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TABLE 5.1
Physical and Orbital Characteristics of Mercury
Chapter 5

Mercury
Mean Distance From Sun (Earth=1)
        0.387
Period of Revolution
88d
Period of Rotation
59 d
Inclination of Axis
~2°
Equatorial Diameter
4,880 km
Mass (Earth = 1}
0.055
Volume (Earth = 1)
0.06
Density
5.44 g/cm3
Atmosphere (main components)
O, Na, K (thin)
Surface Temperature
100 to 700 K
Surface Gravity (Earth = 1)
0.37
Magnetic Field (Earth = 1)
0.01
Surface Area/Mass
23 x 1011 m2/kg
Known Satellites
0
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Introduction
The primary goal of the Mariner 10 mission was to obtain data about Mercury, a planet that had never before been visited by a spacecraft, Twin television cameras and six other instruments constituted the vehicle's 675-kg scientific payload. They provided new insight into the nature of the planet closest to the Sun and how it fits into the overall picture of the solar system. On March 23, 1974, Mariner 10 began photographing Mercury, and by April 3 it had collected an unprecedented store of scientific data, including more than 2000 high-resolution television pictures. The spacecraft passed within about 725 km of Mercury's surface at the point of closest approach. By a lucky coincidence, Mariner 10 was placed in an orbit around the Sun that returned the spacecraft to Mercury twice more at six-month intervals. Nearly complete photographic coverage of the illuminated half of the planet was obtained, with some photo­graphs showing features as small as 150 m in diameter. Although years of study of these photographs and more thorough investigation by future space probes will be required before a detailed picture of the geology of Mercury can be devised, the images available reveal much about the nature of the small planet closest to the Sun.
Major Concepts
1.  The processes that shaped the surface fea­tures of Mercury were remarkably similar to those that shaped the Moon. The major land­forms are as follows: (a) impact craters and cratered terrains, (b) intercrater plains, (c) multiring basins, and (d) sparsely cratered smooth plains presumably flooded with lavas.
2.  Impact craters range in age from old, highly eroded features to young, rayed craters sur­rounded with haloes of bright ejecta and prominent systems of secondary craters.
3.  There are at least two generations of plains on Mercury, both of which are probably lava flows.
4.  Prominent fault scarps extending across the surface of Mercury are believed to be the result of global contraction that occurred as the planet cooled. Grabens are rare, and large rift valleys have not been observed.
5.  Mercury has a large metallic core, compared to its size, that may be partly molten, so its
internal structure differs significantly from that of the Moon.
6.  Mercury's geologic systems were driven dominantly by thermal energy from within the planet and the infall of meteoritic debris. Apparently the lithosphere is now thick and immobile. Lacking surface fluids, the surface of Mercury has changed little in the last billion years.
7.  A preliminary interpretation of the major events in the history of Mercury includes (a) accretion arid initial differentiation, (b) a period of intense bombardment, (c) the start of crustal shortening, (d) impact of large meteorites to form multiring basins, (e) formation of plains material—presumably by floods of basalt, and (f) subsequent meteorite impact at a much lower frequency. Because we have no rock samples from Mercury that can be radiometrically dated, there is no absolute time scale for Mercury,
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Mercury
119
The Planet Mercury
Several of Mercury's physical features distin­guish it from the other planets, as is summarized in Table 5.1. Icy Pluto, in the outer solar system, is the only principal planet smaller than Mercury, but the solar system contains many objects that are much smaller (the asteroids and planetary satel­lites). Mercury is the planet closest to the Sun, and stored in the compositions of its rocks is much important information about the chemical composi­tion and early differentiation of the inner solar system. This information remains largely un­tapped, but some models of the formation of the solar system suggest that Mercury should be rich in refractory materials. Mercury is also much denser than would be expected by strict analogy with the Moon; it is probably the most iron-rich planetary body. Mercury represents an extreme in another respect as well. Its surface environment is very harsh; with essentially no atmosphere to mod­erate its surface, temperatures may rise to almost 700 K during the day and at night drop to less than 100 K. Some areas near the north and south poles may get little or no sunlight and are permanently frigid. The rotation period is 59 terrestrial days long and a year is 88 days long. This represents a 2:3 ratio, where there are 2 mercurian years to exactly 3 mercurian days. Such coincidence is called spin-orbit coupling and it probably evolved during the early history of the planet as a result of the constant tidal tug of the Sun on the planet.
Because of Mercury's small size and proximity to the Sun, its surface features were almost totally unknown before the Mariner 10 voyage. Now, with images of essentially half its surface, we are able to interpret the major events in the planet's geologic history. We now have important information about the general mode of planetary development be-cause Mercury represents a unique "end-member" with a small size and a presumed refractory element-rich composition. The concepts of plan-etary evolution that were developed earlier for the Moon will be tested here; new principles will be developed that can then be applied to larger, more complex planets such as Mars. .
are immediately obvious from the photo mosaics of Mercury shown in Figure 5.1. Indeed, it is difficult for many nonspecialists to tell the surface features of the Moon apart from Mercury. The mosaics were made from a series of computer-enhanced pictures taken at a distance of approximately 230,000 km and are similar in resolution to telescopic pictures of the Moon.
Several terrain units with distinctive histories, are very extensive (Figure 5.2). They include cra­tered terrains, intercrater plains, the Caloris Basin, and smooth plains.
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Major Geologic Provinces
The pictures of Mercury beamed back to Earth by the Mariner 10 spacecraft show large tracts of heavily cratered terrain and broad areas covered by lightly cratered smooth plains like the lunar maria. These and other similarities with the Moon
Figure 5.1
The cratered surface of Mercury is similar in many respects to that of the Moon. This photomosaic, taken by Mariner 10, the only spacecraft to visit the innermost planet, shows densely cratered terrains, a large multiring impact basin (Caloris Basin), younger smooth plains, and rayed craters. In comparison to other planetary bodies, Mercury has not had an exciting history, but it occupies a special place at one end of the spectrum of planetary types.
