Because Mercury is so much nearer to the Sun, it receives ten times as much solar energy per unit surface area when it is at perihelion than does the Moon. Because of the coupling between Mercury’s orbital motion and rotation the meridians at 0° and 180° longitude receive two and a half times more radiation overall than do longitudes 90° and 270° Mercury’s rotation axis is almost perpendicular to its orbit and there are no-seasons as on the Earth or Mars, but there is instead a seasonal variation with longitude.

Photographs of Mercury have only been obtained for longitudes between 10° and 190° (all longitudes are measured westwards), yet this hemisphere is clearly divided into two by the varying nature of the surface. Between longitudes 10° and about 100° the surface could easily be mistaken for the Moon. One significant difference, though, is the presence of conspicuous smooth areas, or plains, in the highlands, something not seen in the generally heavily-cratered lunar highlands. There are also many fewer craters between 20 and 50km in diameter on Mercury than on the Moon. One reason for these differences is the higher gravity on Mercury which confines the material ejected in an impact to an area only one sixth of the size covered by a similar impact on the Moon. It is therefore likely
that recent impacts on Mercury have not obliterated earlier features to the same extent as on the Moon.

Another important difference is the large number of shallowly scalloped cliffs, called LOBATE SCARPS that run for hundreds of kilometers across Mercury. They are not present on the Moon or Mars and suggest that the interior of Mercury has shrunk so that the sur¬face became wrinkled like an old apple. This shrinking may have occurred as the large iron core slowly cooled and contracted. The very large craters on Mercury are well preserved and are probably three or four billion years old, indicating that there has been no migration of plates on the planet since that time.

The other half of the observed surface, between longitudes 100° and 190° shows large areas of smooth plains. These must be younger than the heavily-cratered areas or they too would be covered with impact craters. Like the lunar maria, they appear to be lava flows. A particularly notable feature of this part of Mercury is Caloris Basin, 1400km in diameter, which is entirely filled with smooth plains material and must have been formed by an impact com¬parable to the one that produced Mare Imbrium on the Moon.

The surface history of Mercury was probably very similar to that of the Moon, despite the quite different interiors. After formation of the planet, there must have been a planet-wide obliteration of surface features to give a smooth surface which we can still see today in the smooth plains between the craters. This was followed by a heavy bombardment that produced the craters. Towards the end of this phase there was the impact that gouged out the Caloris Basin and, possibly, other giant basins were similarly formed in the so-far unphotographed hemisphere. After Caloris Basin was formed widespread outflows of lava filled this basin and other low ground to give the large smooth maria-like plains. This was the last active phase of the surface history and it has been followed by a quiescent period lasting to the present. The smooth ground is marked only by a light peppering of impact craters, many of which show conspicuous rays. Remarkably, the distribution of these on Mercury, the Moon and Mars shows that over the last three billion years or so all three of these planets have been subjected to the same total amount of bombardment.

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