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AGU session: SOFIA, An Asset for Planetary Science
Monday, December 14, 2020 - 1:00pm to 2:00pm PST
AGU attendees will be able to access the AGU poster session: SOFIA, An Asset for Planetary Science at the AGU Fall meeting. With contributions from Z. Landsman (UCF), E. Young (SWRI), I. de Pater (UC Berkeley), C. Honniball (NASA Goddard). The iPosters will be up all day, and the live Q&A session will be held at 1pm-2pm Pacific time at Poster Pod 2.
The instrument suite onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) provides unique access to the infrared sky (4-600 microns) for planetary scientists. This wavelength range is particularly relevant to solid surfaces studies, as it contains signature bands of mineral groups and organics, observed for example on asteroids Ceres and Hygiea. SOFIA provides unique diagnostics of crystalline/ amorphous composition, and of cometary dust grain size distribution. In addition, its high spectral resolution allows one to measure abundances of molecular species in planetary and cometary atmospheres, including the D/H ratio in water (Mars, comet Wirtanen). Finally, with the ability to deploy anywhere on Earth, SOFIA has been a key resource in occultations observing campaigns (Pluto, Triton, Varda,...). This session welcomes highlights of the latest SOFIA Solar System results. It will include a Q&A on the role of SOFIA for the planetary community, and an update on observatory status.
Zoe Landsman (UCF): A Mid-Infrared Survey of M-type Asteroids with SOFIA+FORCAST
The M-type asteroids are a taxonomic class associated with the iron meteorites, although there appears to be compositional diversity in this taxon. Near-infrared spectroscopy shows the presence of both mafic silicates and phyllosilicates among the M-types, while radar studies indicate high bulk density associated with metal content for a subset of this class. One such high-metal M-type asteroid is (16) Psyche, the target of the NASA mission Psyche. Understanding the nature and potential diversity within this taxon constrains models of material formation and transport in the solar system and provides a framework to interpret results from Psyche. We are conducting a mid-infrared spectroscopic study of the M-type population, as these wavelengths are ideally suited to constrain both mineralogy and thermal properties of asteroids. Using SOFIA+FORCAST, we have obtained 8-μm to 13-μm and 18-μm to 28-μm spectra for five M-type asteroids: (69) Hesperia, (75) Eurydike, (83) Beatrix, (347) Pariana, and (413) Edburga. We are fitting a thermophysical model to these spectra, a process through which we will derive a thermal inertia for each asteroid. Thermal inertia is related to the thermal conductivity of the surface, which is a function of grain texture and composition (i.e., silicate or metallic). Dividing a mid-infrared asteroid spectrum by its modeled thermal flux can reveal an excess of emission due to the presence of silicate grains. We are also interpreting preliminary emissivity spectra derived by fitting a simple thermal model that does not include thermal inertia explicitly. Diameters and albedos derived from simple thermal modeling are consistent with results from NEOWISE1.
Eliot Young (SWRI): Occultations from SOFIA
Occultations occur when a relatively near object (like Pluto) passes in front and blocks light from a more distant object. These events can be useful in determining an object's size, its precise celestial coordinates, the vertical temperature, pressure and density profiles of its atmosphere, the vertical distribution of aerosols, and in cases where Fresnel fringes can be resolved, the distance to the occulting object. We are currently in an occultation golden age: the Gaia star catalog provides us with unprecedentedly accurate stellar positions, but these positions will be degraded by stellar proper motions in the years to come. A SOFIA occultation campaign often requires extensive pre-flight observations to determine a very accurate ephemeris for the occulting object, as well as adaptive optics observations of the star itself to make sure it is not double. For objects with sizes of a few 10s of km (like Arrokoth), a milliarsecond error in the object's RA and DEC relative to the occulted star is equivalent to missing the event by more than the width of the shadow path. Other science goals, like observing central flashes, also require that observers be placed within a few 10s of km of the center of the shadow path. SOFIA is an effective platform from which to observe occultations. It can be deployed with extreme precision to locations where ground-based observations are impossible, as it was for Arrokoth (then 2014 MU69). The permanent guide camera, FPI+, has properties that are well-suited to occultation observations: high quantum efficiency, low read noise and high frame rates. SOFIA's large aperture means that occultations of faint stars may still provide lightcurves with productive signal-to-noise ratios. We report on a few examples of SOFIA's successful occultation campaigns, including the first occultation detection of Arrokoth and occultations of Triton and Pluto with central flash lightcurve features. We briefly describe where central flash features come from and why they are so sensitive to haze opacity and temperature profiles. Finally, we discuss SOFIA's prospects for upcoming occultations.