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Figure 5.2
The major geologic provinces of Mercury are shown on this map prepared from Mariner 10 photographs. The rims of impact basins more than 200 Ian in diameter are shown with dashed lines. Plains are shaded gray. The ejecta and secondary craters surrounding the major craters are shown by radial lines. Scarps appear as hachured lines.
Cratered Terrains and Intercrater Plains
Craters on Mercury appear similar to their lunar counterparts, but the heavily cratered regions of Mercury have broad areas of gently rolling plains, impact craters, and basins; the lunar highlands are more evenly cratered. This mercurian terrain is called the intercrater plains and is the most wide­spread type of terrain on Mercury. Clusters of impact craters are very common in these areas. Secondary craters are distinct in shape, are relatively shallow, and are aligned in long chains. Their rims are com­monly ill defined and form a linear or grooved fabric. Primary craters of the same size are circular, bowl shaped, and deeper with well-defined sharp rims (compare Figure 5.3 with Figure 5.7). The topo­graphic complexity of these areas is particularly well developed around some of the ancient mercurian basins. In some places, these heavily cratered units are transected by high scarps, somewhat like the lunar wrinkle ridges. Also in places, huge, bright streaks, apparently unrelated to craters, extend for thou­sands of kilometers (Figure 5.1). As on the Moon, this heavily cratered terrain must represent one of the oldest surfaces on the planet in spite of its simpler appearance (caused by the large areas of intercrater plains). Although no absolute ages can be deter­mined for Mercury's features, the heavily cratered
regions probably formed at the same time as the lu­nar highlands and record the same period of intense bombardment. The color of the light reflected from the surface also suggests a similar composition to the feldspar-rich highland crust of the Moon.
The relative ages of the intercrater plains and the oldest, most degraded craters and basins are not firmly established. In some places, the plains clearly overlie ancient craters; in other areas, the craters and their ejecta are obviously younger than the plains. It appears that although most craters are younger than the intercrater plains, a few are older. These relationships could be explained by a major thermal event, in Mercury's early history in which widespread volcanism occurred at the same time as intense bombardment, or possibly even by a thermal event involving planetwide surface "soft­ening." Viscous flow of the surface might rapidly obliterate craters as they, formed. One fact is clear: The intercrater plains represent a significant pe­riod of time in the early history of Mercury, a time during which many of the earlier impact structures were erased and the planet was resurfaced.
Caloris Basin
The huge multiring Caloris Basin, 1300 km in diameter, dominates much of the photographed
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Figure 5.3
Intercrater plains are the most widespread terrains on Mercury. They consist of smooth to gently rolling plains with a high population of craters less than 15 km in diameter. Many form chains or clusters suggestive of secondary origin. Plains occur between and around areas with larger impact structures that form the densely cratered terrain. It is believed that the intercrater plains are of volcanic origin, although there is insufficient evidence to make the conclusion certain.
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 area of Mercury (Figure 5.4). Liike the lunar basins, it was created by the impact of an asteroid-sized object early in Mercury's development. Caloris is half again larger than Imbrium Basin on the Moon but is very similar to it in general form. The perimeter of Caloris Basin is defined by a rugged. ridge rising 2 km above the floor (Figure 5.5). A subdued outer scarp has a diameter of about 1450 km (Figure 5.6). Although it is discontinuous, this outer ring appears to separate an inner zone of hilly or blocky ejecta deposits from an outer, distinctly lineated zone. These lineated areas consist of a well-developed system of valleys and ridges and are similar in many ways to the ejecta that sur­round the Imbrium basin. Both the lunar and mercurian terrains were probably formed in the same way, by erosion and deposition of ejecta thrown from the crater. The lineated terrain extends be­yond the rim to a distance roughly equal to the diameter of the basin and is there buried or em­bayed by extensive smooth plains that completely surround the eastern half of the basin. The lineated terrain is best expressed to the northeast of Caloris Basin and is possibly the most rugged topography on Mercury. An extensive field of secondary crater chains, clusters, and gouges has been mapped beyond the lineated ejecta.
Younger smooth plains material covers the ba­sin floor inside the main scarp that forms the rim (Figure 5.6). If Caloris has inner rings like lunar basins, they are buried beneath the fill. The Caloris plains are extensively ridged and fractured and are unique among the planets—similar features have not been found in other basins of Mercury, the Moon, Mars, or the satellites of the outer planets. Morphologically, the ridges resemble wrinkle ridges on the lunar maria but are much higher and have a polygonal pattern from the intersection of crudely radial and concentric networks. The frac­tures range in width up to 9 km, appear to be flat floored and grabenlike, and cut the older ridges. The width of the fractures increases toward the center of the basin. There are few arcuate trends that could be interpreted as flooded craters on the floor of the basin (ghost craters) and that are common in some of the lunar basins. This relation­ship suggests that the basin was covered with plains material soon after it was formed and, unlike the Imbrium Basin on the Moon, was not modified by impact before filling.
The impact of the body that formed Caloris was so great that the effects were apparently felt on the opposite side of the planet as well. A peculiar terrain of hills and linear valleys (Figure 5.7) occu­pies a region more than 500 km across, centered on the exact opposite side of the planet from Caloris. Perhaps the intercrater plains and pre-existing
crater rims were broken up by focused seismic waves originating from the impact site.; At a point opposite the basin, vertical movements of several kilometers may have been caused by this event. Smooth plains have partially buried this terrain and are therefore younger.
Smooth Plains
Another major geologic unit of Mercury consists of scattered areas of smooth plains that resemble the
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Figure 5.4
The Caloris Basin, with its concentric rings and radial ejecta, is the largest impact structure viewed by Mariner 10. It is 1300 km in diameter and in many ways is comparable with the Orientale Basin on the Moon. Unlike other multiring basins the floor is rilled with smooth plains and is highly ridged and fractured. Both ridges and fractures display a radial and concentric pattern. The impact that created Caloris Basin was a key event in Mercury's history because it modified the landscape over an enormous area and probably led to the formation of an unusual hilly terrain on the opposite side of Mercury, as a result of focusing of seismic waves in that area.
