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6 - The Geologic History of Mercury
- Edited by Sean C. Solomon, Larry R. Nittler, Carnegie Institution of Washington, Washington DC, Brian J. Anderson
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- Book:
- Mercury
- Published online:
- 10 December 2018
- Print publication:
- 20 December 2018, pp 144-175
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Summary
We assess Mercury’s geologic history, focusing on the distribution and origin of terrain types and an overview of Mercury’s evolution from the pre-Tolstojan through the Kuiperian Period. We review evidence for the nature of Mercury’s early crust, including the possibility that a substantial portion formed by the global eruption of lavas generated by partial melting during and after overturn of the crystalline products of magma ocean cooling, whereas a much smaller fraction of the crust may have been derived from crystal flotation in such a magma ocean. The early history of Mercury may thus have been similar to that of the other terrestrial planets, with much of the crust formed through volcanism, in contrast to the flotation-dominated crust of the Moon. Small portions of Mercury’s early crust may still be exposed in a heavily modified and brecciated form; the majority of the surface is dominated by intercrater plains (Pre-Tolstojan and Tolstojan in age) and smooth plains (Tolstojan and Calorian) that formed through a combination of volcanism and impact events. As effusive volcanism waned in the Calorian, explosive volcanism continued at least through the Mansurian Period; the Kuiperian Period was dominated by impact events and the formation of hollows.
12 - Mercury’s Hollows
- Edited by Sean C. Solomon, Larry R. Nittler, Carnegie Institution of Washington, Washington DC, Brian J. Anderson
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- Book:
- Mercury
- Published online:
- 10 December 2018
- Print publication:
- 20 December 2018, pp 324-345
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- Chapter
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Summary
Images from the MESSENGER spacecraft show that irregular, flat-floored depressions with high-reflectance interiors and haloes are common on the surface of planet Mercury. 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 impact melts.