Although Mercury is remarkably similar to the Moon, it is different from it in many respects. This paradox was not unexpected based on observations from Earth predating the Mariner 10 mission. On the one hand it was known that Mercury reflects sunlight and radar waves in the same manner as does the Moon. This similarity combined with the probable absence of any appreciable atmosphere suggested a cratered surface and a lunar-like regolith of pulverized rock mantling the surface of the planet as the result of meteoritic bombardment. On the other hand, the bulk density of the planet was known to be almost the same as that of Earth and about 60 percent greater than that of the Moon, implying that Mercury was a body greatly enriched in the heavy elements and, like Earth, perhaps having an iron-rich core.
The surface of Mercury, like that of the Moon, was indeed found to be pockmarked with impact craters. However, not expected was the discovery that Mercury, unlike the Moon, has a weak but nevertheless Earth-like magnetic field whose origin is undoubtedly related to a large iron-rich core.6 Paradoxically, Mercury has a Moon-like exterior and an Earth-like interior.
The illuminated surface observed by Mariner 10 as it first approached Mercury is dominated by craters and basins. This region of Mercury, included in the Victoria, Kuiper, Discovery, and Bach quadrangles (H-2, H-6, H-11, and H-15), shows a heavily cratered surface that at first glance could be mistaken for the lunar highlands. In marked contrast to this view of Mercury, the surface photographed after the flyby, as the spacecraft receded from Mercury, exhibited features totally different from those shown on the incoming views, including large basins and extensive relatively smooth areas with few craters. This coverage fell in the Borealis, Shakespeare, Beethoven, Tolstoj, and Michelangelo quadrangles (H-1, H-3, H-7, H-8, and H-12). The smooth surfaces are clearly younger than the heavily cratered ground seen in the incoming views of Mercury. The most striking feature in this region of the planet is a huge circular basin, 1300 km in diameter, that was undoubtedly produced from a tremendous impact comparable to the event that formed the Imbrium basin on the Moon. This prominent Mercurian structure in the Shakespeare ( H-3 ) and Tolstoj ( H-8 ) quadrangles, named Caloris Planitia, is filled with material forming a smooth surface or plain that appears similar in many respects to the lunar maria. Mercury, much like the Moon, can thus present two totally different faces; one is a heavily cratered surface like the highlands on the back side of the Moon, and the second shows a region of large basins filled with smooth plains similar to the front side of the Moon.8
Both the heavily cratered regions of Mercury and the craters themselves, however, differ from their lunar counterparts. Mercury's heavily cratered surfaces exhibit relatively smooth areas or plains between the craters and basins, whereas the lunar highlands display closely packed and overlapping craters. In many cases, these "intercrater" plains appear to predate that time when most of the large Mercurian craters were formed.8, 13 The lunar and Mercurian heavily cratered surfaces are probably different because the force of gravity on Mercury is twice that on the Moon.14 The ballistic range of material ejected from a primary crater on Mercury is less than that on the Moon and, consequently, covers, depending on the ejection velocity, an area from a fifth to a twentieth smaller for craters of the same size. As a result, ejecta deposits and secondary craters on Mercury are confined more closely around the primary crater than on the Moon; thus, the early cratering record stored in the surface features of Mercury may be better preserved than on the Moon.14 Ejecta-forming secondaries from the most recent large basin events on the Moon have been superposed on the earlier record of primary craters, increasing the density of craters and obliterating the earlier activity.
The difference in the gravity fields is also probably responsible for the variation in the geometry of craters of the same size on the two bodies. 14 In both cases, the smallest craters are bowl-shaped and with increasing size exhibit central peaks and develop terraces on their inner walls. At the larger sizes, the central peaks become complex structures and undergo a transition into an inner mountain ring that is concentric with the crater rim. Although this progressive change in crater geometry is the same on both the Moon and Mercury, the change from one type to another occurs with smaller diameters on Mercury and apparently reflects gravitationally induced modifications to the original excavation crater.
An additional important difference between the heavily cratered surfaces of Mercury and the Moon are the lobate scarps or cliffs that are several kilometers high and extend for hundreds of kilometers across the Mercurian surface. The scarp named Discovery, by which the H-11 quadrangle is known, is one of the best examples of this feature. Its shape and transection relationships suggest that scarps are thrust faults resulting from compressive stresses, perhaps due to cooling and shrinkage of the iron- rich core, and causing crustal shortening on a global scale.15 Regardless of the mechanism for forming these escarpments, their presence in the large, well preserved craters establishes an approximate relative time scale for their age and eliminates the possibility that planet-wide melting or Earth-like movement of crustal plates has taken place since the heavily cratered ground was created.