Mercury  123
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Figure 5.5
The rim of Caloris Basin is marked by an annulus of hilly or knobby topography and linear ridges formed by ejecta. Two rings are visible in this view. Caloris Basin appears to have formed near the end of the period of intense bombardment on Mercury.
Figure 5.6
The floor of Caloris Basin is covered with smoother plains material, probably of volcanic origin. It is unique in that it is deformed by a complex system of ridges and fractures that form a polygonal pattern. The fractures transect the ridges, indicating that they are younger. The largest crater is about 10 km in diameter.
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(A) This region is broken into valleys and hills up to 2 km high that are interspersed with smooth plains. Similar terrains have been found on the Moon's antipode to Imbrium and Oriental Basins. The large crater to the left is Petrarch.
Ejecta
(B) It is believed that this terrain is the result of focused seismic waves caused by the impact that formed Caloris Basin. From The New Solar System.
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Figure 5.7
The hilly and lineated terrain located on the antipode of Caloris Basin is one of the most peculiar areas viewed on Mercury.
Mercury        125
lunar maria. This type of terrain covers about 15 percent of the photographed portion of the planet and is distinct in that it is very smooth and only sparsely cratered. Impact structures larger than 10 km in diameter are rarely developed on this geo­logic unit. The smooth plains are quite level and often fill major depressions. Marelike wrinkle ridges are common. The largest area of smooth plains lies in and around Caloris Basin (Figure 5.5). Numerous other small patches in and around other large craters are scattered across the planet (Fig­ures 5.8 and 5.9). A concentration of smooth plains lies in the northern hemisphere, possibly reflecting a global asymmetry similar to the distribution of maria on the Moon. Although the smooth plains resemble the lunar maria, they lack a strong con­trast in brightness with the surrounding terrains, so their boundaries are not always clear and distinct. Stratigraphic relations between the smooth plains and other terrain types are clear, however, and indicate that the smooth plains are the young­est major terrain on the surface of Mercury. This conclusion is, of course, supported by its sparse crater population. In addition, the frequency of superposed small craters is approximately the same wherever the smooth plains occur, indicating that most of the terrain is about the same age. However, this is also true for the lunar maria, which were erupted as  lava flows over a 1-billion-year time span. Throughout large areas, the plains material is probably relatively thin, much thinner than the mare basalts that fill the lunar basins. This conclusion is based on the fact that parts of the rims of numerous craters protrude through the cover of plains material so that on a regional scale the cover appears to be incomplete and discontinu­ous (Figure 5.9).
Suggestions for the origin of the mercurian smooth plains include their formation by ballistic erosion and deposition of ejecta associated with the formation of major impact basins, notably Caloris. As impact-energized debris surged away from Caloris, it may have ponded in depressions, creat­ing smooth plains in the same way that the lunar highland plains were formed (like those that fill Ptolemaus, Figure 4.18). In addition, the plains are light colored like the plains in the lunar highlands and unlike the basaltic lavas seen on other planets. However, most of the plains are younger than Caloris, the youngest known impact basin large enough to create plains of ejecta. Small patches of smooth plains within craters may also arise by mass wasting from the walls.
On the other hand, many geologists believe that the smooth plains were formed by the extrusive outpouring of lava much like that which formed
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Figure 5.8
Smooth plains on Mercury closely resemble the lunar maria but lack a strong color contrast with their older surroundings. Their surfaces are only sparsely cratered and are commonly deformed by ridges. Most geologists believe that the smooth plains were formed by the extru­sion of basaltic lava.
the lunar maria, although evidence for this is not conclusive. Evidence favoring a volcanic rather than impact origin for the smooth plains includes (1) the large volume of material that accumulated to form smooth surfaces, (2) differences in the
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Figure 5.9
Crater rims protruding above the smooth plains indicate that on a regional basis the smooth plains material is relatively thin and discontinuous.
volume of plains material in craters and basins of the same size, (3) the striking similarity in morphology and distribution of the smooth plains to the lunar maria, (4) the age differences between the smooth plains and the basins they occupy, and (5) the spectrum of light reflected from the surface suggests that basalt is present. The major obstacle to accepting a volcanic origin is that there are no obvious associated volcanic features (vents, flow fronts, or sinuous rilles). This is possibly due to the fact that even on the best Mariner 10 photographs, details of lava domes or thin flow fronts cannot be resolved. However, some small hills that dot the surface of the plains may be low, shield-type volcanoes. If volcanic eruptions formed the smooth plains, the process of extrusion must have been similar to that which produced the lunar maria—quiet fissure eruptions of fluid basaltic magma that ponded in depres­sions, covering most of the vents through which lava rose to the surface.
The ultimate origin of the mercurian smooth plains will not be known until we have samples of rocks to examine. In the meantime, it is probably safest to conclude that the plains of Mercury were produced by several of the processes described above.
Impact Craters and Basins
The dominant landforms on Mercury are craters of all sizes and states of degradation (Figure 5.10). These range in size from small pits at the very limit of resolution (about 1 km in diameter) to large multi-tiring basins like Caloris, 1300 km across. In many ways, these impact features are similar to those found on the Moon. The craters represent a wide range in age—from older, highly degraded depressions to young, fresh craters surrounded by halos of bright ejecta and extensive ray systems. However, close ex­amination reveals that the mercurian craters differ from lunar craters in several important aspects.
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Figure 5.10
Craters on Mercury are similar to those on the Moon. They range in size from less than 100 meters (highest resolution obtained by Mariner 10) to large basins over 1000 km in diameter. As shown in this image, small craters are simple and bowl shaped, but with increasing size, craters develop central peaks, terraced inner walls, peak rings, ejecta deposits with radial structures, and swarms of secondary craters.
