SMOOTH PLAINS FORMATION ON MERCURY

Walter S. Kiefer and Bruce C. Murray, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125

Smooth plains are sparsely cratered regions which cover about 15% of the part of Mercury imaged by Mariner 10. Based primarily on their areal distribution, inferred volume, and stratigraphic relationships with Caloris basin ejecta, a volcanic origin was suggested by the Mariner 10 Imaging Team 12Definitive volcanic features have not been observed, however, leading Wilhelms to suggest that the smooth plains are dominantly ballistically emplaced impact ejecta deposits, analogous to the lunar Cayley Plains (3). This lingering uncertainty prompted us to reexamine Mariner 10 imagery of several smooth plains units in order to better constrain the history and mode of formation of Mercury's smooth plains. Borealis Planitia is a smooth plains unit which completely surrounds the basin Goethe, which is also covered by smooth plains. Borealis Planitia is covered by a number of low ridges, some of which extend to Goethe's rim. These ridges clearly formed after the Goethe impact and apparently once extended onto Goethe's floor. Because these ridges are no longer visible inside Goethe. Goethe's smooth plains must have buried the ridges and therefore must be younger than the ridges. Since the Borealis Planitia smooth plains predate the ridges and the Goethe smooth plains postdate the ridges, at least two separate episodes of smooth plains emplacement must have occurred in this region. The inferred two stage emplacement history, with the later stage restricted to the interior of the Goethe basin, is inconsistent with a ballistic origin but is consistent with a volcanic origin. Evidence also exists for multiple stages of smooth plains emplacement in the Shakespeare and Van Eyck basins. These basins formed prior to the Caloris impact but their smooth plains fill overlies Caloris ejecta and must therefore be younger than Caloris. A low, west facing scarp occurs on Shakespeare's smooth plains and can be traced continuously for almost 100 km. There is no morphological evidence which indicates a tectonic origin for this scarp, so we suggest that it is probably a volcanic flow feature. This indicates that the Shakespeare smooth plains formed as a result of two(or possibly more) volcanic episodes. A similar situation occurs at Van Eyck, where a series of low, west facing scarps may also represent volcanic flows. These scarps only occur in local areas, however, so their interpretation as flow features is less certain than the interpretation of the scarp in Shakespeare. If the volcanic flow interpretation of the Van Eyck scarps is correct, then the Van Eyck smooth plains, like those in Shakespeare, are probably the result of two or more volcanic episodes. Smooth plains also occur inside several craters in the hilly and lineated terrain. Much of the observed dissection of landforms in this area is believed to be due to mass wasting caused by Caloris impact induced seismicity(4), and a portion of the smooth plains in this region may be due to material mass wasted off of crater rims. However, several lines of evidence suggest to us that most of these smooth plains have a volcanic origin. Shadow height measurements of the crater Petrarch's rim indicate that the volume of smooth plains material on Petrarch's floor is too large to have a solely mass wasting origin. Color and albedo contrasts between crater rims and their smooth plains covered floors in this area(5,6) indicate that these smooth plains are probably compositionally different than the surrounding crater rims. This is inconsistent with a mass wasting origin but is consistent with a volcanic origin for the smooth plains.

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System SMOOTH PLAINS FORMATION ON MERCURY Kiefer, W.S. and Murray, B.C.

Based on these observations, we conclude that several discrete episodes of smooth plains formation occurred on Mercury and that at least some smooth plains are volcanic. Watkins has examined cratering statistics for the smooth plains and also concluded that they formed at several different times(7). While we accept Watkins' basic result that the smooth plains are of several different ages, we disagree with several details of his model. Watkins claimed that the Goethe smooth plains are older than the Borealis Planitia smooth plains, whereas we asserted above that the opposite relative age relationship was correct. Because of the small crater counting area in the Goethe basin and the associated large statistical noise level, we believe that our stratigraphically derived relative age relationship is more reliable than Watkins' cratering age. Watkins' data indicates that most smooth plains are post-Caloris in age, but he also reported four areas of smooth plains which he claimed formed prior to the Caloris impact(his SP 1 unit). However, these areas are affected by large numbers of secondary craters and by preexisting craters which were incompletely obliterated by smooth plains formation. Thus, the cratering statistics for these areas are unreliable and it cannot be said with certainty that smooth plains formation occurred on Mercury prior to the Caloris impact. Existing thermal models of Mercury have been constrained primarily by the existence of a magnetic field and by observed limits on planetary volume change(8). The occurrence of multiple volcanic episodes after the Caloris impact provides an important new constraint on thermal models for Mercury.

References

1) Strom et al., J. Geophys. Res. 80, 2478-2507. 1975. 2) Murray et al.. J. Geophys. Res. 80, 2508-2514, 1975. 3) Wilhelms, Icarus 28. 551-558, 1976. 4) Schultz and Gault, The Moon 12, 159-177, 1975. 5) Hapke et al., Proc. 11th Lun. Plan. Sci. Conf, 817-821, 1980. 6) Dzurisin, Ph.D. Thesis, Part 2, Calif. Inst. Technology, 1977. 7) Watkins, NASA TM 81776, 37-39, 1980. 8) Solomon. Phys. Earth and Planetary Interiors 15, 135-145, 1977.

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