ALMA Thermal Observations of a Proposed Plume Source Region on Europa

Abstract

We present a daytime thermal image of Europa taken with the Atacama Large Millimeter Array. The imaged region includes the area northwest of Pwyll Crater, which is associated with a nighttime thermal excess seen by the Galileo Photopolarimeter Radiometer and with two potential plume detections. We develop a global thermal model of Europa and simulate both the daytime and nighttime thermal emission to determine if the nighttime thermal anomaly is caused by excess endogenic heat flow, as might be expected from a plume source region. We find that the nighttime and daytime brightness temperatures near Pwyll Crater cannot be matched by including excess heat flow at that location. Rather, we can successfully model both measurements by increasing the local thermal inertia of the surface.

ALMA Thermal Observations of Europa

“…All derived thermal inertias are less than that of solid water ice, and the low end of our fitted thermal inertia range is consistent with thermal inertias derived for the Saturnian satellites (Howett et al 2010). Similarly, the depressed millimeter emissivities we derive are consistent with those derived for Kuiper Belt and trans-Neptunian objects (Brown & Butler 2017;Lellouch et al 2017).…”

“…However, we do not see any obvious correlation between residuals, derived thermal inertias, and albedo. Furthermore, this map does produce a locally elevated thermal inertia surrounding the crater Pwyll, which is consistent with nighttime PPR data of the same region (Spencer et al 1999;Trumbo et al 2017) and the tendency of crater ejecta to exhibit elevated thermal inertia elsewhere in the solar system (e.g., Mellon et al 2000;Hayne et al 2017).…”

“…As we are unable to explain why the daytime PPR data is inconsistent with the ALMA daytime data in areas of overlap, we opt to forego using any PPR data and instead focus on our selfconsistent ALMA data set. Despite this, our analysis does reproduce the high thermal inertia near Pwyll crater that was derived using both ALMA data and Galileo PPR nighttime data (Rathbun & Spencer 2017;Trumbo et al 2017). If we apply the same minor albedo adjustment near Pwyll as Trumbo et al (2017), which is within our albedo errors, we arrive at a similar thermal inertia using only the ALMA data.…”

“…The end result is a radiative flux map of the surface that can be output for the geometries and times of the ALMA observations, smoothed to the ALMA resolution, and converted to brightness temperature. A complete description of the model can be found in Trumbo et al (2017).…”

“…In particular, our model does not include sunlight penetration with depth in the regolith, thermal emission from depth in the regolith, or surface roughness. As described in Trumbo et al (2017), our model assumes that the solar flux is absorbed in the topmost model layer, which is a standard assumption for several thermal models (e.g., Spencer et al 1989;Spencer 1990;Hayne & Aharonson 2015) and particularly valid for low-albedo surfaces. However, sunlight may penetrate deeper into a relatively high-albedo particulate surface like that of Europa (Brown & Matson 1987;Urquhart & Jakosky 1996), resulting in heat at depth in the regolith.…”