Earth's Moon appears seismically quiet: its major volcanic and tectonic activity is confined to its distant past, as evidenced by the lack of new large-scale features on the surface. However, recent images from the Lunar Reconnaissance Orbiter (LRO) have revealed smaller features that had escaped earlier notice. Several regions exhibit small ravines known as graben that are free of cratering or other marring, which indicates relatively recent formation.
A new paper in Nature Geoscience (by Thomas R. Watters, Mark S. Robinson, Maria E. Banks, Thanh Tran, and Brett W. Denevi) suggests these shallow graben may have formed within the last 50 million years. While this activity is not precisely new, it postdates the last major tectonic activity, which ended roughly 1.2 billion years ago. Since graben form under extension—the stretching of rock by internal pressures—the authors argue that the Moon's interior may still have a significant molten component, and that its cooling and contraction is producing new features on small scales.
More significantly for the Moon's history, they propose it may not have been fully molten when it formed, as that would produce compression, which tends to erase graben
The Moon's surface features are roughly divided into mares (literally "seas," pronounced MAR-ays) and highlands. Mares, the dark-colored regions, are basins filled with basalt rock and are the remnants of ancient lava flows. The highlands are more heavily cratered, but also exhibit more signs of tectonic activity. The Moon's near side has significantly more mares than its far side, which is more heavily cratered.
Cratering is a way to measure the age of surface features: since meteorite impacts were more common in the past, the more battered a region is, the older it must be. Similarly, the absence of craters indicates a relatively recent formation, since the Moon lacks an atmosphere and thus the weathering that erases the signs of impacts on Earth.
Even though the Moon's far side is more heavily cratered, many of the new graben discoveries lie on the lunar far side. The authors of the current study point out that the pristine condition of the graben in the LRO images is a strong indicator of their relative youth. In addition, the graben are relatively shallow, with some as little as 1 meter in depth. Older features are typically deeper, since old, shallow rifts would be erased over time as they fill with debris.
In addition, some of the newly-discovered graben cut across craters or distort the edges of nearby craters, showing them to be more recent than the impact. Other, smaller features are in mare regions, again revealing that they postdated the lava flows in those basins.
The researchers rule out formation of graben by impact stresses, since there are no recent craters in the vicinity. This means the young channels must be formed by tectonic processes. Comparison to similar features shows them to be consistent with the contraction of the Moon, which could come about as its interior cools and shrinks. Measurement of shallow moonquakes by instruments used during the Apollo missions is also consistent with seismic activity associated with the graben.
The Moon's interior certainly was warmer in the past than it is today, according to any reasonable model of its formation and evolution. But it's harder to know exactly how molten it was, and thus how its interior is differentiated and what has changed. The authors of the current study argue that recent graben formation is incompatible with a fully molten interior during early times, since that would create more compressional stresses rather than the stretching that forms valleys like these. However, volcanic activity is also associated with this sort of stressing, and there is no evidence of any volcanism in the eras when the graben must have formed.
Whatever the implications for models of the lunar interior, the relatively pristine graben certainly indicate the Moon is not as seismically quiet as it seems. With detailed observations from the LRO and other missions such as GRAIL (Gravity Recovery and Interior Laboratory), it will be possible to pin down exactly how the Moon continues to change in time, and refine our understanding of both the surface and interior of Earth's satellite.
Nature Geoscience, 2012. DOI: 10.1038/ngeo1387 (About DOIs).
Listing image by Photograph by NASA/GSFC/Arizona State University/Smithsonian Institution
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