Caloris Basin, the youngest large impact basin known on Mercury1, has unique qualities that differ it from other areas on Mercury as well as lunar equivalents. Using Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) data researchers have been able to look at this basin in its’ totality1 for the first time, revealing widespread evidence of volcanism and tectonic deformation1.
Images taken in January 2008 by MESSENGER of Caloris Basin show a complex pattern of deformation unlike that found in any other basin in the solar system. High-resolution images indicate evidence of volcanic vents around the inner margin of Caloris basin2. Impact craters show evidence for embayment, Dr. Watters and other researchers are confidant that the interior smooth plains of Caloris are volcanic on origin.
Caloris Basin’s geologic history is different from its lunar counterparts2, the mare basins. In contrast to lunar maria, the interior smooth plains of Caloris are higher in albedo than the underlying basin material2. This correlates to a difference in spectra between the Moon and Mercury basin materials, where lunar basins exhibit high ferrous silicate content and Caloris basin silicates had relatively low amounts of FeO1,2.
Figure 1. Lambert conformal tectonic map of Caloris basin showing graben (black) and wrinkle ridges (red) where graben are found in and around the basin, outnumbering the wrinkle ridges.
In addition to its volcanic modification, the discovery of a radial graben system (Pantheon Possae) near the center of the Caloris Basin was completely unexpected. Whereas dominant tectonic landforms on Mercury are constractional, the Caloris basin has the only widespread extensional features. The pattern of deformation seen in Caloris is very different than in lunar basins. Graben ranging in length from ~5 to ~110 km and in width from less than one to ~8 km crosscut Caloris and crosscut wrinkle ridges in the basin1. The graben are found on both the eastern and western sides of the basin1. The presence of graben about the interior of Caloris basin, shown above in Figure 1, sets it apart from lunar counterparts that have internal wrinkle ridges, but mainly basin exterior graben1. The contrasting graben locations in lunar basins versus Caloris basin represent differing tectonic histories for the two bodies. On the Moon, the dispersal of graben resulted from loading of the lithosphere by dense mare basaltic material, leading to the compression inside basins and extension outside1. For Caloris Basin, researchers such as Dr. Tom Watters believe the graben resulted from the uplift of the interior plains1.
By drawing comparisons with lunar basins, one realizes how unique Caloris Basin’s geological history is. Though the geological features such as embayed craters and graben are not uncommon among the terrestrial planets, it is their location, prevalence, and relation with surrounding features that made and continues to make Caloris a region of intense investigation.
1. Murchie et al. (2008), Geology of the Caloris Basin, mercury: A View from MESSENGER.
2. Head et al. (2008), Volcanism on Mercury: Evidence from the First MESSENGER Flyby.
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