The most common topographic features on Mercury are the craters that cover much of its surface. Although lunarlike in general appearance, Mercurian craters show interesting differences when studied in detail.
Mercury’s surface gravity is more than twice that of the Moon, partly because of the great density of the planet’s huge iron-sulfur core. The higher gravity tends to keep material ejected from a crater from traveling as far—only 65 percent of the distance that would be reached on the Moon. This may be one factor that contributes to the prominence on Mercury of secondary craters—those craters made by impact of the ejected material, as distinct from primary craters formed directly by asteroid or comet impacts. The higher gravity also means that the complex forms and structures characteristic of larger craters—central peaks, slumped crater walls, and flattened floors—occur in smaller craters on Mercury (minimum diameters of about 10 km [6 miles]) than on the Moon (about 19 km [12 miles]). Craters smaller than these minimums have simple bowl shapes. Mercury’s craters also show differences from those on Mars, although the two planets have comparable surface gravities. Fresh craters tend to be deeper on Mercury than craters of the same size on Mars; this may be because of a lower content of volatile materials in the Mercurian crust or higher impact velocities on Mercury (since the velocity of an object in solar orbit increases with its nearness to the Sun).
Craters on Mercury larger than about 100 km (60 miles) in diameter begin to show features indicative of a transition to the “bull’s-eye” form that is the hallmark of the largest impact basins. These latter structures, called multiring basins and measuring 300 km (200 miles) or more across, are products of the most-energetic impacts. Several dozen multiring basins were tentatively recognized on the imaged portion of Mercury; Messenger images and laser altimetry contributed greatly to the understanding of these remnant scars from early asteroidal bombardment of Mercury.
Caloris
Basin and surrounding region
The ramparts of the Caloris impact basin span a diameter of about 1,550 km (960 miles). Its interior is occupied by smooth plains that are extensively ridged and fractured in a prominent radial and concentric pattern. The largest ridges are a few hundred kilometres long, about 3 km (2 miles) wide, and less than 300 metres (1,000 feet) high. More than 200 fractures that are comparable to the ridges in size radiate from the centre of Caloris; many are depressions bounded by faults (grabens). Where grabens cross ridges, they usually cut through them, implying that the grabens formed later than the ridges.
Two types of terrain surround Caloris—the basin rim and the basin ejecta terrains. The rim consists of a ring of irregular mountain blocks approaching 3 km (2 miles) in height, the highest mountains yet seen on Mercury, bounded on the interior by a relatively steep slope, or escarpment. A second, much smaller escarpment ring stands about 100–150 km (60–90 miles) beyond the first. Smooth plains occupy the depressions between mountain blocks. Beyond the outer escarpment is a zone of linear, radial ridges and valleys that are partially filled by plains, some with numerous knobs and hills only a few hundred metres across. The origin of these plains, which form a broad annulus surrounding the basin, has been controversial. Some plains on the Moon were formed primarily by interaction of basin ejecta with the preexisting surface at the time a basin formed, and this may also have been the case on Mercury. But the Messenger results suggest a prominent role for volcanism in forming many of these plains. Not only are they sparsely cratered, compared with the interior plains of Caloris, indicating a protracted period of plains formation in the annulus, but they show other traits more clearly associated with volcanism than could be seen on Mariner 10 images. Decisive evidence of volcanism was provided by Messenger images showing actual volcanic vents, many of which are distributed along the outer edge of Caloris.
Caloris is one of the youngest of the large multiring basins, at least on the observed portion of Mercury. It probably was formed at the same time as the last giant basins on the Moon, about 3.9 billion years ago. Messenger images revealed another, much smaller basin with a prominent interior ring that may have formed much more recently (it was named Raditladi).
The antipodal region
On the other side of the planet, exactly 180° opposite Caloris, is a region of weirdly contorted terrain. It is interpreted to have been formed at the same time as the Caloris impact by the focusing of seismic waves from that event to the antipodal area on Mercury’s surface. Termed hilly and lineated terrain, it is an extensive area of elevations and depressions. The crudely polygonal hills are 5–10 km (3–6 miles) wide and up to 1.5 km (1 mile) high. Preexisting crater rims have been disrupted into hills and fractures by the seismic process that created the terrain. Some of these craters have smooth floors that have not been disrupted, which suggests a later infilling of material.