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Impact Cratering and Gravity
Perhaps the major physical differences between the Moon and Mercury that would influence crater morphology are their diameters and masses. Mer­cury is much larger and more dense and therefore has a surface gravity about twice lunar gravity. Af­ter the initial studies of the Mariner 10 photos, it was thought that Mercury's fresh craters were sub­stantially shallower than lunar craters with the same diameters. The differences were attributed to Mercury's greater gravitational pull. It was also , thought that the progression of changes in crater morphology (for example, the transition from simple to complex craters) occurred at smaller diameters on Mercury. Although controversy continues, nei-ther of these early conclusions has been completely borne out by later studies, and significant differ­ences (within about 10 percent) in depth-to-diameter ratios or morphologic relations do not ap­pear to exist. Apparently, the effect of the planet's gravitational field is not as important as many other factors. Small mercurian craters are simple and bowl shaped. With increasing size, terraces on the crater walls becomes apparent and central peaks develop, then irregular clusters of peaks appear. The largest impact features are basins with inner rings (Figure 5.10), just as on the Moon.
However, other gravity-induced differences for impact craters appear to be real. The extent of the ejecta blanket and secondary craters around a pri­mary crater is systematically smaller for a given crater size on Mercury than on the Moon (Figures 5.11 and 5.12). The fields of secondary craters ap­pear to be better preserved on Mercury as well. The distance traveled by material ejected from a crater is due in part to the higher gravitational attraction of Mercury as compared to the Moon, which pulls these ejected objects down to the surface faster and produces shorter travel distances, thus explaining the more pronounced clustering of secondary cra­ters. The ejecta blanket surrounding a mercurian crater must also be thicker, as it is spread over a smaller area and will have an increased ability to degrade or bury nearby craters. The zone of sec­ondary craters is often marked by long linear grooves, which usually radiate from the center of the crater (Figure 5.12). These grooves are pro­duced by the impact of closely spaced ejecta frag­ments, occur much closer to the crater, and are more pronounced than their lunar counterparts. Mercu­ry's higher gravitational pull may likewise give ejected blocks higher velocities and produce larger, more prominent secondary craters when the blocks hit the surface. Although the resultant feature is slightly different, the cratering process is funda­mentally the same on both planets.
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Figure 5.11
Secondary craters on Mercury are relatively small but are well preserved. As shown in this image, secondary craters appear in linear chains radiating from the point of major impact. Note also the terraced inner rim, central peak, and ejecta blanket of the large crater, which is similar to the morphology of equivalent-size craters on the Moon.
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Mercury
Moon
Figure 5.12
Secondary impact craters are commonly elliptical and in some areas form elongate grooves and ridges that impart a "wormy" texture to the surface. They appear to be preserved better than their lunar counterparts, perhaps because they were formed by ejecta with higher velocities and are therefore deeper. Because Mercury's gravity is stronger than the Moon's, impact ejecta travels only half as far on Mercury for an impact of similar size.
Multiring Basins
Large multiring basins, similar to those on the Moon, are also found on Mercury. The most com­mon are relatively small, ranging from 200 to 600 km in diameter (Figure 5.13). These craters usually have two well-preserved rings and an ejecta blan­ket with numerous secondary craters. The largest
impact structure photographed by Mariner 10 is the Caloris Basin discussed earlier.
Two important observations have been made regarding the impact basins on Mercury. First, the intercrater plains are not saturated with small craters; second, there is a similar lack of large impact basins on Mercury. Even accounting for the
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the one in Figure 5.14, are very shallow and show clear evidence of advanced isostatic compensation. This has led some to believe that Mercury's crust cooled more slowly than the Moon's, remained plastic longer, and was able to adjust rapidly to erase all signs of impact, just as thick mud oozes to remove signs of disturbance. A third alternative to explain the small number of mercurian basins was alluded to in the description of the intercrater plains; These plains appear to have formed during the early bombardment, and their emplacement may have destroyed many older basins.
Crater Degradation
As on the Moon, the dominant erosional processes on Mercury are caused by cratering. Degradational
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Figure 5.13
Small ringed basins on Mercury represent a transition from large craters with clusters of central peaks to large multiring basins over 1000 km in diameter.
part of Mercury that has not been photographed, almost twice as many basins over 400 km in diam­eter have been found on the much smaller Moon. These observations can be explained by several competing hypotheses. For example, there are sev­eral indications that the numbers of meteorites passing through all areas of the solar system were not the same. Some scientists have concluded that the lack of large basins on Mercury indicates that fewer meteorites were available to impact Mercury than the Moon. This may explain why many old surfaces are not saturated with craters and why secondary craters and other small features have not been eroded by subsequent impact. Another way to explain this apparent lack of large basins centers on an observation regarding the state of isostatic adjustment of old basins. Many basins, like
Figure 5.14
Shallow craters on Mercury show evidence of ad­vanced isostatic adjustment, possibly because Mercury's lithosphere was warm and plastic during the period of bombardment. The secondary crater field around the double-ring crater Ma Chin-Yuan (170 km across) is none­theless well preserved. Arrows show crater rim.
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131
sequences have been established for mercurian craters that show the morphologic changes with increasing age. The freshest craters have well-defined rims, hummocky ejecta blankets, and sys­tems of bright rays comparable to Copernicus and Tycho on the Moon. Numerous rayed craters from 1 to 50 km in diameter dot the surface (Figure 5.1)., Subsequent bombardment breaks down the crater rim and churns up the ejecta blanket or completely buries it beneath other ejecta deposits. Ultimately, the crater is transformed into a low-rimmed de­pression with large numbers of younger, super-posed impact features. Many of the original crater features become completely obliterated or barely recognizable (Figure 5.14). Degradation of mercu­rian craters by impact from secondary fragments does not extend as far from the primary crater as on the Moon because of shorter  ballistic ranges. In spite of the shorter range, cratering processes active over billions of years have produced a thick layer of soil or regolith on the surface. Optical and radar measurements made from Earth indicate that it is similar in composition and physical prop­erties to the lunar regolith.
