Mercury

On March 29, 2011, NASA's MESSENGER spacecraft began sending its first close-up images of planet Mercury from orbit (MESSENGER news release and feature release; and Kenneth Chang, New York Times, March 30, 2011). After three flybys, MESSENGER inserted itself into orbit around Sol's innermost planet beginning March 18, 2011, at 12:35 AM UTC (March 17, 2011, at 8:35 PM EDT). No images was send back initially because many instruments were turned off for about a week to protect them from heat damage by the fierce sunlight that irradiates the planet and is re-radiated back out from its surface. The probe settled into an elongated, 12-hour orbit around Mercury that approaches within some 200 kilometers (120 miles) of its surface at the closest point and 15,000 km (around 9,300 miles) at the farthest. The orbit is also moving MESSENGER very close over the planet's north pole, where it will try to detect ice in craters shaded from the Sun which has been suggested by ground-based radar observations. The spacecraft will begin regular observations April 4, 2011 by using seven instruments to investigate the composition of the planet's surface, measure its topology, and mesure the planet's magnetic field (NASA Science News; and Maggie McKee, New Scientist, March 16, 2011).

MESSENGER, JHU/APL, CIW, NASA





Larger and jumbo images.





NASA's MESSENGER imaged Mercury
in crescent exposure by sunlight, as it
orbited Sol's innermost planet on May
2, 2011 (more).

To maintain Mercury's tight inner orbit around the Sun, the planet must move much faster than Earth needs to in its more distant Solar orbit so that a spacecraft must gain about 65,000 miles per hour (105,000 km per hour) to "catch" it. Due to fuel and propulsion restrictions, however, MESSENGER is relying on a complicated trajectory which required several gravitational assists from Venus as well as Mercury in its attempt to become the first Human spacecraft to orbit the planet. Shortly after the first flyby, NASA estimated that MESSENGER broke 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).

MESSENGER, JHU/APL, CIW, NASA


Larger and jumbo illustrations.


NASA's MESSENGER spacecraft
inserted itself into orbit around
Mercury on March 17, 2011, late in
U.S. mainland time zones (more).

Located at only about four tenths of the Earth-Sun distance from Sol, Mercury is the closest planet to the Sun and the second smallest planet. (See an animation of Mercury's orbit around the Sun, with a table of basic orbital and physical characteristics.) Its diameter of 4,880 kilometers (about 3,033 miles) is about 40 percent smaller than Earth's diameter (smaller even than Jupiter's biggest moon Ganymede and Saturn's biggest moon Titan) but also roughly 40 percent larger than Earth's Moon. However, Mercury has only about six percent of Earth's mass.

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.

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 (5.3 versus 3.3 gm/cm³). Based on its high metal content and excluding the effects of gravitational compression, Mercury is the densest major planetary body in the Solar System, with about 1.2 times the uncompressed density of the Earth which averages 4.4 gm/cm³ (Murchie et al, Scientific American, March 2011). Accounting for gravitational compression, however, Mercury is the second densest after Earth, with 98 percent of its "mean" or compressed density (5.43 versus 5.52 gm/cm³, see NASA Planetary Factsheet).

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).

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.