The image shown above was taken on March 29, 2011, and relayed to Earth later that same day. The bright rayed crater near the top is Debussy. Thousands of other craters (older ones looking darker, newer ones brighter and with rays) are also seen in the image. Most of Mercury's surface looks just like this, but as shown in images returned in earlier flybys there are many interesting volcanic and possibly tectonic features visible on other parts of the planet. In the first few hours after entering orbit the spacecraft took several hundred images of Mercury, which were still being downloaded to NASA servers as this note was posted on the morning of March 30, 2011. Later that day NASA planned to have a press session showing some of the most interesting images obtained during the first few hours of MESSENGER's orbit. The spacecraft is designed to last 18 to 24 months in the harsh environment at that distance from the Sun, and should map the entire surface of the planet and study its weak magnetosphere and extremely thin atmosphere and ionosphere in detail before attitude-control gas is exhausted, and/or one or more system failures put an end to its mission. By that time there should be hundreds of thousands of images and thousands of terabytes of data obtained for what had been the least studied of any inner planet. (Note: Although it may not be obvious, this is a color image. Like the Moon, Mercury is an almost completely colorless object, more gray than any other color. Images of Mercury and the Moon which show obvious colors are always false-color images in which the colors are greatly exaggerated, or (as on a map) added specifically to highlight different features.)

 A (false-color) composite image of Mercury taken by the Mercury MESSENGER spacecraft (Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/NASA) as it flew by Mercury on January 14, 2008. This image was taken at a distance of 17,000 miles, about 80 minutes before closest approach, which was only 125 miles above the planet's surface (the close approach was used not only for mapping purposes, but also to slow the spacecraft, to reduce the amount of fuel required to enter an orbit around the planet in 2011). The wavelenths used to create the color image do not correspond to those used for color perception by the human eye, so the actual color of the planet is different from that shown here; but differences in color do represent subtle differences in the actual appearance of the planet.
 MESSENGER images are the first direct photos of the planet in more than 30 years (from 1975 to 2008), and will provide detailed information about all of the planet's surface by the time the mission is over, in the mid-2010's.

 An image of Mercury taken by the MESSENGER spacecraft (Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/NASA) during its October 6, 2008 flyby shows a previously unexplored part of the planet. An extensive system of bright rays from a relatively young crater near the left (Northern) side of Mercury crosses numerous ray patterns extending outward from other craters. In this view much of the visible hemisphere has the Sun high in the sky, so shadows which would emphasize relief are noticeable primarily at the bottom (western) side of the planet, and the numerous craters seen there would not be so easily visible if they were on the more directly lit portions of the planet.

 Two photomosaics show Mercury as Mariner 10 neared it (on the right), and moved away from it (on the left). The Sun is nearly overhead where the two images meet, so the surface appears flat and nearly featureless save for brightness differences. At the sides of the images the Sun is rising or setting and long shadows cast the craters and other surface features into prominent relief.
 The Mariner 10 spacecraft made three flybys of Mercury, in March and September of 1974, and again in March of 1975, but because of the peculiar rotation of Mercury and the time between successive flybys (exactly two Mercurian years), the same side of Mercury was facing the Sun each time Mariner 10 flew by, even though Mercury turns every part of its surface to the Sun every 176 days.
 Mercury's surface is covered with large impact craters, but not nearly as many as seen on the far side of the Moon. This suggests that during the earliest part of the period of heavy bombardment in the inner Solar System, 4.3 to 4.5 billion years ago, the planet was too warm for its surface to maintain crater structures, or was so geologically active that volcanism erased all evidence of that early bombardment. But by the time, 4.0 to 4.2 billion years ago, that impacts were less frequent, the surface was strong enough to maintain crater walls, and volcanism had decreased to the point where it had little effect on the craters' structure. Here and there lava flows are still visible between the craters, as well as large scarps (called rupes) presumably caused by shrinking of the planet's surface as it cooled. But unlike our Moon, there is little evidence of volcanic activity after the end of the era of heavy bombardment, as there are no large smooth plains similar to the lunar maria, which are more or less devoid of craters. (Note added Dec 2014: Although Mercury does not have evidence of the very extensive volcanism that caused the lunar maria, MESSENGER spacecraft studies have shown that considerable vulcanism occurred for some time after the end of the period of heavy bombardment; it is just not as obvious as on the Moon or, for that matter, Mars.)

