Planetary Group

The planetary group seeks to understand the origin and evolution of the Solar System. They accomplish this primarily by exploring the various constituents (comets, asteroids and planets) of the Solar System theoretically, observationally, and experimentally.

(text copied from 2017 department review with small edits)

5.1 Planetary science Profs. Deming, Hamilton, Richardson and Sunshine; Drs. Bauer, Bodewits, Farnham, Feaga, Kelley, Knight, Kolokolova, Protopapa
The Department has experienced strong growth in planetary science. There are now 12 Ph.D. scientists in the Department working on campus in planetary (and/or exoplanetary) science, 4 of whom are on the professorial faculty. In addition, there are a number of other astronomy department planetary scientists who work primarily at NASA's GSFC. Planetary research accounts for about one quarter of Departmental Ph.Ds. A primary focus for planetary science in the Department is the study of small bodies (comets, asteroids, and TNOs). These objects are relatively primitive and reveal information about the formation of our Solar System. Other topics include the study of orbital dynamics of planets and their moons, compositional and dynamical studies planetary rings, asteroid evolution and granular dynamics, the composition of planetary surfaces, and the atmospheres of exoplanets.

5.1.1 Solar System Small Bodies Group The UMD small bodies group (UMD-SBG) represents the largest collection of cometary scientists in the country (and perhaps the world). UMD-SBG efforts include participation in various missions, observations with ground-based and space-based telescopes, laboratory research, and the Small Bodies Node of the Planetary Data System.
Spacecraft missions: Over the past decade the UMD-SBG has maintained a strong presence in planetary mission activities. Faculty members have been involved with the Deep Impact (DI)/EPOXI, Stardust-NExT, Dawn, Rosetta, Chandrayaan, New Horizons, and Osiris Rex missions and are now participating in the pre-flight phases of DART and Lucy.
Ongoing analyses of data from the UMD-led Deep Impact/EPOXI mission (Prof. A'Hearn, PI) to comets 9P/Tempel 1 and 103P/Hartley 2 continue to yield a better understanding of the processes under which comets formed in the early Solar System. The team have used studies of the DI impact experiment to measure the physical and mechanical characteristics of the nucleus of Tempel 1, showing that comets are weak, highly porous objects. The experiment also revealed that water ice exists within a meter of surface, indicating that the surface has a very low thermal conductivity. Team members have also shown that, in Hartley 2, CO2 emission drags significant amounts of H2O ice particles and dust grains into the coma, leading to new paradigms about the processes that govern cometary activity, and providing important constraints on models of interstellar dust grains.
The DI spacecraft also served as a remote observatory for two other remarkable comets: C/2009 P1 (Garradd) was shown to have a higher proportion of CO than most comets, which surprisingly increased through perihelion rather than showing the usual symmetry around perihelion. High-cadence observations of the Oort cloud/sungrazer C/2012 S1 (ISON), while it was still far from the Sun, revealed small outbursts, undetectable from Earth-based observations. These outbursts suggest the comet's surface contains highly volatile materials that are vaporizing as it is warmed by the Sun for the first time. ISON continued to be heavily studied by the community (including the UMD-SBG) until the comet disintegrated around perihelion
UMD-SBG members participated on the Moon Mineral Mapper (M3) instrument on the Indian Chandrayaan mission mapping the mineralogical composition of over 95% of the lunar surface, which continues to support analyses of the compositional evolution of the Moon. Moreover, M3 discovered the presence of OH/H2O on the surface of the Moon. Simultaneously, UMD-SBG researchers were able to confirm this important discovery using the IR spectrometer on the DI spacecraft to map the distribution of OH-bearing molecules on the surface and characterize their spatial and temporal variability. More detailed studies of previously unanalyzed DI lunar data are on-going. Recently UMD-SBG members participated in the Rosetta mission to comet 67P/Churyumov- Gerasimenko (C-G), making use of the unique, close proximity measurements around the nucleus to explore physical characteristics not attainable from remote observations. Key results include a study of several large round pits on the comet's surface, which suggest sinkholes and imply that the nucleus has structural heterogeneities deep below the surface. Narrowband observations of the innermost coma also showed a decrease in brightness around perihelion, leading to the discovery that interactions between solar wind electrons and the rapidly increasing levels of cometary gas are cooling the electron temperatures below the threshold needed to dissociate the gas molecules.
Studies of cometary dust combined Rosetta magnetic field measurements and extensive computer modeling of light scattering by ensembles of complex aggregated particles to explain circular polarization in cometary comae. This research has extended to studying other types of dust, including interplanetary and circumstellar, and also found its application in astrobiology through developing a new technique to search for pre-biological organics in space. Finally, UMD-SBG scientists used Rosetta's far-UV spectrometer to map the compositional variations across the nucleus, and constrain the low-abundance of surface water ice, and establish the nuclear phase function in the UV. UV measurements were also used to examine the diurnal and seasonal variations of atomic H, O, C, and S and molecular CO, and to characterize the timing and volatile release during several outbursts.
Most recently, SBG members played an important role in the New Horizons mission to Pluto, investigating its compositional heterogeneity. These analyses led to the first detection of water ice. Compositional maps of ice on Pluto shed light on the behavior of seasonal frosts and provide observational constraints for volatile transport models.
Profs. Hamilton and Richardson are Co-Is for the AIDA/DART mission, which will rendezvous with the binary asteroids Didymos and impact the secondary as a demonstration of kinetic impactor technology. Prof. Richardson's group is supporting the mission by modeling the internal structures of the binary components based on observational constraints. Prof. Hamilton is studying perturbations on the mutual orbit of the Didymos binary, and assessing the lifetime of dust in the vicinity. Prof. Richardson is also active in the JAXA Hayabusa2 and NASA OSIRIS-REx asteroid sample-return missions, which are scheduled to arrive at their respective targets in 2018. Richardson's group is supporting these missions by conducting numerical simulations of lander and sampler interactions with the presumed surface regolith (loose granular material).
Prof. Hamilton is also involved with the New Horizons and Juno missions, discussed under dynamical processes. Prof. Deming is a Co-I for the TESS mission, discussed under Exoplanets.
Remote Observations: In addition to spacecraft missions, UMD-SBG is active in remote observational studies. These are important as a means of characterizing the behavior of small bodies as a function of time, solar insolation, and other variables, but they also provide a means of extrapolating in situ mission results to the broader population of objects. UMD- SBG small body research utilizes ground-based telescopes: DCT, Gemini, NASA's InfraRed Telescope Facility, the airborne SOFIA telescope, and space-borne telescopes (including Swift Gamma Ray Burst Mission, Hubble Space Telescope, Spitzer Space Telescope, and the SOlar and Heliospheric Observatory). One of the strengths of the Department's ground-based research programs is its guaranteed access to the 4.3-meter DCT, the largest telescope in the world equipped with a full set of narrowband filters specifically optimized for cometary observations. UMD-SBG remote studies cover a variety of topics, and to maximize DCT time, the group systematically pools its observation requests to share risk and more efficiently collect data. Observational studies that make use of DCT include: monitoring and comparing the activity of different dynamical age comets to determine how they change over the course of repeated solar passages; characterizing the fundamental properties of both past and potential future spacecraft targets, to provide context for the mission results; and investigations of 'hybrid' objects, to explore the relationships between asteroids and comets; and compositional studies of various other planetary bodies (e. g., Galilean satellites). UMD-SBG members also coordinated an observing campaign to study comet C/2013 A1 (Siding Spring), which made a historically close approach to Mars. As part of this study, the group analyzed the hazards to Mars-orbiting spacecraft posed by the comet's dust environment and presented the results to NASA. This study allowed NASA to minimize risk to the spacecraft from the comet.
In other telescopic studies, UMD-SBG members use Swift to monitor Oort cloud comets at regular intervals to investigate how they evolve during their first solar passage. Swift was also used to show that the sudden brightening of asteroid 596 Scheila was the result of a collision with a much smaller asteroid. The Spitzer Space Telescope is used in multiple programs, including examining the role of CO2 in the persistent post-perihelion activity observed in Jupiter-family comets, measuring the seasonal variations of CO2 and dust mass loss as a proxy for nucleus heterogeneity, and searching for orbital trends sensitive to cumulative insolation as a proxy for nucleus layering. The IRTF is also used in a systematic survey to look for and analyze water-ice grain halos in cometary comae, and to derive the properties of the ice and evaluate conditions that favor the presence of these halos.
Members of the UMD-SBG have been at the forefront in observational campaigns for re- cent high-profile comets, helping to coordinate, organize and disseminate information about observations obtained by the community, and reporting on interesting events to prompt follow- up observations. UMD-SBG members have participated in or led campaigns for comet Siding Spring and ISON, and are currently leading the campaign for comet 46P/Wirtanen, 45P/Honda- Mrkos-Pajdusakova and 41P/Tuttle-Giacobini-Kresak. Laboratory Studies: To support their remote compositional assessment of small bodies across a range of wavelengths (far UV through mid-wave IR), members of the UMD-SBG perform a number of laboratory studies. Coordinated spectral and compositional studies of meteorites in conjunction with colleagues at the Smithsonian Institutions continue to provide fundamental connections between key compositional trends in meteorites and spectra that have been used to map the asteroid belt. These include quantification of spinel and calcium-aluminum-rich inclusions to identify some of the most ancient asteroids in the main belt, quantification of the degree of aqueous alteration in carbonaceous chondrites and their parent bodies, and the spectral identification of amorphous glass in the most pristine (unaltered) meteorites. UMD-SBG members have also begun measurement effort to build the first library of mineralogically relevant spectra in the far UV to support analyses of data that have already been collected for C-G and the Moon. Finally, to support quantitative interpretation of spectroscopic measurements of Pluto and other TNOs, UMD-SBG members have established a program to collect optical constants of methane and nitrogen mixtures at appropriate temperatures. These optical constants were derived from transmission measurements of crystals grown from the liquid phase at Northern Arizona University and are being used to help understand the processes responsible for volatile loss and retention on TNOs and investigate the seasonal behaviors of their atmospheres.

