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Ganymede's Surface Properties from Millimeter and Infrared Thermal Emission

de Kleer, Katherine and Butler, Bryan and de Pater, Imke and Gurwell, Mark A. and Moullet, Arielle and Trumbo, Samantha and Spencer, John (2021) Ganymede's Surface Properties from Millimeter and Infrared Thermal Emission. Planetary Science Journal, 2 (1). Art. No. 5. ISSN 2632-3338. doi:10.3847/psj/abcbf4.

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We present thermal observations of Ganymede from the Atacama Large Millimeter Array (ALMA) in 2016–2019 at a spatial resolution of 300–900 km (0.”1–0.”2 angular resolution) and frequencies of 97.5, 233, and 343.5 GHz (wavelengths of 3, 1.3, and 0.87 mm); the observations collectively covered all Ganymede longitudes. We determine the global thermophysical properties using a thermal model that considers subsurface emission and depth- and temperature-dependent thermophysical and dielectric properties, in combination with a retrieval algorithm. The data are sensitive to emission from the upper ~0.5 m of the surface, and we find a millimeter emissivity of 0.75–0.78 and (sub)surface porosities of 10%–40%, corresponding to effective thermal inertias of 400–800 J m⁻² K⁻¹ s^(−1/2). Combined with past infrared results, as well as modeling presented here of a previously unpublished night-time infrared observation from Galileo's photopolarimeter–radiometer instrument, the multiwavelength constraints are consistent with a compaction profile whereby the porosity drops from ~85% at the surface to 10⁺³⁰₋₁₀% at depth over a compaction length scale of tens of centimeters. We present maps of temperature residuals from the best-fit global models, which indicate localized variations in thermal surface properties at some (but not all) dark terrains and at impact craters, which appear 5–8 K colder than the model. Equatorial regions are warmer than predicted by the model, in particular near the centers of the leading and trailing hemispheres, while the midlatitudes (~30°–60°) are generally colder than predicted; these trends are suggestive of an exogenic origin.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
de Kleer, Katherine0000-0002-9068-3428
Butler, Bryan0000-0002-5344-820X
de Pater, Imke0000-0002-4278-3168
Gurwell, Mark A.0000-0003-0685-3621
Moullet, Arielle0000-0002-9820-1032
Trumbo, Samantha0000-0002-0767-8901
Additional Information:© 2021 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. Received 2020 August 16; revised 2020 November 13; accepted 2020 November 16; published 2021 January 22. This work was supported in part by the Heising-Simons Foundation 51 Pegasi b postdoctoral fellowship to K. de Kleer. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2016.1.00691.S, ADS/JAO.ALMA#2018.1.01292.S, ADS/JAO.ALMA#2011.0.00001.CAL. 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.
Funding AgencyGrant Number
Heising-Simons Foundation51 Pegasi b Fellowship
Subject Keywords:Galilean satellites ; Jovian satellites ; Radio interferometry ; Planetary surfaces ; Surface processes
Issue or Number:1
Classification Code:Unified Astronomy Thesaurus concepts: Galilean satellites (627); Jovian satellites (872); Radio interferometry (1346); Planetary surfaces (2113); Surface processes (2116)
Record Number:CaltechAUTHORS:20210122-151827531
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Official Citation:Katherine de Kleer et al 2021 Planet. Sci. J. 2 5
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:107679
Deposited By: Tony Diaz
Deposited On:22 Jan 2021 23:42
Last Modified:16 Nov 2021 19:05

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