In summary,, mercurian impact features differ from lunar craters and basins in three important ways. First, the ejecta thrown out of mercurian craters does not appear to have traveled as far as on the Moon. Considering the larger strength of the mercurian gravitational field (almost twice the Moon's) this appears to be logical. Second, even the densely cratered terrain of,the surface is not satu­rated with craters. The population of projectiles may have been smaller at Mercury's orbit or basins formed early may have been removed by some process. Third, many of the ancient mercurian basins are very shallow and ill defined.
ridges found on the Moon. These characteristics seem to indicate that the faulting or flexing oc­curred as the result of compression in the mercu­rian crust. Thrust faults in which one block of rock is pushed or thrust over another seem to have been produced. Thrust faults usually have shallow dips as compared to normal faults. The scarps transect the older intercrater plains and craters, whereas younger craters and portions of the smooth plains cross the scarps. These cross-cutting relations indi­cate that the scarps began forming sometime near the final phase of the heavy bombardment and continued developing after some smooth plains were formed.
An important characteristic of the scarps is their global distribution. Maps prepared from Mari­ner 10 photos show that the scarps extend from pole to pole over most of the visible surface (Figure 5.2) and trend in a more or less north-south direc­tion. The relatively uniform global distribution of the scarps suggests that the entire planet was subjected to compressive forces that resulted in crustal shortening after the early period of differ­entiation and intense bombardment.
There are several ways in which this global deformation may have occurred. As mentioned ear­lier, Mercury's rotational period has probably changed substantially over the course of geologic time as it evolved toward the 3 days per 2 years stable relationship. This despinning or slowdown may have induced substantial changes in the shape of the planet, creating compressive forces in the surface layers hear the equator and causing the flexures and faulting observed in the form of the scarps. An important difficulty with this model is that it predicts extension in the polar regions, but no evidence of extension in the form of grabens has yet been found. Another method for planetwide compression is suggested by calculations of Mercu­ry's probable thermal history, which predict that substantial contraction occurred as it cooled after differentiation. Cooling of a large metallic core or cooling of silicates in the lithosphere and conse­quent contraction is adequate to explain many of these features. A change in the radius by only 2 km (0.1 percent) is sufficient to cause crustal compres­sion of approximately the same magnitude as that observed on the surface.
The smooth plains inside Caloris Basin display features obviously produced by structural deforma­tion including the ridges and grabenlike fractures. Both the ridges, with a compressive origin, and the fractures, formed by tensional stresses, have simi­lar radial and concentric patterns (Figure 5.6) and were most likely caused by minor vertical move­ments of the interior of the circular basin. Initial subsidence of the basin as it filled with smooth
Tectonic Features                    :
Even though some of its surface features are quite similar to those on the Moon, Mercury appears to have been subjected to a style of tectonic defor­mation not found on the Moon or other terrestrial planets. Evidence of extensional stress is found only in small areas near Caloris Basin. On Mercury, the dominant tectonic features are a series of lobate escarpments or scarps—steep, clifflike slopes. These are often more than 1 km high and hundreds of kilometers long. The general nature of these scarps is shown in Figure 5.15. They may be irregu­lar, arcuate, or 16bate in outline and generally have rounded crests that greatly differ from the sharp crests and straight ridges formed by vertical faults and graben margins on the Moon. Mercurian scarps are similar to but larger than the wrinkle
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Figure 5.15
Fault scarps on Mercury transect and offset craters, clearly indicating crustal deforma­tion. The large scarps are probably thrust faults, some of which are 2 Ion high and 500 km long. They were probably caused by crustal shortening associated with cooling and contracting of the planet after the period of intense bombardment. Heating and planetary expansion cause extension and grabens to occur at the surface.
plains material probably formed the ridges. Subse­quently, it appears that the floor was uplifted and fractured. In most cases, the fractures cut the ridges and are therefore younger, which is consis­tent with this model. An explanation for the uplift is not in hand. However they were formed, these structural features were most likely produced by relatively local stress concentration similar to the type of tectonics operative on the Moon that pro-
duced the linear rilles and wrinkle ridges and probably ate riot closely related to global processes like those that produced the global scarp system.
The Internal Structure of Mercury
From the results of the Mariner 10 mission and other Earth-based studies, the diameter (4,880 km)
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and the density (5.44 g/cm3) of Mercury are well known. Data returned from Mariner 10 also demonstrate the presence of a significant magnetic field, with a strength of less than 1 percent of Earth's at the surface. These few facts and a knowledge of the surface history help scientists to determine the type of internal structure Mercury possesses. The high density implies that the planet has a very dense interior, most likely as a result of high concentrations of iron. Using mathematical models to determine how the interior of Mercury may have evolved, scientists think that the core may be around 3500 km in diameter and that it developed in the first billion years of Mercury's history. This large core occupies about 75 percent of the radius (Figure 5.16). If these calculations are correct, Mercury's iron core occupies the largest fraction of any planetary volume.
The discovery of Mercury's magnetic field came as a surprise and its origin is still unknown, but the most likely explanation is that the outer portion of the core is still molten. Convective movements within this electrically conductive liquid zone may create a magnetic dynamo that produces the mag­netic field. Heat released from a crystallizing inner core may drive convection. A pure iron core should have solidified long ago. Perhaps the core in Mercury is iron sulfide, which has a lower melting point than pure iron. A less likely possibility is that the core is at this time entirely solid, but a remanent or inherited field is present. An earlier molten core
may have established a magnetic field that perma­nently magnetized a relatively thin surface layer several hundred kilometers thick. Orbiting space­craft may be able to determine which of these two possibilities is the case.
