We present best-fit values of porosity—and the corresponding effective thermal inertiae—determined from three different depths in Europa's near-subsurface (∼1–20 cm). The porosity of the upper ∼20 cm of Europa's subsurface varies between 75% and 50% (Γeff ≈ 50–140 J m−2 K−1 s−1/2) on the leading hemisphere and 50%–40% (Γeff ≈ 140–180 J m−2 K−1 s−1/2) on the trailing hemisphere. Residual maps produced by comparison with these models reveal thermally anomalous features that cannot be reproduced by globally homogeneous porosity models. These regions are compared to Europa's surface terrain and known compositional variations. We find that some instances of warm thermal anomalies are co-located with known geographical or compositional features on both the leading and trailing hemisphere; cool temperature anomalies are well correlated with surfaces previously observed to contain pure, crystalline water ice and the expansive rays of Pwyll crater. Anomalous regions correspond to locations with subsurface properties different from those of our best-fit models, such as potentially elevated thermal inertia, decreased emissivity, or more porous regolith. We also find that ALMA observations at ∼3 mm sound below the thermal skin depth of Europa (∼10–15 cm) for a range of porosity values, and thus do not exhibit features indicative of diurnal variability or residuals similar to other frequency bands. Future observations of Europa at higher angular resolution may reveal additional locations of variable subsurface thermophysical properties, while those at other wavelengths will inform our understanding of the regolith compaction length and the effects of external processes on the shallow subsurface.
Subsurface Thermophysical Properties of Europa's Leading and Trailing Hemispheres as Revealed by ALMA
Abstract
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© 2024. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
This material is based upon work supported by the National Science Foundation under grant No. 2308280. This research was also funded in part by the Heising-Simons Foundation through grant #2019-1611. Funding for this paper was provided by the NASA ROSES Solar System Observations program (through Task Order 80NM0018F0612) for A.E.T., K.d.K., A.A. Contributions from A.A. were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). We acknowledge support from the National Science Foundation Graduate Research Fellowship under grant #DGE-1745301 to M.C.
We would like to acknowledge the North American ALMA Science Center staff for their expertise and help reducing data associated with this project, and in particular to R. Loomis and L. Barcos-Muñoz for their assistance during an ALMA face-to-face visit. We would also like to acknowledge A. Moullet and R. Moreno for their contributions to the original ALMA proposal on which these observations were based.
This paper makes use of the following ALMA data: ADS/JAO.ALMA#2016.1.00691.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
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Additional details
- National Science Foundation
- AST-2308280
- Heising-Simons Foundation
- 2019-1611
- National Aeronautics and Space Administration
- 80NM0018F0612
- National Aeronautics and Space Administration
- 80NM0018D0004
- National Science Foundation
- NSF Graduate Research Fellowship DGE-1745301
- Caltech groups
- Astronomy Department, Division of Geological and Planetary Sciences