ALMA Thermal Observations of Europa
We present four daytime thermal images of Europa taken with the Atacama Large Millimeter Array. Together, these images comprise the first spatially resolved thermal data set with complete coverage of Europa's surface. The resulting brightness temperatures correspond to a frequency of 233 GHz (1.3 mm) and a typical linear resolution of roughly 200 km. At this resolution, the images capture spatially localized thermal variations on the scale of geologic and compositional units. We use a global thermal model of Europa to simulate the ALMA observations in order to investigate the thermal structure visible in the data. Comparisons between the data and model images suggest that the large-scale daytime thermal structure on Europa largely results from bolometric albedo variations across the surface. Using bolometric albedos extrapolated from Voyager measurements, a homogenous model reproduces these patterns well, but localized discrepancies exist. These discrepancies can be largely explained by spatial inhomogeneity of the surface thermal properties. Thus, we use the four ALMA images to create maps of the surface thermal inertia and emissivity at our ALMA wavelength. From these maps, we identify a region of either particularly high thermal inertia or low emissivity near 90° west and 23° north, which appears anomalously cold in two of our images.
© 2018. The American Astronomical Society. Received 2018 March 21; revised 2018 August 9; accepted 2018 August 13; published 2018 September 18. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.01302.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. This research was supported by grant 1313461 from the National Science Foundation, as well as by a NASA Earth and Space Science Fellowship. The authors thank John R. Spencer, Julie A. Rathbun, James T. Keane, and Katherine R. de Kleer for useful conversations. The authors would also like to thank the anonymous referee, whose insightful comments led to the improvement of this manuscript. Software: ALMA pipeline (Muders et al. 2014), CASA (McMullin et al. 2007), AIPS (Bridle & Greisen 1994).
Accepted Version - 1808.07111
Published - Trumbo_2018_AJ_156_161.pdf