5.1.2 The Small Bodies Node
The Small Bodies Node (SBN, Prof. A'Hearn, PI) of NASA's Planetary Data System is operated by the UMD-SBG. The SBN is charged with permanently archiving and making readily available all data from NASA missions to small bodies in the Solar System and any other data relevant to planning such missions or interpreting the results from them. This includes both scientific missions (typically in the Discovery and New Frontiers Programs) and Planetary Defense missions, such as the DART mission now nearing Phase B. The SBN has operated, with Prof. A'Hearn as PI, since 1990, with operations at UMD and the Planetary Science Institute in Tucson. Members of the UMD-SBG participate in SBN to provide scientific expertise on the data in the archive and to assess whether data are well-documented and sufficiently useful to be worth archiving. The SBN has archived data from every cometary and asteroidal mission flown to date. It has also pioneered rigorous peer review of datasets and developed special tools for users, including tools to read PDS-formatted objects into IDL and to display data under the new PDS4 standard. The interplay between the archiving and the mission roles of the scientists (currently Rosetta, New Horizons, DART, as well as many previous missions) is crucial to the scientific success of the archive. Based on the management experience at SBN, the Minor Planet Center (at CfA), which is funded entirely by NASA through the Planetary Defense Coordination Office, has been merged into the SBN as a subnode. This will lead to additional staff both at UMD and at CfA.

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