Melissa Ann McGrath

OH in Saturn's magnetosphere: Observations and implications

The discovery of OH in Saturns inner magnetosphere changed our view of this region from one where plasma dominated the physics to one where neutrals are dominant. We revisit Hubble Space Telescope observations of OH and derive revised OH brightnesses for observations in 1992, 1994, and 1995. These OH observations as well as Voyager observations are used as constraints on a model of neutral and plasma interactions. We find that the neutral source required to produce the observed OH brightnesses is 1.4×10 27 H 2 O s -1 , with a sharp peak in the neutral source rate near 4.5 R S . A good fit to the data requires OH densities of over 700 cm -3 at 4.5 R S . Rapid diffusion times, about 5 days at 6 R S , are required to match the observed ion densities. We find that the plasma and neutral composition vary with distance from Saturn, and make predictions for the ion and neutral densities as a function of radius.

PROBING FOR EVIDENCE OF PLUMES ON EUROPA WITH HST/STIS

Roth et al (2014a) reported evidence for plumes of water venting from a southern high latitude region on Europa - spectroscopic detection of off-limb line emission from the dissociation products of water. Here, we present Hubble Space Telescope (HST) direct images of Europa in the far ultraviolet (FUV) as it transited the smooth face of Jupiter, in order to measure absorption from gas or aerosols beyond the Europa limb. Out of ten observations we found three in which plume activity could be implicated. Two show statistically significant features at latitudes similar to Roth et al, and the third, at a more equatorial location. We consider potential systematic effects that might influence the statistical analysis and create artifacts, and are unable to find any that can definitively explain the features, although there are reasons to be cautious. If the apparent absorption features are real, the magnitude of implied outgassing is similar to that of the Roth et al feature, however the apparent activity appears more frequently in our data.

Active Cryovolcanism on Europa

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 Europas internal water and ice and for seeking evidence of Europas habitability. ~

Morphology of Ganymede's FUV auroral ovals

We study the morphology of Ganymedes FUV aurora by analyzing spectral images obtained over the past two decades by the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The ...

The Far‐UV Albedo of Europa From HST Observations

We present an analysis of Europas far-UV spectral albedo using observations during the 1999-2015 time period made by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Disk-in ...