  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. ~
