October 13, 1999

Messenger: Mission to Mercury

Click on the images for a larger view and more detailed information.

Image Credts: Mariner 10 mission, Jet Propulsion Laboratory, and NASA.

Mercury is a planet of extremes. It is the closest planet to the Sun. It is the smallest of the terrestrial planets (which, besides Mercury, include Venus, Earth, and Mars), with a diameter of just 0.38 that of Earth. In fact, it is smaller than Ganymede and Titan, which are moons of Jupiter and Saturn. It has the greatest temperature range of any of the terrestrial planets, ranging from -300° F to 800° F (90 K to 700 K).

Mercury is also the least known of all of the planets in the solar system, excepting only Pluto. The only spacecraft to visit Mercury, Mariner 10, surveyed only 45% of its surface during three flybys in 1974 and 1975.

NASA plans to change this paucity of data on Mercury. As part of its Discovery Program, NASA is developing the "Mercury Surface, Space Environment, Geochemistry and Ranging" mission, or Messenger for short. Messenger, which will carry seven science instruments, will image the entire planet and send back data that should help scientists answer many of the questions they have been asking for the past 25 years:

• Why is Mercury so dense? What is the structure of its interior?

  Mercury has the greatest density (5.4 times that of liquid water) of all of the planets after correcting for self-compression by gravity. This suggests that Mercury has a large iron core surrounded by a relatively thin rocky mantle and crust. Mercury's core is estimated to be roughly 75% of the size of the planet. In comparison, the size of the Earth's core is a little more than half of its full size.

• What geological processes produced Mercury's surface features?

  Mercury's surface is the oldest of the terrestrial planets. It is heavily cratered, like the highlands of the Moon; but there are also smooth and less-cratered areas, which may be the result of lava flows, like those of the lunar maria. Mercury also possesses enormous cliffs, some of which are more than a mile in height, extend for hundreds of miles, and cut through craters and other surface features. These cliffs may have been formed by a shrinkage of Mercury, but scientists are not sure.

• Does Mercury harbor water ice in some of its polar craters?

  Radar images of Mercury indicate that the large craters at the planet's poles are highly reflective. The most straightforward explanation is that the reflective material is water ice. Because Mercury's rotation axis is nearly perpendicular to the plane of its orbit around the Sun, the interiors of the polar craters are in permanent shadow and, despite the planet's closeness to the Sun, extremely cold (lower than -300° F). Two of Messenger's instruments will measure whether Mercury's polar craters contain hydrogen, a constituent of water, or whether the reflective material is something else, such as sulfur that over eons has escaped from surface minerals.

• What is the precise composition of Mercury's thin atmosphere? Where does this atmosphere come from?

  Mercury has an extremely thin atmosphere. Scientists know of five constituents of this atmosphere: hydrogen, helium, oxygen, sodium, and potassium. But there are probably other elements as well. Since Mercury is so hot (over most of its surface), the atmospheric gases escape into space and, in contrast to the other terrestrial planets, which have stable atmospheres, must be constantly replenished. Scientists think that this replenishment is hydrogen and helium carried to the planet by the solar wind, gases blasted off Mercury's rocky surface by the solar wind, and material emitted from the planet's interior. Messenger will study the relative importance of these and, possibly, still other sources of atmospheric gases.

• What is the nature of Mercury's magnetic field? Is it dynamic and does it vary like the Earth's?

  Mercury, like Earth, has a global magnetic field (the other terrestrial planets, Venus and Mars, do not; nor does the Moon). Mercury's magnetic field is probably a dipole magnetic field, like the Earth's, with well-defined north and south poles, but it is much weaker than Earth's. Scientists hope that Messenger will help them to better understand Mercury's magnetic field, its strength, its origin, and how it interacts with the solar wind.

Messenger is scheduled for launch in the spring of 2004. To get to fast-moving Mercury with the minimum expenditure of fuel (thus leaving more weight for instruments), the spacecraft will use gravity assists by first looping around Earth, Venus, and Mercury, then entering into a stable orbit around Mercury in the fall of 2009.

More Cool Stuff

For more information on NASA's Messenger, go to the project's homepage at the Johns Hopkins University Applied Physics Laboratory:

http://sd-www.jhuapl.edu/MESSENGER/

Messenger is part of NASA's Discovery Program, whose goal is to conduct a series of frequent, highly focused, cost-effective missions to answer critical questions about the solar system. To learn more about this program and its missions, go to:

http://nssdc.gsfc.nasa.gov/planetary/discovery.html

The Planetary Data System (PDS), which is sponsored by NASA's Office of Space Science, archives and distributes digital data from past and present NASA planetary missions, astronomical observations, and laboratory measurements. The site offers authoritative information on the solar system and many useful links for both the professional and amateur astronomer:

http://pds.jpl.nasa.gov/pds_home.html

As part of its Learning Technologies Project (LTP), NASA supports a number of educational Web sites that have excellent material on the space sciences:

http://observe.ivv.nasa.gov/nasa/education/edu/edudocs/topic_space.html