Post by Dr. Paul K. Byrne, Carnegie Institution of Washington, USA
Channel-like landforms termed “valles” (sing. “vallis”) have been observed on the Moon, Mars, and Venus, and recent results from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission show that the innermost planet hosts its own brand of valles, too. Resembling the broad outflow channels on Mars and Venus, five shallow, linear depressions form a channelized network at high latitudes in Mercury’s northern hemisphere. These valles are situated adjacent to expansive northern volcanic plains that cover some 6% of the planet’s surface, and likely conducted voluminous, low-viscosity lavas from these plains southward.
The vallis in Image 1 is characteristic of Mercury’s broad channels. It features a wide, shallow cross-section, steep walls, a smooth floor relative to the more cratered terrain nearby, and streamlined kipukas—islands of older, high-standing terrain that have been completely surrounded by younger lavas (Image 2). This example is ca. 20 km across and almost 90 km long, and opens out into the 140-km-diameter Kofi basin. The kipukas at the eastern end of the vallis describe a splay-like pattern that probably formed when lavas encountered the topographic barrier of the now-eroded Kofi basin rim. The streamlined kipukas suggest that this channel was shaped, at least in part, by the thermally and mechanically erosive action of lavas.
Geochemical data returned by MESSENGER indicate a high-magnesian, mafic to ultramafic composition for much of Mercury’s lavas, and similar compositions are known to be conducive to lava erosion on other terrestrial planets. Most of Mercury’s broad valles, however, are also radial to the mighty Caloris basin to the southeast—at 1,640 km in diameter, the largest preserved impact structure on the planet (Image 3). Caloris is encircled by radial troughs carved by the ballistic emplacement of large ejecta blocks during the basin’s formation, and so the valles may have started out as impact-sculpted furrows. Under this scenario, the later emplacement of lavas from the northwest filled, shaped, and preserved the landforms we see today.
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