MESSENGER's View of Hollows on Mercury, and Links to the Planet's High Volatile Content
MESSENGER, a mission in NASA's Discovery Program, was the first spacecraft to orbit Mercury and only the second to visit the planet. MESSENGER orbited this planetary oddball for four Earth years (2011-2015), mapping and determining the elemental composition of the surface, probing areas of permanent shadow at the poles, and making measurements of topography, the exosphere, magnetic field, and gravitational field.
One of the most surprising results of the mission was the discovery of an unusual landform. Images from MESSENGER show that small, irregular, flat-floored depressions with high-reflectance interiors and haloes are common. These landforms, called hollows, are among Mercury's youngest non-impact features and may be forming today. Hollows are unique to Mercury, with no close equivalent on other planetary bodies. Clues to understanding hollows come from consideration of morphological features associated with ice-bearing surfaces on Mars and icy satellites, and of processes leading to loss of sulfur from asteroids. Evidence suggests that hollows form when sublimation or destruction of a volatile-bearing phase weakens the host rock, causing collapse and scarp retreat. The phase susceptible to loss may be a sulfide mineral or graphite. Loss of the volatile component could be driven by solar heating, exposure to solar ultraviolet radiation, exposure to the solar wind, sputtering by magnetospheric ions, and micrometeoroid bombardment. The depth to which hollows grow may be controlled by accumulation of a protective lag deposit. The volatile-bearing phase that is lost appears to be a pervasive component of the host rock, but in some cases the hollow-forming phase may have been concentrated by volcanic processes or differentiation of melts.
Biography
David T. Blewett joined the Johns Hopkins Applied Physics Laboratory as a member of the Senior Professional Staff in September 2007. Prior to that he was a Principal Scientist at NovaSol, a small employee-owned high-tech company in Honolulu, Hawaii of which he was a co-founder. His planetary research emphasizes remote sensing, geological analysis, and spectral algorithm development using data from planetary spacecraft including Mariner 10, Clementine, Galileo, Lunar Prospector, NEAR, LRO, MESSENGER and Dawn. He was selected as a MESSENGER Participating Scientist in 2007, and a Dawn at Vesta Participating Scientist in 2010. Dr. Blewett has focused on geological and compositional analysis of UV-NIR spectra and multispectral images, including study of space weathering on the Moon, Mercury and Vesta. He served for three years as Co-Chair of the MESSENGER Geology Discipline Group and as a member of the PI's Science Steering Committee. Dr. Blewett fielded all the questions from the public that were submitted via the MESSENGER Q&A website. In 2015 and 2016 he had two-month Visiting International Scholar fellowships from the Chinese Academy of Sciences, hosted by the Key Laboratory for Lunar and Deep Space Exploration in Beijing. During the fellowship, he lectured on his planetary research and collaborated on analysis of data from the Chang'E lunar missions. He was an Antarctic Search for Meteorites field party member during the 1988-89 austral summer. He is the namesake of asteroid 8997 Davidblewett.
Dr. Blewett obtained M.S. and Ph.D. degrees in Geology and Geophysics at the University of Hawaii, and a B.A. in Astronomy and Astrophysics from the University of Pennsylvania.
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