Active Cryovolcanism on Europa? (1704.04283v1)

Abstract: Evidence for plumes of water on Europa has previously been found using the Hubble Space Telescope (HST) using two different observing techniques. Roth et al. (2014) found line emission from the dissociation products of water. Sparks et al. (2016) found evidence for off-limb continuum absorption as Europa transited Jupiter. Here, we present a new transit observation of Europa that shows a second event at the same location as a previous plume candidate from Sparks et al. (2016), raising the possibility of a consistently active source of erupting material on Europa. This conclusion is bolstered by comparison with a nighttime thermal image from the Galileo Photopolarimeter-Radiometer (PPR) which shows a thermal anomaly at the same location, within the uncertainties (Spencer et al. 1999). The anomaly has the highest observed brightness temperature on the Europa nightside. If heat flow from a subsurface liquid water reservoir causes the thermal anomaly, its depth is ~1.8-2 km, under simple modeling assumptions, consistent with scenarios in which a liquid water reservoir has formed within a thick ice shell. Models that favor thin regions within the ice shell that connect directly to the ocean, however, cannot be excluded, nor modifications to surface thermal inertia by subsurface activity. Alternatively, vapor deposition surrounding an active vent could increase the thermal inertia of the surface and cause the thermal anomaly. This candidate plume region may offer a promising location for an initial characterization of Europa's internal water and ice and for seeking evidence of Europa's habitability. ~

Cryovolcanism and the Potential for Habitability on Europa

The paper "Active Cryovolcanism on Europa?" by Sparks et al. explores the continued examination of Europa's geological activity, specifically focusing on cryovolcanism. Leveraging observations from the Hubble Space Telescope, the authors present evidence supporting the transient presence of water vapor plumes on Europa. Such phenomena suggest that Europa could harbor essential ingredients and conditions supportive of life, enhancing its status as a significant astrobiological interest in our Solar System.

Key Observations and Findings

The paper builds upon earlier evidence of water plumes detected using two distinct HST techniques. Roth et al. (2014) initially identified line emissions from dissociation products of water, and Sparks et al. (2016) reported evidence for off-limb continuum absorption during Europa's transit across Jupiter. The current investigation documents a subsequent event occurring at the same geographical position as a plume candidate previously noted by Sparks et al. (2016), implicating the existence of a potentially persistent source of eruptive material.

Significantly, this observation is supplemented by thermal data from the Galileo Photopolarimeter-Radiometer (PPR), which revealed a thermal anomaly in the same region. This anomaly registers the highest brightness temperature on Europa's nightside, suggesting an increase in subsurface heat flow. Simple modeling places a subsurface liquid reservoir at a depth of approximately 1.8-2 kilometers. The anomaly may result from either direct heat conduction from subterranean water bodies or surface thermal inertia modifications due to recent cryovolcanic activity. The scenario wherein plumes influence surface thermal characteristics via vapor deposition is also considered.

Implications of Findings

The convergence of optical and thermal data bolsters the hypothesis of an active cryovolcanic system on Europa. Such activity raises intriguing questions about the viability of a subsurface ocean and potential habitability. If water reservoirs are indeed shallow, they might offer relatively accessible sites for future exploration missions aimed at probing Europa's potential to support life.

Several theoretical models and the data point towards complex geological and thermal processes occurring beneath Europa's icy crust. These processes may include localized melting due to tidal forces, forming transient melt-water reservoirs that occasionally connect to the surface, thus causing periodic venting. Such convection-driven vapor release would align with observed surface anomalies.

Future Directions and Considerations

While compelling, these findings necessitate further validation. The paper acknowledges potential inconsistencies, specifically artifacts due to HST's observational limits or coincidental alignments of data sets. Nevertheless, the evidence presents a substantial incentive for ongoing and future missions. Exploration initiatives such as the Europa Clipper are poised to provide the required detailed in-situ assessments, delivering critical insights into the moon's geology and habitability.

In conclusion, the integration of observational methodologies and thermal analysis manifests a nuanced understanding of Europa's active cryovolcanic processes. This work contributes significantly to our comprehension of icy body dynamics in the Solar System and lays foundational knowledge to potentially unravel the enigma of life's existence beyond Earth.