Post by Carolyn Ernst, Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.
Prior to 2008, less than half of Mercury’s surface had been imaged at close range, during the flybys of Mariner 10 in the mid-1970s. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft completed three flybys of the planet in 2008 and 2009 on its way to insertion into orbit about Mercury on 18 March 2010 and viewed most of the planet’s surface that had never before been seen by a spacecraft. These MESSENGER images have helped to confirm some Mariner-10-based hypotheses and have elicited new science questions to be investigated.
Images taken during MESSENGER’s third Mercury flyby revealed a 290-kilometer-diameter peak-ring impact basin named Rachmaninoff (Prockter et al., 2010) (Image 1). Peak-ring, or double-ring, impact basins are characterized by an outer basin rim and an interior ring of contiguous peaks. They are common on Mercury, formed at sizes intermediate between complex craters and large multi-ring basins. The well-preserved appearance of Rachmaninoff indicates that it is younger than most basins on Mercury, likely having formed after the period of late heavy impact bombardment (which ended ~3.8 Ga).
The smooth plains within the basin’s inner ring differ from the surrounding units in reflectance, color, and structure (Images 1 and 2). These plains are observed to embay and overlie units related to the formation of the basin. Therefore, the plains must postdate the impact and are unlikely to be formed of impact melt. Furthermore, the interior plains are less cratered than the plains between the outer and inner rims, suggesting that the former were emplaced a substantial time after the basin formed. These relationships imply that the smooth plains within the peak ring formed from effusive volcanic activity. The very low density of superposed craters indicates that these interior smooth plains are products of relatively young volcanism, the youngest documented on Mercury to date (Prockter et al., 2010).
Also of note is a set of narrow graben that lie within the inner smooth plains. Mercury underwent an episode of global contraction following the end of the late heavy bombardment (Strom et al., 1975). Contractional features are found all over Mercury, whereas extensional features such as graben are rare and appear to be largely confined to the interior of impact basins (Watters et al., 2009). The graben within Rachmaninoff are likely the result of basin floor uplift. The confinement of the graben to the inner plains suggests a relationship between the volcanic activity and the floor uplift (Prockter et al., 2010).
Whereas pre-MESSENGER interpretations suggested that volcanism on Mercury ended early in the planet’s history, MESSENGER’s images of Rachmaninoff reveal that some volcanism extended well beyond that time, probably into the second half of Solar System history. This surprising discovery is likely to be matched by many others once MESSENGER enters into orbit around the innermost planet.
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Strom, R.G., Trask, N.J., Guest, J.E., 1975. Tectonism and volcanism on Mercury. J. Geophys (Solid Earth). Res. 80, No. 17, pp. 2478-2507.
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