The smooth plains

The smooth plains are younger than the heavily cratered terrain and mostly occur around the Caloris Basin. They appear to be about the same age as Caloris or perhaps a little younger. Craters are sparse and they closely resemble the Cayley Plains on the Moon. There is no difference in the albedo of the intercrater plains and the smooth plains, the albedo values being 0.15 ± 0.02 for both. In general, there seems to be little variation in albedo across the surface of Mercury.

This lack of strong contrast between the albedos of the heavily cratered terrain and both the plains units is in strong contrast to the differences obvious on the Moon between the rugged light feldspathic highlands and the smooth dark plains of basalt that form the maria. Very large wrinkle ridges occur on the floor of the Caloris Basin that could argue for the presence of lava. However the planet is exposed to an intense solar particle flux, so that caution is warranted in making comparisons with the lunar surface.

The origin of the plains: a Cayley Plains analog?

The debate over the origin of both sets of the mercurian plains parallels that surrounding the origin of the lunar Cayley Plains and bears on the question of whether there are primary and secondary crusts on Mercury. Some authors regard the smooth plains units as analogues of the lunar maria, formed by fluid volcanic lava flows. Others note that both types of the mercurian plains resemble the Cayley Plains that are common in the lunar highlands. These lunar plains were identified, following the Apollo 16 mission, as debris sheets, impact melt sheets or fluidized ejecta flows from major basin-forming collisions, although they were often previously interpreted as ash flows or ignimbrites derived from the eruption of siliceous volcanics.

The large area of the plains units comprises the best evidence for a volcanicorigin, as well as the apparent lack of source basins for an origin of basin ejecta.Thus the intercrater plains are extensive and there appears to be an apparent paucity of multiring basins, which could have supplied ejecta. Such ejecta on Mercury have a more restricted ballistic range than on the Moon, due to the higher gravity of Few visible morphological indicators of volcanism can be recognized on Mercury. Perhaps the most persuasive evidence for volcanism on Mercury is the presence of some darker albedo areas within craters (e.g. Tyagaraja).In addition, recalibration of the Mariner 10 photos indicates that distinctive geological units are present on Mercury. These are interpreted to be consistent with volcanic deposits, thus suggesting that Mercury has had a complex geological history. There is also a doubtful spectral interpretation of basalt. Although wrinkle ridges are present on the smooth plains, they show differences from those on the lunar maria and “in any case, ridges are not necessarily diagnostic of volcanic origin – they may merely indicate deformation of any coherent material ”.

Likewise, in marked contrast to the lunar maria, there is apparently little differ-ence in age between that of the Caloris Basin and the smooth plains that surround it.

There are no post-collision/pre-plains craters in Caloris that are analogous to Archimedes, Iridum or Plato on the Moon that formed on the Imbrium Basin before it was flooded with mare basalt.

The lunar terrain which most closely resembles that of the mercurian intercrater plains is the so-called Pre-Imbrian “ pitted terrain”, southwest of the Nectaris Basin (35– 65° S; 10 –30° E). Their distinction from Cayley Plains is mainly in a higher density of craters. These lunar plains have been suggested to represent an early phase of volcanism as it is difficult to assign them to particular multiringed basins, if they are basin ejecta. However, there seems to be no compelling evidence to interpret this lunar pitted terrain as other than the result of impact-produced debris from basin formation. The lack of identifiable sources may be simply due to the destruction of old basins by new ones, a view consistent with an extended period of basin formation. This is the same problem that is encountered on Mercury. The absence of contrast in albedo between all these units is certainly not consistent at first sight with a basaltic composition for the mercurian plains.

Finally, there is the problem that all analogies with the Moon require caution, as that body has a distinctly lower bulk density and hence a different composition, as well as a different interior structure. Thus the mantle of Mercury may be very different than that of the Moon. Lavas erupted from it may not resemble lunar basalts in albedo. There is accordingly a complex situation with respect to inter-pretations of the mercurian photographs. The Moon provides the only viable analogy, but due to planetary density differences and probable mantle compositional differences, mercurian volcanism, if present, may be sufficiently different to make photogeological interpretations and comparisons difficult.

Further caution is needed in interpreting the mercurian surface. Space weathering processes that affect planetary surfaces are likely to be severe on Mercury. Solar radiation and the flux of solar wind particles and solar energetic particles are an order of magnitude more intense on Mercury than on the Moon, although subject to some shielding by the magnetic field of the planet. Their effects may render conclusions based on our current understanding of reflectance spectroscopy invalid for Mercury. So although we judge that the evi-dence for extensive basaltic style volcanism on Mercury is slender and the inter-crater plains seem more likely to be debris sheets from basin impacts, this whole debate may be yet another example of arguments at the limits of resolution, analogous to the martian canal problem.

Primary and secondary crusts on Mercury?

Can we distinguish between primary or secondary crusts on Mercury? There are two options from the currently limited amount of data. The apparent resemblance between the composition of the mercurian and the lunar highlands crust suggests that the entire crust might be primary. This interprets the various plains units to have originated as debris sheets from basin-forming impacts as we concluded above and that no subsequent melting occurred in the mercurian mantle.

The alternative view is that the heavily cratered terrain represents a primary crust, while the intercrater plains, that cover 45% of the surface photographed by Mariner10, are lavas forming a conventional secondary crust, similar to the lunar example.

Coincident with, or a little younger than the formation of the Caloris Basin, further volcanism produced the smooth plains. The various plains units apparently cover a larger percentage of the mercurian surface than the 17% of the lunar surface covered by maria. Thus if the plains are due to volcanism, extensive partial melting of the mercurian mantle occurred on a shorter timescale than on the smaller Moon. The formation of the plains units ceased around 4 Gyr about the time of the Caloris Basin-forming impact, in contrast to the lunar example where mare volcanism persisted for another billion years. These speculations will hopefully be resolved by Messenger. Because of the inferred violent history of this planet, compositional data from this mission may also test whether refractory lithophile elements have survived in chondritic proportions.