Julie Ann Rathbun

Cassini Observes the Active South Pole of Enceladus

Cassini has identified a geologically active province at the south pole of Saturns moon Enceladus. In images acquired by the Imaging Science Subsystem (ISS), this region is circumscribed by a chain of folded ridges and troughs at ∼55°S latitude. The terrain southward of this boundary is distinguished by its albedo and color contrasts, elevated temperatures, extreme geologic youth, and narrow tectonic rifts that exhibit coarse-grained ice and coincide with the hottest temperatures measured in the region. Jets of fine icy particles that supply Saturns E ring emanate from this province, carried aloft by water vapor probably venting from subsurface reservoirs of liquid water. The shape of Enceladus suggests a possible intense heating epoch in the past by capture into a 1:4 secondary spin/orbit resonance.

Ice diapirs on Europa: Implications for liquid water

Early examinations of Galileo images of Europa revealed features that look like low topographic domes. These small (5–10 km in radius) domes have been interpreted as surface manifestations of diapirs. As a way to probe the subsurface structure of Europa, we investigate the possibility that thermally-driven ice diapirism created these surface features. We use a previously developed analytic model for rising diapirs to estimate the initial radii of the diapirs, their initial depth, the temperature of the medium through which they rise and their formation timescales. We assume that the diapirs originate at the boundary layer between solid ice and whatever underlies it, whether it be liquid water or solid rock. Our results show that the diapirs originate at a depth of no more than a few tens of kilometers. Since the H2O layer on Europa is substantially thicker than this, our results support the view that liquid water recently existed beneath the ice on Europa at this location, at a depth of no more than a few tens of kilometers. Further, the ice must have been warm (250–270 K) and was therefore likely to have been convecting.

Loki, Io: A periodic volcano

Loki is the most powerful volcano in the Solar System. It has been observed to be in continuous though variable activity since 1979. Synthesis of more than a decade of groundbased data suggests that Loki eruptions are cyclic, with a 540 day period. Application of a simple lava cooling model to temperatures in Loki Patera, and eruption start and end times, implies that brightenings are due to a resurfacing wave propagating across the patera. The data are most consistent with lava lake overturn, but resurfacing by lava flows cannot be ruled out. A porosity gradient in the lake crust could cause lava lake overturn to occur periodically on the timescale observed.

Ground‐based observations of volcanism on Io in 1999 and early 2000

Ground-based observations of volcanism on Io during the period of the 1999 and early 2000 Galileo close flybys have detected several types of activity, providing information which complements the spacecraft observations. At Loki a brightening began between August 25 and September 9 and continued through February. On August 2 a major outburst was observed near (14°N, 74°W) whose brightness corresponds to an area of approximately 350 km 2 at a temperature of 1100 K. Observations of eruptions in late June (9906A) and in late November (9911A, at Tvashtar) provide temporal and photometric constraints on activity also seen by Galileo. High-resolution adaptive optics images provide further information on the fainter sources distributed across the surface.

Spatially resolved m-band emission from io's Loki Patera-fizeau imaging at the 22.8 m LBT

The Large Binocular Telescope Interferometer mid-infrared camera, LMIRcam, imaged Io on the night of 2013 December 24 UT and detected strong M-band (4.8 μm) thermal emission arising from Loki Patera. The 22.8 m baseline of the Large Binocular Telescope provides an angular resolution of ∼32 mas (∼100 km at Io) resolving the Loki Patera emission into two distinct maxima originating from different regions within Loki’s horseshoe lava lake. This observation is consistent with the presence of a high-temperature source observed in previous studies combined with an independent peak arising from cooling crust from recent resurfacing. The deconvolved images also reveal 15 other emission sites on the visible hemisphere of Io including two previously unidentified hot spots.

Formation of Beta Regio, Venus: Results from measuring strain

Beta Regio is an area of rifting and volcanism on Venus, constituting a topographic rise. A shield volcano, Theia Mons, lies near the center of the region and is surrounded by several radially oriented rifts. We use Magellan altimetry, gravity, and synthetic aperture radar data of the area to constrain some subsurface parameters. First, we derive hoop strain. Using altimetry data and a fault dip angle derived from the split crater Somerville, we determine the extension in the rifts surrounding Beta Regio. We then derive the hoop strain accommodated by the rifts from the extension in these rifts. Except near Theia Mons, the hoop strain follows the shape expected from a mantle upwelling. The difference near the volcano, we believe, is due to volcanic infilling. We then model three observable quantities, the newly derived strain along with gravity and uplift, using two separate modeling techniques, one for the strain and uplift and another for the gravity. The model results show that the data are consistent with the view that a relatively low density contrast region now exists below Beta and has caused the uplift and rifting in the region.

Io's hot spots in the near-infrared detected by LEISA during the New Horizons flyby

The New Horizons spacecraft flew past Jupiter and its moons in February and March 2007. The flyby provided one of the most comprehensive inventories of Ios active plumes and hot spots yet taken, including the large 350 km high eruption of Tvashtar. Among the suite of instruments active during the flyby was the Linear Etalon Infrared Spectral Array (LEISA), a near-infrared imaging spectrometer covering the spectral range 1.25 to 2.5 µm. We have identified 37 distinct hot spots on Io in the nine LEISA spectral image cubes taken during the flyby. We describe the thermal emissions from these volcanoes and fit single-component blackbody curves to the hot spot spectra to derive eruption temperatures, areas, and power output for the hot spots with sufficient signal-to-noise. Of these, 11 hot spots were seen by LEISA more than once, and East Girru showed short-term variability over a few days, also seen by other New Horizons instruments. This work presents a comprehensive look at the global distribution of Ios volcanism at the time of the flyby. From these measurements, we estimate the global power output of high-temperature (>550 K) volcanism on Io to be ~8 TW. This work provides the first short-wavelength near-infrared survey with global coverage at all longitudes on the nightside of Io without sunlight contamination at these wavelengths. A major conclusion from this study is that 90% of all the volcanoes observed in the New Horizons LEISA near-infrared data in 2007 were also observed during the Galileo epoch, suggesting these are all long-lived hot spots.

The Global Distribution of Active Ionian Volcanoes and Implications for Tidal Heating Models

Tidal heating is the major source of heat in the outer Solar System. Because of its strong tidal interaction with Jupiter and the other Galilean Satellites, Io is incredibly volcanically active. We use the directly measured volcanic activity level of Ios volcanoes as a proxy for surface heat flow and compare to tidal heating model predictions. Volcanic activity is a better proxy for heat flow than simply the locations of volcanic constructs. We determine the volcanic activity level using three data sets: Galileo PPR, Galileo NIMS, and New Horizons LEISA. We also present a systematic reanalysis of the Galileo NIMS observations to determine the 3.5 micron brightness of 51 active volcanoes. We find that potential differences in volcanic style between high and low latitudes make high latitude observations unreliable in distinguishing between tidal heating models. Observations of Ios polar areas, such as those by JUNO, are necessary to unambiguously understand Ios heat flow. However, all three of the data sets examined show a relative dearth of volcanic brightness near 180 W (anti-Jovian point) and the equator and the only data set with good observations of the sub-Jovian point (LEISA) also shows a lack of volcanic brightness in that region. These observations are more consistent with the mantle-heating model than the asthenospheric-heating model. Furthermore, all three of the data sets are consistent with four-fold symmetry in longitude and peak heat flow at mid-latitudes, which best matches with the combined heating case of Tackley et al. (2001).