Abstract

Determining whether Callisto hosts a subsurface ocean, and the implications for material and energy transport through its hydrosphere, is of high interest to NASA's Europa Clipper and ESA's Juice missions. A recent reanalysis of Galileo magnetometer data strengthens the ocean hypothesis, but definitive confirmation awaits new measurements. We assess how Callisto's interior properties and presence or absence of an ocean affect observable quantities, including tidal response and dissipation, rotation-rate variations, and spin-axis obliquity. In addition to the well-known influence of an ocean, we find that these quantities are strongly affected by viscoelastic deformations. As a result, measurements of a single quantity will likely be insufficient to confirm the presence of an ocean, but the combination of two of them will provide a conclusive test. Based on the expected performances of Juice and Europa Clipper investigations, pairing Callisto's tidal response with either tidal dissipation or obliquity will provide a robust test.

Abstract

The internal structure of Callisto has been a long-standing question since the Galileo mission. Previous analyses suggested a largely undifferentiated interior. We present an updated interior structure for Callisto derived from a reanalysis of Galileo's Doppler tracking data using new signal processing techniques. We present two solutions: a ‘classic’ hydrostatic model and our favored model, which accounts for non-hydrostatic contributions from major impact basins. The latter yields a normalized Moment of Inertia (MOI) of 0.345 ± $\pm $ 0.005, lower than the canonical value. Combined with magnetic induction data, our interior modeling indicates Callisto is more differentiated than previously believed. Our results favor a ∼10–120 km thick ice shell, a ∼300 km subsurface ocean—assuming the ocean partly generates the magnetic induction response—and a large rocky core with low average density. This density indicates a chondritic composition with significant mass fraction of organics, suggesting an interior configuration similar to that of Titan.

Abstract

The complex refractive index (CRI) of ice is one of the main parameters driving the optical properties of ice clouds and frozen surfaces, hence their climate impact. While already characterized at short wavelengths, ice CRI has never been measured across the infrared range at temperatures relevant to Earth. Here, we present the first comprehensive experimental determination of the temperature-dependent CRI of hexagonal ice in the infrared (25–5,000 cm−1 $\mathrm{c}{\mathrm{m}}^{-1}$), and for temperatures 150–270 K. The method relies on high-accuracy synchrotron-based transmission measurements, combined with a modeling of the optical system and an advanced retrieval procedure. The temperature dependence of the bands of ice, particularly the connectivity band, have been measured for the first time, highlighting differences from existing extrapolated databases often exceeding 20% $\%$ in strongly temperature depending regions. The obtained data set fills major gaps in both the spectral and temperature dimensions, and will help improve models of frozen surfaces and ice clouds.

Abstract

The Siple Coast region of West Antarctica is known for its fast flowing, topographically unconstrained ice streams that evolve due to subglacial conditions. In this work, we analyze the nature of the Siple Coast subglacial hydrologic system and its interactions with the ice sheet using two-way coupled subglacial hydrology and ice flow modeling. Our modeling is consistent with observations of a water saturated till layer underlying the Siple Coast ice streams. We find that future thinning of ice upstream of Ice Stream B can lead to a hydraulic potential low that promotes water amalgamation and drainage through the ice stream, leading to pulses in ice motion. The inclusion of an ice-dynamics driven variable melt rate and the alteration of glacier driving stress due to subglacial lake geometry lead to stabilized motion of Ice Stream B. These feedbacks demonstrate the importance of coupled modeling for projecting the evolution of ice dynamics.

Abstract

ICESat-2 has demonstrated solid performance in retrieving sea ice freeboard during winter, when sea ice surface types are well-defined and leads provide reliable sea surface height references. Summer retrievals are challenging because melt ponds are not explicitly distinguished in the ATL07 surface classification and may be identified as ice or leads. Here we assess ICESat-2 summer sea ice observations using airborne LiDAR and imagery collected along near-coincident ground tracks during a July 2022 calibration and validation campaign. Over a ∼50-km Arctic sea ice transect, we compare ICESat-2 surface classifications, elevations, and freeboards with high-resolution airborne measurements. Melt ponds are generally not selected as sea surface height references in the ICESat-2 freeboard retrieval. ICESat-2 and airborne surface heights agree well (R ≥ ${\ge} $ 0.9) and there are minimal freeboard differences (−0.05 ± $\pm $ 0.12 m to 0.01 ± $\pm $ 0.10 m). ICESat-2 summer freeboard retrievals are largely unaffected by melt pond misclassification, supporting the potential for year-round sea ice thickness monitoring.

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