The rigid outer shell of Mercury, consisting of a mantle and crust, may be called the lithosphere. It is probably around 500 to 600 km thick and consists of iron and magnesium silicates (Figure 5.16). Spec­tral data obtained from Earth indicate that the crust may be quite similar to the lunar highlands, with an impact regolith of anorthositic composition. This is also consistent with the bright nature of the mercurian crust and may indicate that the early crust developed by crystallization of a magma ocean like that of the Moon.
Mercury possesses a very tenuous atmosphere consisting of oxygen, potassium, and sodium vapor. The pressure of the atmosphere is not sufficient to support wind-related processes. This discovery suggests that Mercury is not totally devoid of moderately volatile elements like sodium, but then neither is the volatile-poor Moon. Potassium and sodium may come from feldspars in the crust. The only other body in the solar system with a similar sodium atmosphere is lo, which develops its atmo­sphere by active volcanism. Another puzzling ob­servation regarding volatiles on Mercury comes from recent radar observations of Mercury. With the use of radio telescope dishes, radio beams can be bounced off Mercury; maps of the brightness of the reflected energy show a very bright area at the north pole. Although other materials could be re­sponsible, a smooth polar cap of water ice would have similar characteristics. Is it possible that Mercury, the closest planet to the Sun, has polar caps of water ice? Some areas near the poles may be permanently in shadow arid never warm up, providing a cold sink for the accumulation of vola- tiles. But where did the water come from? The other information we have about Mercury tells us that it is depleted in volatile materials like water.
In summary, the presence of a magnetic field and the high density of the planet indicate that Mercury is differentiated, with a large iron core that is probably still molten, unlike the Moon's, and a silicate crust and mantle like the Moon's. Much more remains to be discovered about the structure and composition of Mercury. Such data should be eagerly sought because of Mercury's unique posi­tion in the solar system.
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Figure 5.16
The internal structure of Mercury is postulated on the basis of its size, density, composition, and surface fea­tures. The best models predict an iron core 3600 km in diameter, containing 80 percent of the planet's mass. The overlying silicate rock layers are probably differentiated into a mantle and a crust.
Geologic Evolution of Mercury
The geologic history of a planet depends on many factors, including its size (mass and radius) and its
134 Chapter 5
chemical composition (determined by its position in the solar nebula). As it ages, each planet passes through three general stages: (1) a highly active period of crustal formation and mobility; (2) a volcanic stage accompanying a thickening sub-crustal lithosphere; and (3) a terminal quiescent state when the lithosphere is too thick to allow magma to puncture it or to move laterally. The rate at which a planet evolves through these steps depends on how quickly it cools, which in turn depends on the planet's size and composition. In this sense, Mercury provides geologists with an important reference marker: (1) It is the planet closest to the Sun and thus it may have an "ex­treme" chemical composition dominated by high temperature, refractory elements, and a lack of more volatile materials like water. (2) Mercury is larger than the Moon and should have evolved at a slightly different tempo. (3) Although smaller in radius, Mercury has nearly the same mass and surface gravity as Mars and almost the same bulk density as Earth, thereby showing how differences in these qualities affect a planet's subsequent development.
Only part of the surface of Mercury has been photographed, but geologists, utilizing the same methods and techniques as those used to study the Moon, have been able to establish a preliminary geologic time scale and develop a working hypoth­esis for the geologic evolution of Mercury. The large impact basins, such as Caloris Basin, like the Imbrium basin on the Moon, provide a useful refer­ence for the major geologic events. It is clear from superposition and from crater frequencies that a period of intense bombardment occurred before the formation of the Caloris Basin, and the volcanic (smooth plains) material was emplaced afterward, followed by minor cratering. These periods of time have been given formal names taken from promi­nent craters of various ages. Prom oldest to young­est they are pre-Tolstojan, Tolstojan, Calorian, Mansurian, and Kuiperian. Comparisons of the cra­ter densities on these terrains with those on the Moon can be used to estimate the absolute ages of these stages: pre-Tolstojan (4.6 to 4.0 billion years ago), Tolstojan (4.0 to 3.9 billion years), Calorian (3.9 to 3.5 or 3.0 billion years), Mansurian (3.5 or 3.0 to 1.0 billion years). Careful study of the figures in this chapter will reveal these fundamental relative age relationships. Figures 5.17, .5,18, and 5.19 show schematically how the interior and surface of Mer­cury may have evolved.
Stage I: Accretion and Differentiation.
Mercury probably accreted from materials that condensed at high temperature from the nebula
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Figure 5,17
A graphic representation of Mercury's thermal history shows that a massive core must have formed early during the period of accretionary heating. At the same time much of the mantle probably melted. Rapid cooling later produced a thick rigid lithosphere and resulted in contraction of the planet.
that gave birth to the Sun and the rest of the planets. Where Mercury formed, only refractory elements were condensed as minerals and much of the iron was metallic—not in less-dense silicates minerals. Thus, the high density of Mercury could be explained by a large proportion of metallic iron. Moreover, Mercury is apparently water-poor. Silicate condensates that contain water, a volatile substance, formed farther from the Sun, apparently in the vicinity of the asteroid belt. Ices condensed only in the outer solar system.
Heat deposited in Mercury by accretionary impacts, and radioactive decay drove the internal differentiation of the planet. By analogy with the Moon, much of the outer part of Mercury probably began to melt soon after its formation about 4.5 billion years ago. Light silicate minerals eventually crystallized and formed the crust. Denser silicates' accumulated as the mantle. For a planet with the size and density of Mercury, this silicate shell (the crust and mantle) could only be 600 to 700 km thick.
Mobile crustal plate interactions may have been limited to this early period of crustal forma­tion. A rigid lithosphere must have developed well before the end of heavy bombardment because craters that formed during this episode are pre­served. As heat was radiated from the planet into space, the depth of the molten zone increased and the rigid lithosphere formed above it.
