Surface structure

There are several major landscape forms on Mercury. These are the heavily cratered terrain, the intercrater plains, the Caloris basin and the smooth plains as well as a unique set of lobate fault scarps. These are curving fault scarps formed by compres-sional forces. While the origin of the cratered terrain is not in doubt, con-troversy swirls around the origin of the plains, recalling similar disputes over the formation of the Cayley Plains in the lunar highlands. The plains units on Mercury are of two types. There are both older intercrater plains of Pre-Tolstojan age andyounger “smooth ” plains of Calorian age. Apart from the production of younger impact craters, there is no sign of more recent geological activity. Table 4.1 gives the stratigraphic sequence for Mercury.

The heavily cratered terrain

The oldest portions of the surface of Mercury are heavily cratered and bear some resemblance to the lunar highlands in the numbers of craters and impact basins, over 50 of the latter being recognized.

However, although the planet, like the Moon, has also undergone an early heavy bombardment, there are few craters with diameters less than 50 km, the smaller ones having been covered during the formation of the intercrater plains. An age around 4000 Myr is usually assigned to the mercurian surface on the basis of the lunar highlands analogue and the absence of similar terrains on the Earth. Although the similarity of the cratering record in the inner Solar System suggests a common population of impactors, attempts to establish a chronology based on cratering that would extend throughout the Solar System have not been successful.

The intercrater plains

The intercrater plains are of Pre-Tolstojan age. They occupy about 45% of the mercurian surface that was visible to Mariner 10 and so form a major geological unit.

About 15 old degraded impact basins, such as Eitoku-Milton, appear dimly. They predate the intercrater plains, suggesting that the deposition of the plains obliterated an older heavily cratered surface. The plains contain a large number of craters in the range of 5–16 km diameter. These appear to be mostly secondary craters resulting from the massive bombardment that formed the heavily cratered terrain. If the intercrater plains are volcanic in origin, Mercury must have been producing lavas of low viscosity, representative of a secondary crust. If so, they do not appear to have been iron-rich, as their albedo is twice as high as that of the lunar maria, while the infrared and microwave reflectance spectral data (discussed later) suggest very low surficial abundances of TiO₂ and FeO.

The Caloris Basin: a mercurian cataclysm?

In addition to the old degraded basins and craters that are partly covered by the intercrater plains, a number of fresher and apparently younger impact basins are recognized. The type example is the Caloris Basin, 1550 km in diameter, one of the latest and largest basins on Mercury. This great feature recalls Mare Orientale on the Moon, but is somewhat larger (Orientale is 900 km in diameter). Antipodal to the Caloris Basin is a hilly terrain. This appears to have formed from seismic waves focused from the Caloris impact.Crater-counting techniques give an estimate of 3.77– 3.85 Gyr for the Caloris Basis. These ages, if valid, provide evidence that the massive cratering episodes recorded on the mercurian surface persisted for several hundred million years following accretion of the planet. However definitive answers to these questions require radiometric dating of returned samples, a woefully distant pro-spect. Other evidence suggests that the mercurian cratering record is consistent with the concept of a cataclysm. This conclusion is reached by comparing the distribution of crater sizes on Mercury and the Moon that is related to the relative sizes of the impactors. The post-Caloris craters on Mercury match the distribution of sizes on the lunar maria. The sizes of the older basins and craters on Mercury match those on the lunar highlands. If correct, this would indicate that most of the observed cratering record is due to the cataclysm, and that older ages (> 4.0 Gyr) inferred on the surface of Mercury are incorrect