Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.u21a0021g&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #U21A-0021
Other
5480 Volcanism (6063, 8148, 8450), 6235 Mercury, 8040 Remote Sensing, 8440 Calderas, 8450 Planetary Volcanism (5480, 6063, 8148)
Scientific paper
From images obtained by the MESSENGER spacecraft during its first flyby of Mercury on 14 January 2008, we have identified five impact craters that exhibit pit craters on their floors. Pit craters are rimless depressions that are determined to have formed by non-impact process. Among the arguments in favor of this interpretation are that the features are rimless steep-sided craters that exhibit no observable ejecta and that they are irregularly shaped, with the long axis of the pit crater often concentric to the host impact crater. Impact craters hosting pit craters we term pit-floor craters, which vary in size from 52 to 120 km in diameter. The associated pit craters range in size from 20 to 38 km in diameter, but there is no evident correlation between pit-floor crater diameter and size of pit crater. Pit craters are found on other planets, occurring along rift zones and commonly in chains. The majority of pit craters observed on Mercury, however, do not match the characteristics of pit craters in these other planetary settings. There are three important differences: First, pit craters on Mercury have only been found of the floors of impact craters and within basins, which suggests that pit craters must be related to the impact process in some way. Second, pit craters identified on Mercury to date are distinctive for their large size. Pit craters on other planetary objects are smaller, typically <2 km in diameter. By comparison Mercury's pit craters range upward from 6 km wide and can be nearly 40 km in length. Pit craters in this size range on the other planets are actually calderas. And third, they occur as isolated craters rather than within chains. Lava flows are not observed to be associated with pit craters on Mercury, and there is little evidence to support formation of the pit craters observed via explosive venting of volatiles. They are however, proximally located to smooth plains deposits. A spatial association between the pit-floor craters and smooth plains deposits nonetheless suggests a genetic relationship. Our interpretation of this relation is that pit craters provide evidence for a process of endogenic crater modification, whereby shallow subsurface magma bodies form collapse pits or calderas in association with near-surface magma reservoirs. In this scenario for pit formation, the mechanism of formation is the result of piston-like collapse over a broad magma body or evacuated chamber. Stoping of roof material by magma below pit-floor craters creates a large cavity, into which the highly fractured crater floor above collapses after magma is withdrawn. At present there is not enough evidence to indicate whether an individual pit crater formed all at once (i.e., in a single collapse or explosion) or in episodes over a protracted period of time. Moreover, it is not known whether all the pit-floor craters were made over a brief and contemporaneous interval or at different times. During its second Mercury flyby on 6 October 2008, MESSENGER will view about 30% of the surface at close range for the first time and may yield additional examples of these enigmatic features.
Blewett Dave T.
Denevi Brett Wilcox
Gillis-Davis Jeffery J.
Robinson Mark S.
Solomon Stanley C.
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