Online Astronomy eText: The Planets
Pictures of Mercury
(also see The Rotation of Mercury, The 'Weather' on Mercury, The 'Atmosphere' of Mercury)
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The series of black dots in the upper right hand corner of this image of the Sun is a multiple exposure of the May 7, 2003 transit of Mercury. 23 images were taken, about 15 minutes apart, on a single frame of film, during a period of more than 5 hours. As small as Mercury looks in this image, it is smaller yet in comparison to the Sun, because it was in between us and the Sun during the transit, and being closer, looked larger than it would have at the same distance as the Sun.
(Dominique Dierick, apod030527) |
Click image for larger version of right half (USGS, Mariner 10, NASA)
Composite image of two photomosaics of Mercury taken 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, the Sun is rising or setting, and long shadows throw the craters and other surface features into prominent relief.
The Mariner 10 spacecraft actually 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), exactly the same side of Mercury was facing the Sun each time Mariner 10 flew by, even though Mercury gradually turns every part of its surface to the Sun, every 176 days. (Right image Mariner 10, NASA, apod010819), Left image Mariner 10, NASA, apod020716) |
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 is available. (Mariner 10, Astrogeology Team, U.S. Geological Survey, apod011124) |
Photomosaic of Caloris Basin area (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 from below to well up, 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 not like our Moon, which always keeps one face to the Earth, but 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 and the chaotic area opposite it (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 degrees. (NASA, JPL, Mariner 10, apod960120) |
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 would have served as a lens, focusing the energy of the shock waves on an area directly opposite the basin -- which is exactly where this chaotic terrain is found. |
Typically heavily cratered area. Note extensive flow features on upper right.
 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) |
Heavily cratered area, but with very shallow crater walls,
suggesting that the material below the surface was too soft to support high walls.
The structure of Mercury 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 seemed to imply a relatively cold, presumably solid interior, while the presence of a weak magnetic field seemed to imply an at least partially molten metallic core. Recent studies of the rotation of the planet, and how that reacts 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)
The trajectory of the Mercury Messenger, which will fly by Mercury in 2008 and 2009, and orbit the planet beginning 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 inadvertantly lost in saving this file; and as a result of the lost time and frustration involved in that loss, will be put off until another day.) (Johns Hopkins, Carnegie Institution, NASA) |
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