts to allocate the available time in an optimal manner, given an input set of observing goals and target values.

QOMIT is capable of maintaining a database of targets, computing their orbits, star-crossings, ephemerides, moon proximity, and other qualities determining their suitability for observations. It generates nightly visibility tables (Fig. 6) and synthetic finder charts. The system is working now, and we only need to complete a user-friendly web interface.

In order to obtain precise detailed photometric light curves for faint outer solar system moving objects, work was done to test and explore new algorithms for high precision photometry. Photometry of moving targets offers additional challenges (i) to aperture photometry because background object contamination varies from image to image, and (ii) to routines that build a PSF model from point sources in the image because trailed field stars do not perfectly represent the PSF of the untrailed target. We find that the newly developed tphot software most accurately and precisely reproduces the object’s true light curve, with particular advantages in centroiding, exclusion of contaminants from the target’s flux, and fitting flux in the wings of the pointspread function.