Caloris Basin and Surrounding Region

The ramparts of the Caloris impact basin span a diameter of about 1,550 km (960 miles). (Estimates of its size from the part of Caloris seen by Mariner 10 were considerably smaller.) 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; 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, which was named Raditladi.

Mercury's Caloris impact basin, as seen in a mosaic of images captured by Mariner 10 during its three flybys. Only the eastern half of the structure is visible; it appears as partial concentric rings stretching from top to bottom (left portion of photo) within relatively smooth plains. The western half of Caloris was on the nightside of the planet during the Mariner encounters. NASA/JPL

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

A double-ringed crater on Mercury filled with plains material, in an image taken by the Messenger probe on Jan. 14, 2008. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington and by volcanic ash are examples of this mechanism.

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

A double-ringed crater on Mercury filled with plains material, in an image taken by the Messenger probe on Jan. 14, 2008. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington 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 at least 3 km (2 miles).

A scarp on Mercury, as seen by the Messenger probe on Jan. 14,2008. The scarp (upper left) curves downward, ending in the large impact crater at the bottom. The region shown is about 200 km (120 miles) across. NASA/Johns Hopkins University Applied Physics Laboratory/ Carnegie Institution of Washington

A scarp on Mercury, as seen by the Messenger probe on Jan. 14,2008. The scarp (upper left) curves downward, ending in the large impact crater at the bottom. The region shown is about 200 km (120 miles) across. NASA/Johns Hopkins University Applied Physics Laboratory/ Carnegie Institution of Washington

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 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, the contraction of Mercury estimated from cooling of its mantle should have produced even more com-pressional features on its surface than have been seen, which suggests that the planet has not finished shrinking.

Surface Composition

Scientists have attempted to deduce the makeup of Mercury's surface from studies of the sunlight reflected from different regions. One of the differences noted between Mercury and the Moon, beyond the fact that Mercury is on average somewhat darker than the Moon, is that the range of surface brightnesses is narrower on Mercury. For example, the Moon's maria—the smooth plains visible as large dark patches to the unaided eye—are much darker than its cratered highlands, whereas Mercury's plains are at most only slightly darker than its cratered terrains. Colour differences across Mercury are also less pronounced than on the Moon, although Messenger images taken through a set of colour filters have revealed some small patches, many associated with volcanic vents, that are quite colourful. These attributes of Mercury, as i v

Part of the surface of Mercury, in a composite image formed from data collected by Mariner 10 during its first flyby in March 1974. Kuiper is the prominent impact crater in the lower right of the image. NASA/JPL/Northwestern University well as the relatively featureless visible and near-infrared spectrum of its reflected sunlight, suggest that the planet's surface is lacking in iron- and titanium-rich silicate minerals, which are darker in colour, compared with the lunar maria. In particular, Mercury's rocks may be low in oxidized iron (FeO). This leads to speculation that the planet was formed in conditions much more reducing—i.e., those in which oxygen was scarce—than other terrestrial planets.

Determination of the composition of Mercury's surface from such remote-sensing data involving reflected sunlight and the spectrum of Mercury's emitted thermal radiation is fraught with difficulties. For instance, strong radiation from the nearby Sun modifies the optical properties of mineral grains on Mercury's surface, rendering straightforward interpretations difficult. However, Messenger is equipped with several instruments, which were not aboard Mariner 10, that can measure chemical and mineral compositions directly. These instruments need to observe Mercury for long periods of time while the spacecraft remains near Mercury, so there can be no definitive results from Messenger's three early and brief flybys of the planet.

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