Plains
Plains—relatively flat or smoothly undulating surfaces—are ubiquitous on Mercury and the other terrestrial planets. They represent a canvas on which other landforms develop. The covering or destruction of a rough topography and the creation of a smoother surface is called resurfacing, and plains are evidence of this process.
There are at least three ways that planets are resurfaced, and all three may have had a role in creating Mercury’s plains. One way, raising the temperature, reduces the strength of the crust and its ability to retain high relief; over millions of years the mountains sink and the crater floors rise. A second way involves the flow of material toward lower elevations under the influence of gravity; the material eventually collects in depressions and fills to higher levels as more volume is added. Flows of lava from the interior behave in this manner. A third way is for fragments of material to be deposited on a surface from above, first mantling and eventually obliterating the rough topography. Blanketing by impact crater ejecta and by volcanic ash are examples of this mechanism.
Some of the evidence tilting toward the volcanism hypothesis for the formation of many of the plains surrounding Caloris has already been described. Other comparatively youthful plains on Mercury, which were especially prominent in regions illuminated by a low Sun during Messenger’s first flyby, show prominent features of volcanism. For example, several older craters appear to have been “filled to the brim” by lava flows, very much like lava-filled craters on the Moon and Mars. However, the widespread intercrater plains on Mercury are more difficult to evaluate. Since they are older, any obvious volcanoes or other volcanic features may have been eroded or otherwise obliterated, making a definitive determination more difficult. Understanding these older plains is important, since they seem to be implicated in erasing a larger fraction of craters 10–30 km (6–20 miles) in diameter on Mercury as compared with the Moon.
Scarps
The most important landforms on Mercury for gaining insight into the planet’s otherwise largely unseen interior workings have been its hundreds of lobate scarps. These cliffs vary from tens to over a thousand kilometres in length and from about 100 metres (330 feet) to 3 km (2 miles) in altitude. Viewed from above, they have curved or scalloped edges, hence the term lobate. It is clear that they were formed from fracturing, or faulting, when one portion of the surface was thrust up and overrode the adjacent terrain. On Earth such thrust faults are limited in extent and result from local horizontal compressive (squeezing) forces in the crust. On Mercury, however, these features range across all of the surface that has been imaged so far, which implies that Mercury’s crust must have contracted globally in the past. From the numbers and geometries of the lobate scarps, it appears that the planet shrank in diameter by as much as 7 km (4 miles).
Moreover, the shrinkage must have continued until comparatively recently in Mercury’s geologic history—that is, since the time Caloris formed—because some lobate scarps have altered the shapes of some fresh-appearing (hence comparatively young) impact craters. The slowing of the planet’s initial high rotation rate by tidal forces (see above Orbital and rotational effects) would have produced compression in Mercury’s equatorial latitudes. The globally distributed lobate scarps, however, suggest another explanation: later cooling of the planet’s mantle, perhaps combined with freezing of part of its once totally molten core, caused the interior to shrink and the cold surface crust to buckle. In fact, many small geologically young scarps have been found, which suggests that the planet has not finished shrinking.
Surface composition
Messenger used X-ray fluorescence spectra to study the surface composition of Mercury. It found a high ratio of magnesium to silicon and low ratios of aluminum and calcium to silicon, which showed that the crust was not rich in feldspar like that of the Moon. The surface is rich in sulfur, about 20 times richer than the surfaces of Earth, the Moon, and Mars. Messenger also found low surface abundances of titanium and iron. Mercury seems to have formed in conditions much more reducing—i.e., those in which oxygen was scarce—than other terrestrial planets.
Origin and evolution
Mercury’s formation
Scientists once thought that Mercury’s richness in iron compared with the other terrestrial planets’ could be explained by its accretion from objects made up of materials derived from the extremely hot inner region of the solar nebula, where only substances with high freezing temperatures could solidify. The more volatile elements and compounds would not have condensed so close to the Sun. Modern theories of the formation of the solar system, however, discount the possibility that an orderly process of accretion led to progressive detailed differences in planetary chemistry with distance from the Sun. Rather, the components of the bodies that accreted into Mercury likely were derived from a wide part of the inner solar system. Indeed, Mercury itself may have formed anywhere from the asteroid belt inward; subsequent gravitational interactions among the many growing protoplanets could have moved Mercury around.
