Improved Determination of Europa's Long‐Wavelength Topography Using Stellar Occultations

Abstract

Europa's global shape is only poorly constrained, with our best models inferred from four Galileo limb profiles (Current values are that its axes (a, b, c) are (1562.6 km, 1560.3 km, 1559.5 km) with uncertainties of roughly 0.2 km (uncertainty estimated by comparing the different shape models in Nimmo et al. (2007)), and the topographic variance at degree 180 (the finest scale in our shape model) has an amplitude of about 20 m (Nimmo et al., 2011)) (Nimmo et al., 2007). Obtaining more precise global and regional topography is important if we want to understand Europa's internal structure and the state of its ice shell (e.g., Ojakangas & Stevenson, 1989), and is an objective of the Europa Clipper mission (Pappalardo et al., 2019). The primary instruments for determining Europa's global shape are the radar instrument REASON (Radar for Europa Assessment and Sounding: Ocean to Near-surface) (Blankenship et al., 2018) and the visible imager EIS (Europa Imaging System) (Turtle et al., 2019). The main limitation in fitting global shape is the fact that neither the radar tracks (black dots in Figure 1) nor the EIS limb profiles (gray dots in Figure 1) have complete global coverage, instead having gaps around the poles and longitudes near 90° and 270° as a consequence of Europa Clipper's resonant flyby geometry. Filling in those gaps has the potential to dramatically improve our ability to fit the global shape and its time variation. A similar problem was encountered by the Abstract Europa Clipper will arrive at Jupiter at the end of this decade and will explore Europa through a series of flybys. One of its many goals is to characterize Europa's topography and global shape using the Europa Imaging System and Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) instruments. In addition, Europa Clipper's UV Spectrograph will observe stars pass behind (be occulted by) Europa. The spectrograph has sufficiently precise timing, corresponding to a topographic precision of order meters, that these occultations can also serve as altimetric measurements. Because of gaps in the REASON radar altimeter coverage imposed by the flyby geometries, the addition of ∼100 occultations results in a substantial improvement in the recovery of Europa's long-wavelength shape. Typically, five extra spherical harmonic degrees of topography can be recovered by combining occultations with radar altimetry.Plain Language Summary Understanding Europa's topography is crucial to understand the moon for a variety of reasons. One way to quantify the topography is global shape, where we describe the entire surface at once. For example, Earth's rotation causes it to bulge at the equator, and we can describe that bulge with a single number: Earth is on average about 40 km larger at the equator than at the poles. A body's global shape as a whole can be described in a similar way, with a series of amplitudes of prescribed shapes (like an equatorial bulge) referred to as "spherical harmonics." Understanding global shape is important because the...

Catalog of Ultraviolet Bright Stars: Strategies for UV Occultation Measurements, Planetary Illumination Modeling, and Sky Map Analyses Using Hybrid IUE-Kurucz Spectra

“…Using the absorption cross sections for each component, these data can be analyzed to provide direct information about the composition of the atmosphere as a function of altitude above the body's surface. In addition, occultation observations can be used to provide astrometric information on the topographic shape of a target based on the time of complete signal obscuration, as is planned to be done on Europa by Europa-UVS (Abrahams et al 2021). This process has been successfully implemented for constraining the sizes of asteroids in our solar system, such as the Trojan asteroids Patroclus and Menoetius (Buie et al 2015).…”

The Europa Clipper Gravity and Radio Science Investigation

“…However, the G/RS investigation does not plan to do so for Europa. Instead, Europa Clipper will measure the shape and size of Europa using EIS images, REASON altimetry, and Europa-UVS stellar occultations (Abrahams et al 2021).…”

Exploring the Interior of Europa with the Europa Clipper

“…Europa Clipper will determine Europa’s shape through a combination of REASON altimetry profiles, EIS limb profiles, and Europa-UVS occultation measurements (Abrahams et al. 2021 ). REASON altimetry profiles have an accuracy on order of 10 m (Steinbrügge et al.…”

“…Abrahams et al. ( 2021 ) has shown that these gaps can be filled by Europa-UVS stellar occultation measurements, substantially increasing the achievable spherical harmonic degree – from 8 to 13 for very conservative assumptions about REASON performance and without limb profiles. The UVS measurements have timing accuracy of as low as 1 ms, depending on the instrument operating mode and desired precision, which for Europa-Spacecraft relative velocities of ∼1 km/s equate to measurements of chords across the body of Europa with a precision of ∼1 m (Abrahams et al.…”

“…The UVS measurements have timing accuracy of as low as 1 ms, depending on the instrument operating mode and desired precision, which for Europa-Spacecraft relative velocities of ∼1 km/s equate to measurements of chords across the body of Europa with a precision of ∼1 m (Abrahams et al. 2021 ).…”