The MESSENGER Venus Flyby: First results from the Mercury Atmospheric and Surface Composition Spectrometer

The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft, NASA's first mission to orbit the planet Mercury, was launched on August 3, 2004, from Cape Canaveral Air Force Station. A gravity-assist maneuver brought the spacecraft past the planet Venus with a closest approach on June 5, 2007. This event provided an important opportunity to observe the Venus clouds and atmosphere with several of MESSENGER's remote sensing experiments. The Mercury Atmospheric and Surface Composition Spectrometer (MASCS), one of seven instruments onboard the spacecraft, consists of a Cassegrain telescope that simultaneously feeds the Visible and Infrared Spectrograph (VIRS) and the Ultraviolet and Visible Spectrometer (UVVS). VIRS is a point spectrometer, covering the wavelength range 320-1450 nm with a field-of-view of 0.023°. The Sun-illuminated portion of Venus was visible to the body-fixed MESSENGER instruments for about 15 minutes, during which time VIRS recorded ~500 individual spectra and MESSENGER's Mercury Dual Imaging System (MDIS) acquired an 11-color sequence before and after the VIRS measurements. These dual observations provide a unique data set for inter-comparing the radiometric calibration of these complementary devices. Visible color ratios from VIRS show evidence for a heterogeneous atmosphere. Nightside near-infrared spectra from VIRS do not show evidence for thermal emission in the known atmospheric window bands. Preliminary analysis of visible and near-infrared spectra from VIRS and multispectral MDIS Wide Angle Camera (WAC) images are internally consistent with observations of the Moon acquired in August 2005.