Some planetary scientists have suggested that during Mercury’s early epochs, after it had already differentiated (chemically separated) into a less-dense crust and mantle of silicate rocks and a denser iron-rich core, a giant collision stripped away much of the planet’s outer layers, leaving a body dominated by its core. This event would have been similar to the collision of a Mars-sized object with Earth that is thought to have formed the Moon (see Moon: Origin and evolution).
Nevertheless, such violent, disorderly planetary beginnings would not necessarily have placed the inherently densest planet closest to the Sun. Other processes may have been primarily responsible for Mercury’s high density. Perhaps the materials that eventually formed Mercury experienced a preferential sorting of heavier metallic particles from lighter silicate ones because of aerodynamic drag by the gaseous solar nebula. Perhaps, because of the planet’s nearness to the hot early Sun, its silicates were preferentially vaporized and lost. Each of these scenarios predicts different bulk chemistries for Mercury. In addition, infalling asteroids, meteoroids, and comets and implantation of solar wind particles have been augmenting or modifying the surface and near-surface materials on Mercury for billions of years. Because these materials are the ones most readily analyzed by telescopes and spacecraft, the task of extrapolating backward in time to an understanding of ancient Mercury, and the processes that subsequently shaped it, is formidable.
Later development
Planetary scientists continue to puzzle over the ages of the major geologic and geophysical events that took place on Mercury after its formation. On the one hand, it is tempting to model the planet’s history after that of the Moon, whose chronology has been accurately dated from the rocks returned by the U.S. Apollo manned landings and Soviet Luna robotic missions. By analogy, Mercury would have had a similar history, but one in which the planet cooled off and became geologically inactive shortly after the Caloris impact rather than experiencing persistent volcanism for hundreds of millions of years, as did the Moon. On the presumption that Mercury’s craters were produced by the same populations of remnant planetary building blocks (planetesimals), asteroids, and comets that struck the Moon, most of the craters would have formed before and during an especially intense period of bombardment in the inner solar system, which on the Moon is well documented to have ended about 3.8 billion years ago. Caloris presumably would have formed about that time, representing the final chapter in Mercury’s geologic history, apart from occasional cratering.
On the other hand, there are many indications that Mercury is very much geologically alive even today. Its dipolar field seems to require a core that is still at least partially molten in order to sustain the magnetohydrodynamic dynamo. Indeed, recent measurements of Mercury’s gravitational field by Messenger have been interpreted as proving that at least the outer core is still molten. In addition, as suggested above, Mercury’s scarps show evidence that the planet may not have completed its cooling and shrinking.
There are several approaches to resolving this apparent contradiction between a planet that died geologically before the Moon did and one that is still alive. One hypothesis is that most of Mercury’s craters are younger than those on the Moon, having been formed by impacts from so-called vulcanoids—the name bestowed on a hypothetical remnant population of asteroid-sized objects orbiting the Sun inside Mercury’s orbit—that would have cratered Mercury over the planet’s age. In this case Caloris, the lobate scarps, and other features would be much younger than 3.8 billion years, and Mercury could be viewed as a planet whose surface has only recently become inactive and whose warm interior is still cooling down. No vulcanoids have yet been discovered, however, despite a number of searches for them. Moreover, objects orbiting the Sun so closely and having such high relative velocities could well have been broken up in catastrophic collisions with each other long ago.
A more likely solution to Mercury’s thermal conundrum is that the outer shell of Mercury’s iron core remains molten because of contamination, for instance, with a small proportion of sulfur, which would lower the melting point of the metal, and of radioactive potassium, which would augment production of heat. Also, the planet’s interior may have cooled more slowly than previously calculated as a result of restricted heat transfer. Perhaps the contraction of the planet’s crust, so evident about the time of formation of Caloris, pinched off the volcanic vents that had yielded such prolific volcanism earlier in Mercury’s history. In this scenario, despite present-day Mercury’s lingering internal warmth and churnings, surface activity ceased long ago, with the possible exception of a few thrust faults as the planet continues slowly to contract.
Clark R. Chapman