Voyager and Galileo SSI views of volcanic resurfacing on Io and the search for geologic activity on Europa

Abstract

Observational evidence and theoretical arguments suggest that Jupiter's satellite Europa could be geologically active and possess an "ocean" of liquid water beneath its surface at the present time. We have searched for evidence of current geologic activity on Europa in the form of active plumes venting material above the surface and by comparison of Voyager and Galileo images to look for any changes on the surface. So far, we have observed no plumes and have detected no definitive changes. The lack of observed activity allows us to estimate a maximum steady state surface alteration rate of 1 km² y⁻¹ in the regions analyzed, assuming alterations will cover contiguous areas of at least 4 km² over a period of 20 years. Assuming this as a constant, globally uniform resurfacing rate leads to a minimum average surface age of 30 million years. Lava flows and plumes are the two main types of volcanic activity that resurface Io. We have used the Galileo Io dataset to observe the detailed sequences of interconnected plume activity, hotspot activity, and new surface deposits at a number of volcanic centers on Io. Red material has faded on a timescale of less than a year, and a green coating has formed on a caldera over a time period of about 3 months. Change detection maps can illustrate the percentage of the surface newly covered by plume deposits and lava flows, and constrain volume and mass resurfacing rates. Areal resurfacing is dominated by plume deposits, but volume resurfacing is dominated by lava flows. Estimates of resurfacing from these change maps range from 0.4 to 12.9 cm/year, assuming a flow thickness of 1 to 10 meters. The minimum resurfacing rate required for the lack of impact craters on Io's surface is about 0.02 cm/year. If high-magnesium (komatiitic) lavas dominate the observed Io heat flux, the maximum resurfacing rate is about 0.69 cm/year. Basaltic lavas could produce a rate of 1.3 cm/year. The komatiitic rate produces an average flow thickness of about half a meter. Thus, we suggest that the average resurfacing rate of lo is between 0.1 and 1 cm/year.