During this epoch the core was formed, as a "rain" of metallic droplets concentrated in the cen­ter of the planet. This redistribution of mass from its initially more homogeneous state provided more
Mercury
135
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Stage I. Accretion and differentiation resulted in the formation of a planet with a large iron core. The molten core and silicate mantle caused global expansion and tensional fracturing in a thin, solid lithosphere.
Stage II. Period of intense bombardment and forma­tion of intercrater plains.
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Stage III. Excavation of Caloris Basin and formation of the associated hilly and lineated terrain on the opposite side of the planet. Convection in the mantle had already allowed the planet to cool sufficiently to cause global contraction, resulting in compressive stress, and thrust faulting at the surface.
Stage IV. Formation of the smooth plains, probably
from volcanic extrusions.
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Stage V. Cooling and contraction are completed and
the planet became tectonically inactive as the lithosphere thickened. The only process to modify the surface signifi­cantly is the occasional impact of meteorites, which create rayed craters.
Figure .5.18
The geologic history of Mercury.
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Chapter 5
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Stage I. Accretion shaped the ancient surface of Mer­cury which was dominated by large multiring basins and large tracts of heavily cratered terrain.
Stage II. Heavy bombardment and the emplacement of intercrater plains, probably as lava flows, buried many of the earlier features.
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Stage III. Formation of Caloris Basin, late in the period of heavy bombardment, modified the surface of the planet over a very large area.
Stage IV. Emplacement of smooth plains continued after formation of Caloris Basin during a period of declin­ing impact rates that extended to the present.
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Stage V. The present surface of Mercury is modified only by occasional impact craters. Cooling and contraction formed scarps.
Figure 5.19
The surface of Mercury has changed dramatically over the course of its history as illustrated in this sequence of diagrams.
Mercury
137
heat that may have aided in forming magmas that extruded on the surface, as the intercrater plains during the early heavy bombardment and enhanced isostatic adjustment of craters. This, difference may be one of the most important Mercury-Moon contrasts, explaining why inter­crater plains are more expansive on Mercury. Even if core formation occurred on the Moon, the amount of energy released would have been small by comparison with Mercury.
Theoretical models of condensation and geo­logic evidence suggest that Mercury never con­tained abundant volatiles and probably never outgassed an atmosphere or hydrosphere as the, interior differentiated. Mercury has sufficient gravity to retain an atmosphere at least as thick as that of Mars, but its envelope of sodium vapor pales by comparison. Nor is evidence of a past eolian regime visible in Mariner 10 photos. Subse­quent research may determine if an atmosphere ever formed by release from the interior and, if so, may explain what happened to it.
Another hypothesis that might explain Mercu­ry's high density and lack of a thick atmosphere appeals to the possibility of a large impact during this stage of its early evolution. A large, perhaps Moon-sized body may have collided with Mercury after core formation. If the impactor was large enough (20 percent of Mercury's mass), it may have stripped away the outer shell of less-dense sili­cates, leaving Mercury smaller and richer in dense iron. The mass of precatastrophe Mercury could have been twice as large before impact. Therefore, the high density of Mercury could reflect its accre­tion history and not necessarily a high condensa­tion temperature. Such a large collision may have purged the volatiles from Mercury's outer portions, making the formation of an atmosphere less likely. If the giant collision scenario holds true for Mer­cury, as it appears to for the Moon, condensation of solids in a thermal gradient around the ancient Sun may not be required to deplete volatile elements.
highlands; based on crater statistics and strati-graphic relations on both planets, they appear to be volcanic rocks emplaced during the late stages of the heavy bombardment. The mercurian inter- crater plains may be more voluminous than their lunar counterparts because core formation in Mer­cury produced much more melting during this early period than was possible for the Moon with its small core. There is no evidence preserved of plan­etary expansion, which presumably would have accompanied this thermal event. Subsequently, the lithosphere cooled, thickened downward, and in time contracted. Calculations of this development show that Mercury's radius may have decreased by 2 km. This contraction may have decreased the surface area and caused global thrust faulting, producing the scarps and ridges so characteristic of the mercurian surface. Many contractional scarps appear to have formed before Caloris Basin formed.
Stage III: Formation of Caloris Basin (Early Calorian Period). The formation of the large multiring Caloris Basin was a major event in the geologic development of Mercury. Ejecta from this basin extends more than 1000 km away from the rim. The excavation of the basin modified the landscape over much of the photo­graphed surface, forming large ejecta deposits and radial ridges and valleys far beyond the outer ring of mountains. Hilly and lineated terrain on the opposite side of the planet probably formed as a result of this tremendous impact. There may be other large multiring basins not seen on Mariner 10 photographs. If a lunar analogy can be drawn, Caloris probably formed about 4 billion years ago.
Stage IV: Formation of Smooth Plains (Middle to Late Calorian Period). The
smooth plains material that fills Caloris Basin and parts of the surrounding areas represents flooding of the earlier basins at a time when the heavy bombardment had greatly decreased. (This material is probably of volcanic origin and was extruded over a period of time, but small variations in crater densities between different areas imply that the period during which flooding occurred was rela­tively short. The smooth plains were emplaced as the final product of the volcanic stage of Mercury's evolution, probably by 2 or 3 billion years ago, shortly after the decline in the cratering rate. Even though the lithosphere was thickening, magmas were apparently still able to reach the surface. Mercury's lack of water (which would have lowered the melting points of many of its component materials, allowing them to stay liquid over a longer
Stage II: Heavy Bombardment and Formation of Intercrater Plains (Pre-Tolstojan and Tolstojan). A period of in­tense bombardment is recorded by the clusters of densely packed large craters and basins. The oldest surfaces on Mercury do not have as many large craters as those on the Moon, and it appears that periods of heavy bombardment on Mercury oc­curred during the emplacement of the intercrater plains (Figure 5.19). The material that forms the intercrater plains could be volcanic, or it may be basin ejecta. Similar deposits occur in the lunar
138 Chapter 5
period of time) may have increased the rate of lithospheric thickening relative to Earth; and the timing of the volcanic events may approximately coincide with similar events on the Moon.
