Robert A. West

Rapid and Extensive Surface Changes Near Titan’s Equator: Evidence of April Showers

The Cassini spacecraft detected signs of recent methane rainfall on Titan’s arid equatorial surface. Although there is evidence that liquids have flowed on the surface at Titan’s equator in the past, to date, liquids have only been confirmed on the surface at polar latitudes, and the vast expanses of dunes that dominate Titan’s equatorial regions require a predominantly arid climate. We report the detection by Cassini’s Imaging Science Subsystem of a large low-latitude cloud system early in Titan’s northern spring and extensive surface changes (spanning more than 500,000 square kilometers) in the wake of this storm. The changes are most consistent with widespread methane rainfall reaching the surface, which suggests that the dry channels observed at Titan’s low latitudes are carved by seasonal precipitation.

No oceans on Titan from the absence of a near-infrared specular reflection

With its substantial atmosphere of nitrogen, hydrocarbons and nitriles, Saturns moon Titan is a unique planetary satellite. Photochemical processing of the gaseous constituents produces an extended haze that obscures the surface. Soon after the Voyager fly-bys in 1980 and 1981 photochemical models led to the conclusion that there should be enough liquid methane/ethane/nitrogen to cover the surface to a depth of several hundred metres. Recent Earth-based radar echoes imply that surface liquid may be present at a significant fraction of the locations sampled. Here we present ground-based observations (at near-infrared wavelengths) and calculations showing that there is no evidence thus far for surface liquid. Combined with the specular signatures from radar observations, we infer mechanisms that produce very flat solid surfaces, involving a substance that was liquid in the past but is not in liquid form at the locations we studied.

Direct observations of the comet Shoemaker-Levy 9 fragment G impact by Galileo UVS

The Galileo Ultraviolet Spectrometer (UVS) team has detected the Shoemaker-Levy 9 fragment G impact on Jupiter in data recently played back from the spacecraft tape recorder. A 20% brightening of the disc-integrated signal of Jupiter was detected at 292 nm during a swath across Jupiter that lasted 1.6 sec and was centered at 1994-July 18 (day 199)/07:33:31 UT (all times in this paper are corrected to be the time of the event as seen from Earth). The emission brightness, when combined with simultaneous Photopolarimeter Radiometer (PPR) measurements at 945 nm, is consistent with thermal radiation at a temperature of 7800 (+500, −600) K emitted over an area of 40 (+60, −25) km². No excess signal was seen during swaths 5 1/3 sec before and after the detection swath.

Adaptive optics imaging of a stellar occultation by Titan

We present resolved images of the occultation of a binary star by Titan, recorded with the Palomar Observatory adaptive optics system on 20 December 2001 UT. These constitute the first resolved observations of a stellar occultation by a small body, and demonstrate several unique capabilities of diffraction-limited imaging systems for the study of planetary atmospheres. Two refracted stellar images are visible on Titans limb throughout both events, displaying scintillations due to local density variations. Precise relative astrometry of the refracted stellar images with respect to the unnocculted component of the binary allows us to directly measure their altitude in Titans atmosphere. Their changing positions also lead to simple demonstration of the finite oblateness of surfaces of constant pressure in Titans mid-latitude stratosphere, consistent with the only previous measurement of Titans zonal wind field.

Titan's Meteorology Over the Cassini Mission: Evidence for Extensive Subsurface Methane Reservoirs

Cassini-Huygens mission, a cooperative endeavor of NASA; ESA; ASI; NASA; Cassini-Huygens grant [NNX13AG28G]; Institut Universitaire de France; UnivEarthS LabEx program of Sorbonne Paris Cite [ANR-10-LABX-0023, ANR-11IDEX-0005-02]; French National Research Agency [ANR-APOSTIC-11-BS56-002, ANR-12-BS05-001-3/EXO-DUNES]