Mercury’s Mighty Valles

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.

Image 1: This vallis has the steep sides, smooth floors, and erosional residuals characteristic of Mercury’s broad valles, and likely channeled lavas from left to right across the image. The image has a field of view of ca. 150 km. The 57°N parallel and 115°E meridian are shown, and Kofi basin is labeled. The image is a portion of MESSENGER’s Mercury Dual Imaging System (MDIS) global monochrome basemap, which has a resolution of 250 meters per pixel.

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.

Image 2: Example of a vegetated Kipuka, Kilauea Volcano, Hawai`i. Image Source: USGS

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.

Image 3: The Caloris basin on Mercury is one of the solar system’s largest impact basins. It formed during the early history of the solar system during the impact of a large asteroid. The basin spans about 1,500 km and is seen in yellowish hues in this enhanced color mosaic captured with the MDIS instrument. Orange splotches around the basin’s perimeter are now thought to be volcanic vents. Image Source: NASA

Further Reading:

Byrne, P. K., et al. (2013), An Assemblage of Lava Flow Features on Mercury. J. Geophys. Res., 117, doi: doi:10.1002/jgre.20052.

Head, J. W., et al. (2011), Flood volcanism in the northern high latitudes of Mercury revealed by MESSENGER. Science, 333, 1853–1856.

Hurwitz, D. M., et al. (2013), Investigating the origin of candidate lava channels on Mercury observed in MESSENGER data: Theory and observations. J. Geophys. Res., 117, doi:10.1029/2012JE004103.

Komatsu, G., et al. (1993), Venusian channels and valleys: Distribution and volcanological implications. Icarus, 102, 1–25.

Leverington, D. W. (2004), Volcanic rilles, streamlined islands, and the origin of outflow channels on Mars, J. Geophys. Res., 109, doi:10.1029/2004JE002311.

Nittler, L. R., et al. (2011), The major-element composition of Mercury’s surface from MESSENGER X-ray spectrometry. Science, 333, 1847–1850.

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