Mercury

During its January 2008 flyby, NASA's MESSENGER spacecraft detected an internal magnetic field consistent with the field from a dipole nearly aligned with the planet’s spin axis (with a dipole tilt around 10°). The MESSENGER observations and data from the two earlier Mariner 10 flybys are consistent enough to describe a dipolar field with an active electrical dynamo in which the magnetic field is produced by electrical currents flowing in an outer core of molten metal (more discussion and images from MESSENGER's January 2008 flyby). More discussion about the magnetic field and Mercury's hypothesized molten or partially molten core can be found below.

MESSENGER, U. Colorado,
JHU/APL, CIW, NASA





Larger illustration.





Depiction of a simulated
magnetosphere illustrates
how Mercury's magnetic
field lines (blue) are
distorted by the Solar
Wind (more).

On January 14, 2008, NASA's MESSENGER spacecraft made its first "flyby" of Mercury, providing the first close-up view of the planet in 33 years (more from MESSENGER and APOD). MESSENGER passed about 124 miles (200 kilometers, or km) above Mercury's night-side surface, which appeared to be heavily cratered. The night-side view included the giant Caloris Basin, including previously unseen western portions. Formed by the impact of a large asteroid or comet, Caloris is one of the largest, and possibly one of the youngest, basins in the Solar System. NASA's new image of the complete basin interior reveals that it is brighter than surrounding regions, suggesting a different composition. Darker smooth plains completely surround Caloris, and unusually dark-rimmed craters are seen inside the basin.

MESSENGER JHU/APL, CIW, NASA




Larger and jumbo images.




This red-light sensitive photograph
provides the first view ever of
Mercury's "backside" from a distance
of 27,000 miles or 27,000 kilometers,
which reveals the full extent of the
Caloris Basin (more).

After two more flybys, the spacecraft should settle into orbit about Sol's innermost planet around March 18, 2011. Shortly after the first flyby, NASA estimated that MESSENGER has broken the known speed record for all spacecraft thus far by achieving a speed of over 140,000 miles per hour (over 225,300 km per hour). The new record is some eight times faster than the speed of NASA's Space Shuttle in low-Earth orbit (more).

NASA (Mercury is smaller than Mars and Jupiter's moon Ganymede, but larger than Earth's Moon)

The closest planet has a very elliptical (or eccentric) orbit and swings from only 46 million km (29 million miles) from the Sun at perihelion out to 70 million km (43.5 million miles) at aphelion. Mercury's orbit around Sol takes only about a fourth of an Earth year (about 88 days) to complete. However, it's daily rotation is much slower than Earth's and takes 59 Earth days to complete.

Because of its small mass and closeness to the Sun, Mercury isn't able to hold on to much of an atmosphere. As a result, temperature variations on Mercury are the most extreme in the solar system ranging from -183°C (-298 °F) to 427 °C (800 °F), although its average surface temperature is 167 °C (333 °F). In contrast, the average surface temperature on Venus is actually hotter at a very stable 464 °C (867 °F) because of its thick atmosphere. Mercury has a very thin atmosphere consisting of surface atoms blasted off its surface by the solar wind. Because Mercury is so hot, however, these atoms quickly escape into space. Thus, in contrast to the Earth and Venus whose atmospheres are stable, Mercury's atmosphere is constantly being replenished.

Courtesy Jet Propulsion Laboratory. Copyright (c) California Institute of Technology, Pasadena, CA. All rights reserved.

From Mercury, the Sun appears two and a half times larger than it does on Earth. The sky would appear black because there's little atmospheric scattering of light. However, one would be able to see two bright "stars," possibly discernible as cream-colored Venus and blue-colored Earth. Although Mercury is not tidally locked to the Sun, its rotational period of almost 59 days is tidally coupled to its orbital period of 88 days, rotating only one and a half times during each orbit in a 3:2 resonance. Hence, a "day" on Mercury (sunrise to sunrise) is 176 Earth days long.

NASA/JPL/Northwestern University

Larger image of Mercury's surface.

This color composite of the terrain around Crater Kuiper at
lower right highlights differences in opaque minerals such as
ilmenite, iron content, and soil maturity (more).

Although Mercury has no known satellites, the planet is similar to Earth's Moon because its surface is heavily cratered and very old, and there is no evidence of plate tectonics. Despite the predominance of cratered areas called "uplands," there are also regions of relatively smooth plains. Some may be the result of ancient volcanic activity but others may be made of debris from meteoric impacts. There is also some unconfirmed evidence of recent volcanism on Mercury, and radar observations of Mercury's north pole show evidence of water ice in the protected shadows of some craters. Unlike the Moon, the surface of Mercury has a slightly pinkish cast, and the planet is much denser than the Moon (5.43 versus 3.34 gm/cm³). In fact, Mercury is the second densest major body in the solar system, after the Earth.

Unlike Earth, Mercury has only a relatively thin silicate mantle and crust, only 500 to 600 km (311 to 373 miles) thick. Inside, however, there is a large iron core whose diameter is 3,600 to 3,800 km (about 2,200 to 2,400 miles). Part of the core is probably molten, but Mercury's weak magnetic field is only one percent of the strength of Earth's. Some astronomers suggest that Mercury lost much of its lighter rocks to the Sun in early catastrophic impacts. Moreover, iron has not been detected on the surface, and so given its presumably large iron core, Mercury may be much more thoroughly differentiated than the other terrestrial planets.