Background
Ocean detection and characterization are key components of both selected and recommended NASA missions to outer solar system moons. However, detecting oceans from spacecraft observations is challenging, particularly in systems that are not conducive to magnetometer-based detections (e.g., the Saturn system). To improve our ability to detect oceans for a diverse suite of moons, we investigate heat flow measurements and interior characterization from long-range imaging as robust techniques for ocean identification. New results from our team members have shown that heat flow may be more diagnostic of an ocean than previously considered. Hence, an in-depth study of how surface heat flow in the presence of an ocean varies spatially, temporally, and across different icy moons, as well as the different methods of measuring heat flow, would be highly valuable for improving ocean detection. Long-range imaging has also been used to capture small rotational changes (i.e., librations) of Mimas and Enceladus, providing strong evidence for oceans in both moons. We will build upon these results to develop an observing strategy for ocean identification, on additional moons, using imaging.
Approach
To make SwRI more competitive in leading and contributing to missions that will discover and explore ocean worlds – including the next New Frontiers and Discovery calls – we (1) develop tools to predict and identify heat flows that are indicative of a subsurface ocean and (2) determine the requirements needed to obtain diagnostic heat flow measurements and long-range images for icy moons throughout the solar system. Specifically, we investigate the diagnostic limits of surface heat flow measurements, stereo imaging and topography of craters, whose shapes are thermally altered by the presence of an ocean, and long-range imaging to evaluate global shape and characterize a moon’s rotation state to develop a cohesive exploration strategy for multi-flyby missions.
Accomplishments
To date, we have produced heat flow and ice shell thickness maps for Jupiter’s ocean-moon, Europa, exploring parameters that control heat flow. We find that the magnitude and variation in heat flow can be used to constrain ice shell thickness and the extent of endogenic heat entering the ice shell. We also find that spatial variations in surface temperature dominate the pattern of heat flow. We are in the process of finalizing and publishing these results, which can then be compared with forthcoming Juno mission data of Europa. In addition, our investigation of Saturn’s small moon, Mimas, has revealed that a sub-surface ocean would increase the temperature measured at the surface by only a few kelvins, requiring improvements in both the measurement precision (over that of the Cassini IR Spectrometer) and models of temperature changes from solar illumination. We are currently investigating alternative instrumentation that can probe below the portion of the ice shell that is affected by solar illumination. We have also determined that the effect of an ocean on relaxation of craters on Mimas would be below the detection limit of Cassini image data and would also require more modeling of crater formation to determine the initial crater shapes to higher fidelity. Our results were presented in three abstracts at the Uranus Flagship Mission Workshop and are being developed into multiple journal articles.