Saturn's Inner Satellites at True Opposition

Details Saturn's Inner Satellites at True Opposition Saturn's Inner Satellites at True Opposition

Jul 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004hst..prop10357v&link_type=abstract

HST Proposal ID #10357

Physics

Hst Proposal Id #10357 Solar System

Scientific paper

We request one HST orbit to observe Janus, Epimetheus, Mimas, and Enceladus with WFPC2 exactly at opposition, when the Earth transits the center of the solar disk seen from Saturn on UT 13/14 January 2005. Data obtained at this unique viewing geometry are essential to determining physical properties of the moon's surface, related to its emplacement and evolution, and critical for the interpretation of photometric data obtained by Cassini at higher phase angles. This single observation will be the capstone of 9 years of legacy HST WFPC2 observations of the saturnian system {Cycles 6-12, R. French, PI} from which we have constructed precise, multiwavelength phase curves which demonstrate how the reflectance of these satellites varies with solar phase angle from 0.07 to 6.4 degrees. Each satellite exhibits a dramatic increase in brightness, or "opposition effect", as phase angles decrease below 1 degree. Since 1998 {Cycle 7} the minimum observable phase angle at opposition has decreased each year to 0.07 degrees in Cycle 12; however, the absolute minimum observable phase angle, about 0.02 degrees {limited by the angular size of the Sun viewed from Saturn}, has not been accessible until Cycle 13. Using the same set of broadband filters for continuity with our previous programs, we will place observations made during the Earth transit on the existing UVBRI phase curves and establish the amplitude of each satellite's opposition surge. From these observations we will determine surface properties such as porosity, grain size distribution and particle opacity using radiative transfer models. While the Cassini spacecraft will obtain images at larger phase angles, it will miss entirely the narrow brightness surge near opposition due to orbital constraints. Because these inner satellites will be either lost in or contaminated by the glare of the fully open rings, they are not accessible to ground-based telescopes. The 2005 opposition prese nts the only opportunity for HST to observe the saturnian system during this rare planetary alignment. The next transit of Earth across the solar disk seen from Saturn occurs in 2020; the next central transit occurs in 2049.

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