Structural modification of the smooth plains in the Caloris Basin produced large ridges and open fractures that may be related to isostatic adjust­ment of the basin's interior. Smooth plains else­where are wrinkled by lobate scarps formed as Mercury continued to contract or are undeformed.
Stage V: Light Cratering (Mansurian and Kuiperian Periods). After the period of smooth plains formation, the surface of Mercury was subjected to light cratering, which formed the bright-rayed craters. The density, distribution, and morphology of these craters resembles the post-mare cratering on the Moon with slightly degraded but still relatively fresh craters formed during the Mansurian Period and rayed craters formed during the Kuiperian Period.
The absence of subsequent modification of the surface of Mercury by tectonism, volcanic activity,  or atmospheric processes is significant because it indicates that after the period of basin flooding, the geochemical and tectonic evolution of Mercury was essentially completed. The extrusion of the, plains material was apparently the end of Mercury's dy­namic history. Mercury's lithosphere may be rigid all the way to its core, with no intervening astheno­sphere. Nonetheless, Mercury appears to have re­mained warm enough to maintain a convecting iron core. There is no, observable deformation of the outer silicate shell of Mercury such as would arise from recent movement caused by the postulated fluid core. Although the unexplored 50 percent of Mercury's surface could reveal evidence of recent internal activity, it is very likely that the only processes available to modify Mercury after the end of its . final period of volcanic activity are degradation of slopes by gravity-driven mass movement and the occasional impact of objects ranging from small meteorites through micromete-orites and cosmic particles.
the solar system. The largest impact structure photographed on Mercury is the multiring Caloris Basin, similar in form, and, probably in age, to the Moon's Imbrium Basin. This large basin is younger than a heavily cratered terrain (similar in many ways to the lunar highlands) that contains inter­spersed smoother plains. Still younger plains fill the Caloris cavity and are found scattered across the rest of the photographed part of the planet. Both generations of plains were probably produced by lava flows--an, indication of the importance of volcanism in the development of the planets. These terrains are transected by distinctly mercurian scarps that appear to be thrust faults created as the planet cooled and contracted.
The most significant differences between the Moon and Mercury are the result of Mercury's larger size and enrichment in iron. Impact crater ejecta are distributed closer to the craters than on the Moon. Perhaps more important, Mercury ap­pears to have cooled more slowly so that plains-• producing volcanic activity during the period of intense bombardment was more long-lived and per­haps more vigorous than on the Moon. Moreover, the interior must be relatively hot to this day because Mercury has a magnetic field that is thought to be generated by convection within a molten metallic core. If the  Moon has a metallic core, it solidified completely billions of years ago. Mercury's iron-rich composition and large core may be the consequence of condensation and accretion of its constituents near the forming Sun. The ab­sence of a significant atmosphere or any surface fluids on Mercury was predetermined by its con­ception in this part of the solar system.
The geology of Mercury reinforces the notion that the tectonic and volcanic activity on a planet , depend on the thermal state of the interior (the temperature distribution at depth). Since most planets were initially quite hot as a result of their accretion, much of their thermal history is domi­nated by cooling. Small planets, like Mercury, with large surface-area mass ratios, cool rapidly and have short thermal histories. Mercury, with a ratio higher than Mars and lower than the Moon, may have had a thermal history intermediate to these planets.
In short, the history of Mercury produced a Moonlike planet whose development was modified in pace and tenor by the distinctive properties of this, the innermost of the planets.
Conclusions
The cratered surface of Mercury is strikingly similar to that of the Moon and attests to the impor­tance of meteorite impact-as a general process in
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139
Review Questions
1.  Compare and contrast the surface of Mercury with the surface of the Moon.
2.  In what ways do the impact craters on Mercury differ from those found on the Moon? Why do they differ?
3.  Is it possible to determine the absolute ages of surfaces and features on Mercury?
4.  Why do surface temperatures on Mercury range from very hot to very cold?
5.  Why is spin-orbit coupling a common phenomena in the solar system?
6.  What composition of volcanic rocks would you expect to find at the surface of Mercury? Why?
7.  How do the plains on Mercury differ from the lunar maria? What was their probable mode of origin and age?
8.  What is the principal evidence that Mercury experi­enced global contraction during its history? When did this happen—early or late? Why did Mercury contract rather than expand?
9. How does the interior of Mercury differ from the interior of the Moon? Is there any evidence that the interior is still molten?
10.  Mercury formed very near the early Sun. What is the evidence that Mercury is rich in refractory elements as a result? Are there other processes that could explain its iron-rich composition?
11.  Outline the major events in the history of Mercury and compare them to the major events in the Moon's history.
12.  Your job is to make recommendations for a manned mission to Mercury. Where should the space ship land to obtain the most information about Mercury? What tasks should the astronauts perform? What instruments should they take with them? What should they bring back with them? Assume the astronauts have a small "rover" and will be on the planet for two weeks.
Key Terms
Antipode
Despinning
Scarps
Spin-Orbit Coupling
Additional Reading
Davies, M. E. et al. 1978. Atlas of Mercury, NASA
SP-423.

Journal .of Geophysical Research. 1975. Vol. 80, No. 17. (This entire issue is devoted to analysis of data returned from Mercury by Mariner 10.)

Murray, B.C. 1975. "Mercury." The Solar System. New York: W. H. Freeman and Co., pp. 37-48.
Strom, R. G. 1984. "Mercury." The Geology of the Terres­trial Planets, NASA SP-469, pp. 13-55.

 

Strom, R. G. 1987. Mercury: The Elusive Planet. Wash­ington, DC: Smithsonian Institution Press.