Stellar Occultations

A stellar occultation occurs when the light from a star is blocked by an intervening body (such as a planet, moon, ring, or asteroid) from reaching an observer. The main reason for observing stellar occultations is that they can be used to probe ring systems and atmospheres in the outer solar system with spatial resolutions of a few kilometers-several orders of magnitude better than the resolution of any other Earth-based method. We are currently conducting a program emphasizing a combination of the highest temporal resolution multi-wavelength observations possible using today's and tomorrow's state-of-the-art instrumentation, along with further analyses of the highest quality archived occultation observations.

Small Bodies in the Outer Solar System

Bodies residing in the outer solar system exhibit unique physical processes, and some of the lessons learned from them can be applied to understanding what occurred in the outer solar system during its formation and early evolution. Pluto, the largest known Kuiper belt object (KBO), and its near twin Triton -- an ex-KBO that has been captured by Neptune -- have nitrogen atmospheres that are in vapor-pressure equilibrium with surface ice. These atmospheres are most sensitively probed from Earth by the technique of stellar occultations, which can provide the temperature and pressure profiles of these atmospheres at a spatial resolution of a few kilometers. Recent results from occultations show that the surface pressure of Triton's atmosphere has been increasing and that the shape of the atmosphere deviates from its expected spherical figure. With the occultation technique we can also learn the sizes of smaller bodies that have formed in the outer solar system: Charon, the Centaurs, and KBOs.

Giant-Planet Occultations

Occultation studies allow an investigation of the temperature, pressure, and number density profiles in the upper atmospheres of Uranus, Jupiter, and Saturn. Recent Uranus occultation observations indicate that the secular trend in stratospheric temperature has reversed. The region probed by occultations lies just below the extremely hot stratosphere discovered by Voyager on Uranus (and the other giant planets), so these changes could be driven by variations in that upper region. Our studies have included analysis of general thermal profiles and investigations of the He fraction in these planetary atmospheres. The Jovian auroral regions are particularly suitable to the occultation studies since inversion of stellar occultation light curves provide temperature profiles of the microbar region--where the bulk of the auroral energy is deposited. These studies can supply a critical independent measure of the temperature in these regions necessary to understand the infrared spectroscopy observations of hydrocarbon abundance. We are also insterested in resolveing the question of whether Titan's hemispheric brightness asymmetry is variable with altitude as well as time by combining occultation and imaging observations.

Please see our occultation publications for additional details. Also, view the latest occultation predictions.