Casey Honniball (NASA Goddard): A Legacy of lunar water through SOFIA
Through its unique instrument suite and operational altitude, the Stratospheric Observatory For Infrared Astronomy (SOFIA) has allowed for molecular water on the sunlit Moon to be detected for the first time. The discovery of water on the sunlit Moon is of high importance for planetary and lunar science. At this time SOFIA is the only observatory capable of detecting and mapping the 6 µm molecular water band on the lunar surface using the FORCAST instrument. Initial detections of water on the Moon with SOFIA were made at high southern latitudes in one region. Using FORCAST on SOFIA we are proposing a Legacy campaign to map water on the sunlit Moon. Maps of water across the Moon at multiple lunar times of day, latitudes, and over a range of compositions will allow us to fully characterize the behavior and processes of molecular water on the Moon. We will characterize the mobility of water and determine its correlation with solar wind intensity and other parameters that may indicate its formation mechanisms. Through SOFIA we will advance our understand of water formation, storage, and retention on the lunar surface and extend this to other airless bodies. Maps created through the Legacy program will inform scientists on the availability of molecular water as a resource and how to extract the water (based its residence location) and may be used for landing site selection during the Artemis program.
Imke de Pater (UC Berekeley): SOFIA FORCAST observations of Jupiter in the JWST-era
Jupiter, the most accessible example for the study of atmospheric circulation on a giant planet, serves as a template for our understanding of the atmospheric dynamics and chemistry of the ever-growing number of extrasolar planets. The atmospheres of giant planets are extremely active, varying on timescales ranging from decades (seasonally evolving chemistry and clouds), to months (variability of storms and banded structures) and even minutes (e.g., asteroidal/cometary impacts and localized storm systems). These evolving atmospheres serve as natural planetary-scale laboratories for studying the fundamental meteorology, chemistry and evolutionary mechanisms that shape the worlds around us. SOFIA’s remote sensing in the far-IR penetrates thick upper-tropospheric hazes to explore the complex, turbulent dynamics of Jupiter's weather layer. In 2014 we observed Jupiter with FORCAST at 17-37 micron to constrain the shape of its continuum emission (Fletcher et al., 2017, Ic. 286, 223), which can only be achieved if obscuration by telluric water vapor is minimized, i.e., from SOFIA or from space. The SOFIA data confirmed the Voyager findings in detecting an equator to pole increase in the para-H2 fraction (fp), with low fp and sub-equilibrium conditions at the equator and high fp and super-equilibrium conditions polewards of 60° latitude. The para-H2 fraction traces mean vertical mixing on timescales of years to decades, depending on the poorly-known hydrogen equilibration time in Jupiter's atmosphere. Equilibrium fp thus implies weak vertical mixing, while sub- or super-equilibrium fractions correspond to mean upwelling or subsidence, respectively (in the upper troposphere where fp is measured). Interestingly, both Voyager and SOFIA measured higher fp values at high northern latitudes than at high southern latitudes, suggesting an asymmetry between the two hemispheres where none is expected on the basis of seasonal variability. We discuss the advantages of a similar experiment simultaneously with JWST/ERS observations planned to be carried out after launch.