Monday, 9 June 2014

A recent volcanic lava flow in the Lowell Crater?

The Lowell Crater is a 66 km diameter crater located in the northwest Oriental Basin, just beyond the Moon's western limb (i.e. not directly observable from Earth). It is thought to be early Copernican in age (about 1.1 to 0.9 billion years old), and sits in the Montes Rook (series of ring-shaped mountain ranges surrounding the Oriental Basin) between the Cordillera Ring and the Outer Rook Ring. A smaller (9 km diameter) crater known as Crater S sits on the eastern edge of the Lowell Crater, and a prominent area of resurfacing extends 17 km inwards from Crater S towards the center of the Lowell Crater.

In a paper published in the journal Planetary and Space Science on 16 September 2013, Neeraj Srivastava of the Physical Research Laboratory Planetary Sciences and Exploration Programme, Diganta Kumar of the Department of Geological Sciences at Gauhati University and Ravi Gupta of the Department of Earth Sciences at the Indian Institute of Technology Roorkee describe the resurfaced area extending from Crater S towards the center of the Lowell Crater in detail, using data collected by the KaguyaLunar Reconnaissance Orbiter and Chandrayaan-l satellites, and discuss the conclusions they derive from this.

LROC-WAC image mosaic showing regional geologic setting of Lowell crater on the Moon. The prominent recent resurfacing examined in this study is enclosed within a rectangle. Srivastava et al. (2014).

The resurfaced area covers about 63 km², confined to a linear channel 3-6 km wide and about 100 m deep, and overlies at least two older lava flows, which also appear to emanate from Crater S. The surface of these lava flows resembles pahoehoe lava (an undulating or ropey surface pattern, caused by fluid lava moving under a congealing surface. The area shows few crater impacts, implying that the surface is relatively young, and most of those that are present appear subdued and lack signs of ejecta, which suggests they may have been covered by the lava flow.

A 3d view from TC data showing the extent of old melts that originated and diverged from Crater S and the fresh resurfacing under consideration here following a rectilinear trend. The older flows are marked with thin arrows and the comparatively fresh ones are indicated by thick central arrow. Srivastava et al. (2014).

The occurrence of lava within impact craters is easily accounted for, as large impacts release enough energy to melt rock around the site of impact. However an impact event causing repeated laval flows separated by long periods of inactivity, as seems to have been the case at the Lowell Crater, is much harder to explain. For this reason Srivastava et al. conclude that the lava flow in the Lowell Crater is likely to be volcanic in origin. 

However the density of impact craters within the resurfaced area (considered a good proxy for age, since impacts occur at steady rate across the entire surface of the Moon) suggests that the lava flow is between 2 and 10 million years old, while all volcanic activity on the Moon is generally accepted to have ceased over a billion years ago, so if this conclusion is correct then our current understanding of magmatic processes on the Moon is obviously in need of some revision.

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The moon is thought to have formed about 4.5 billion years ago, thrown off from the Earth by a collision with another early planetesimal. After this collision it cooled rapidly, with the last of the flood basalts that cover much of the surface of the moon erupting...

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