Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p41b..04s&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P41B-04
Physics
6295 Venus
Scientific paper
During the flyby of Venus on 05 June 2007 by the MESSENGER spacecraft the Mercury Laser Altimeter (MLA) was operated for approximately 30 minutes, including approximately 8 minutes within 800 km of the planet's surface. With almost no expectation of receiving a return from the Venus surface the altimeter was targeted at the Venus atmosphere between 40 and 80 km above the surface, the altitude range of thickest clouds. Previous observations of the Venus atmosphere from nephelometry and particle size spectrometry indicated a structure of the clouds consisting of a haze that increases in opacity with depth, without discrete cloud tops. An objective of the MLA experiment was to attempt to identify heretofore undetected layering within the upper part of the Venus cloud deck. Although the MLA is a ranging device and not an atmospheric lidar, an instrument of similar design on the Mars Global Surveyor (MGS) spacecraft had obtained clear returns from layers within the Martian atmosphere at altitudes up to several tens of kilometers above the surface. At Venus, the MLA operated successfully in science ranging mode for the first time; the instrument recorded photon counts throughout the period of operation. We analyzed in detail a period of approximately 3 minutes when the spacecraft was closest to the planet at less than 400 km and when returns from the atmosphere would be most likely. When the backscattered signal is weak or non-existent, the altimeter dark noise dominates the detector count. Returns from cloud layers, if they exist, would appear as increased detector counts at the altitude of the cloud layer on top of the detector noise. The altimeter returns indicate a significant noise component throughout the 8-minute period of operation, including the 3 minutes bounding closest approach. Analysis of the data by three different methods suggest an increase in returns in two regions between 50 and 70 km altitude range, which, on the basis of statistical analysis, could be attributable to backscattered photons from the Venus atmosphere at a weak level of statistical significance. However, the experiment showed no evidence for discrete layering within the primary cloud deck.
Cavanaugh John F.
McGarry Jan
Neumann Gregory A.
Smith Douglas E.
Solomon Stanley C.
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