 A large=scale version of the inbound Mariner 10 composite reveals a heavily cratered surface presumably mostly dating back to the end of the era of heavy bombardment, more than 4 billion years ago.

 A large-scale version of the outbound Mariner 10 composite shows heavily cratered areas, but also large relatively smooth areas (not as smooth as those on the lunar maria, but not as heavily cratered as the inbound image), suggesting substantial volcanic resurfacing around 4 billion years ago. Just below center far left is an especially large impact structure, the Caloris Basin.

 A map of that portion of Mercury which was facing the Sun when Mariner 10 flew by, constructed from Mariner 10 photographs. The smooth blank areas are areas where no information was available. (Mariner 10, Astrogeology Team, U.S. Geological Survey, apod011124) Click on the map for a large-scale version

 Photomosaic of the region near the Caloris Basin (the Basin is at bottom center), a gigantic impact structure bearing a strong resemblance to Mare Orientale on the Moon. Violent shocks associated with the impact that formed the Basin created ringed structures all around it, and the sudden removal of large amounts of surface material allowed molten material to well up from below and fill in the Basin with still obvious lava flows. The large number of radial streaks bears testimony to the huge numbers of rocks, a mile or more in size, thrown outwards in all directions, gouging the surface where thrown horizontally, peppering it in all directions where thrown at higher angles.
 The name of the Basin refers to its position on Mercury. As discussed in The Rotation of Mercury, Mercury has a very peculiar rotation, which is locked to the Sun in such a way that every other orbit one side or the exactly opposite side of Mercury faces the Sun. At perihelion the Caloris Basin or the chaotic area exactly opposite its position on the Mercurian globe (see the image below) are facing the Sun, and are heated to more than 800 Fahrenheit degrees; whereas at aphelion areas exactly a quarter-way around the planet face the Sun, and are only heated to a little less than 600 Fahrenheit degrees.

 Chaotic terrain thought to be caused by severe shaking associated with the formation of the Caloris Basin. As the shock waves from the formation of the Basin passed through Mercury, the planet's relatively large core served as a lens, focusing the energy of the shock waves on an area directly opposite the basin -- exactly where this chaotic terrain is found.

 A 200-mile long scarp (cliff). Such structures are believed to be caused by crustal compression as a result of shrinkage during cooling of the planet after its formation. (NASA/JPL/Northwestern University)

A heavily cratered area with very shallow crater walls, suggesting that the material below the surface was too soft to support high walls.

 Mercury is denser than we would expect it to be if it were a smaller version of the Earth, and is therefore presumed to have a larger core in proportion to its size (perhaps 70 to 80% of the diameter of the planet, compared to 55% for the Earth's core/diameter ratio). Whether the core is liquid or solid is uncertain -- the large numbers of presumably ancient craters covering the surface seems to imply a relatively cold presumably solid interior, while the presence of a weak magnetic field seems to imply an at least partially molten metallic core. Recent studies of the rotation of the planet, and how the surface flexes in response to tidal forces from the Sun support the partially molten core theory; so it is now thought that the core must contain a substantial amount of sulfur, which lowers its melting temperature. (JPL, NASA)

Flow structures on Mercury represent ancient volcanic activity (around 4 billion years ago). Although ancient the features indicate that the planet was geologically active for half a billion years or more after its formation. More recent MESSENGER studies suggest that at least some geological activity may have persisted for considerably longer periods of time. (MESSENGER, NASA, JHU APL, CIW, apod090527)

A double-ringed impact basin on Mercury. Such basins are rare, but are found on almost all cratered surfaces in the Solar System. Theories of their origin are uncertain, but the flow structures and smoothness of the floor of the inner basin suggest that volcanic outpourings associated with the formation of the main crater were in some way connected to the double basin's origin. (MESSENGER, NASA, JHU APL, CIW, apod091007)

 The trajectory of the Mercury MESSENGER, which flew by Mercury in 2008 and 2009, and entered orbit around the planet in 2011. (A long discussion of the complexities of the planetary flybys and gravitational assists required to achieve this goal, which required a considerable amount of time to write, was lost in uploading files; and as a result of the lost time and frustration involved in that loss rewriting it was put off until another day. However, a placeholder (Orbital Perturbations and Gravitational Assists was created as a prod to enlarge on this sooner rather than later.) (Johns Hopkins, Carnegie Institution, NASA).