The extensive areas of smooth surfaces or plains on Mercury have been classified into three types.13 The most widespread type forms a level to gently rolling ground between and around large craters and basins. These "intercrater" plains are characterized by an extremely high density of superposed small ( 5 to 10 km ) craters, which are frequently elongate, shallow, and suggestive of being of secondary origin. A second type, "hummocky" plains, occurs within a broad ring that is 600 to 800 km wide and circumscribes the Caloris Planitia. These plains consist of low, closely spaced to scattered hills, and have been interpreted.13, 15 to be material ejected during the cratering event that produced the Caloris basin. "Smooth" plains are the third type and form relatively level tracts with a very low population of craters, both within and external to Caloris Planitia as well as in some of the smaller basins (e.g., Borealis Planitia in the Borealis quadrangle). The smooth plains are similar to the lunar maria and, if analogous, result from extensive lava flows that would reflect an extended period of volcanism on Mercury after the Caloris event.15, 16
In addition to the cratered surfaces and plains regions, several other distinctive topographic features occur. A system of linear hills and valleys that extends up to 300 km cut through or modify some parts of the heavily cratered and intercrater areas in the Discovery quadrangle (H-11). These valleys are scalloped and range up to 10 km wide. The best example of this type of feature extends more than 1000 km to the northeast from the mountainous rim, Caloris Montes, in the Shakespeare quadrangle (H-3). Both examples are similar to the so-called lunar Imbrium sculpture. It is generally believed that this type of lineated surface feature resulted from excavations by secondary projectiles when the large basins were formed and, possibly, fracturing and faulting of the planet's crust during the basin formation. The basin associated with the lineations in the Discovery quadrangle is unknown, but it may be found in the darkened hemisphere that was hidden from Mariner 10's cameras.13
Some of the most peculiar and interesting landforms seen on Mercury are in another region in the Discovery quadrangle that has been termed "hilly and lineated." The hills are 5 to 10 km wide and vary from a few hundred meters up to almost 2 km in height. This region included many old degraded craters whose rims have been broken up into hills and valleys. Similar surfaces are known at two sites on the Moon. In all three cases, the regions are antipodal to the youngest large basins ( Imbrium and Orientale on the Moon and Caloris on Mercury). For this reason, there could be a genetic relationship between the formation of the basins and the hilly and lineated terrain. It has been suggested that seismic waves generated by the basin impacts are focused in the antipodal region and are the cause of the peculiar surfaces.17
Well defined bright streaks or ray systems radiating away from craters constitute another distinctive feature of the Mercurian surface, again in remarkable similarity to the Moon. The rays cut across and are superposed on all other surface features, indicating that the source craters are the youngest topographic features on the surface of Mercury.13 The basin and ray systems are shown in Figure 8.
Despite some differences, the striking duplication of surface features between Mercury and the Moon suggests that although an absolute time scale for the development of the Mercurian surface must remain uncertain, the relative sequence of events for the two bodies must have been very similar, if not contemporary. The greatest uncertainty in the Mercurian absolute time scale is: When did the heavy bombardment forming the heavily cratered surfaces (lunar highlands) and the large basins (lunar Imbrium and Orientale) come to an end'?
Within these uncertainties, Mercury's evolution can be divided into five stages or epochs.8, 18 The first epoch includes the interval of time at the earliest stage of the solar system, condensation of the solar nebula into solids, and the accumulation of the solid material into the main mass of Mercury. It is not known whether the planet accumulated heterogeneously or homogeneously; i.e., whether it formed directly as an iron core with a silicate crust, or whether the proto-Mercury was initially a mixture of iron and silicates which subsequently melted and separated into the core and crust configuration. Regardless of how the planet accumulated, all crustal melting must have been completed well before the craters in the heavily cratered surfaces were formed to have preserved their shapes and geometries to the present time. Moreover, if Mercury ever had been enveloped in an atmosphere either during or immediately after accumulation,....
 ....aeolian degradation of craters would have occurred, similar that seen on Mars. Because such degradation has not been recognized, any atmosphere must have disappeared before the oldest cratered surfaces were formed.
The second epoch following accumulation and chemical separation was a period of heavy bombardment by large objects from an unknown source that produced the heavily cratered surfaces and the large basins; this epoch was terminated by the time of the Caloris event. It is not certain whether this last period of heavy bombardment was the terminal phase of the accumulation of Mercury, or whether it was a second episode of bombardment unrelated to the accretionary phase. 19 The "intercrater" plains probably represent an older surface that predates this second epoch,20 or they may have been emplaced during the period of heavy bombardment. Because the lobate scarps are prevalent in the intercrater areas and sometimes pass through and deform some of the older craters, core shrinkage and crustal shortening may have occurred during the end of the first epoch and extended into at least the early part of the second.
A convenient and well delineated point in Mercury's history is the time of the impact that formed the Caloris basin. This massive event marks the onset of the third epoch. It produced the mountainous ring Caloris Montes and the basin Caloris Planitia, as well as the ejecta deposits and sculpturing of the older heavily cratered surface that can be traced more than 1000 km from the ring of mountains. If the Caloris basin were contemporary with Moon's two youngest basins, Imbrium and Orientale, an absolute time for the Caloris event would be about 4 billion years ago.
The start of the fourth epoch followed an indeterminate but probably short, period after the Caloris event. During this time broad plains were formed, most probably as a result of widespread volcanism grossly similar to that which produced the lunar maria. It has been suggested. however, that the smooth plains surrounding the Caloris Planitia (i.e., the Suisei, Odin, and Tir Planitia) are ejecta from Caloris that were melted by the impact.21 If the smooth plains are analogous to lunar maria, this fourth epoch may represent the period of time from 4 to 3 billion years ago. If the plains are impact melt, they must be contemporary with the Caloris event, about 4 billion years in age.
The fifth and final epoch in what can be recognized in Mercurian history probably extends from about 3 billion years ago to the present. Little has happened on Mercury during this period except for a light "dusting" of meteoritic debris which has produced many of the prominent rayed craters. The crater population on the smooth plains is very similar to that on the lunar maria.
The apparent similarity in the sequence of events for the Moon and Mercury is especially significant for interpreting and understanding evolutionary processes of the terrestrial planets. It is now clear that Mercury, in common not only with the Moon, but also with Mars, was subjected to an early, intense crater-producing bombardment ( including basin events ) that was followed by volcanism and, in turn, by a greatly reduced impact flux. Because the orbital distances to the Sun for these three bodies are significantly different, their cratering records suggest that a similar impact history is basic to all terrestrial planets. If this is correct, then an important step has been made in developing a theory of the origin and evolution of the planets. By implication, for example, the Earth in its early history must also have displayed a surface of craters and basins. Thus, from the observations of Mariner 10 there is evolving a new, more complete and unified understanding of our own planet and the solar system in general.