Northpole of Mercury, where captured comets may have deposited water-ice in sufficient dark cover to preserve it, as is suspected to be the case on our own moon as well. The bright pockets are thought to be water-ice deposits. Image width is approximately 450 kilometers on a side with a resolution of 1.5 kilometers (1 mile). Credit: Arecibo Radar
The Mercury mission, MESSENGER, completed its third trajectory correction maneuver since launch - and its last of 2004 - trimming its speed and tweaking its course toward the Earth flyby next August.
The 48-second burst from MESSENGER's hydrazine-fueled thrusters reduced the spacecraft's velocity by just over 7 miles per hour (3.2 meters per second) relative to the Sun - easing it into a cruising speed of about 62,030 miles (99,827 kilometers) per hour. The maneuver started at ; mission operators at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, began tracking it about two minutes later, when the first signals indicating thruster activity reached the NASA Deep Space Network tracking station near Madrid, Spain.
MESSENGER, now nearly 22.8 million miles (36.7 million kilometers) from Earth, is in good health and operating normally. Detailed checkouts of the science instruments and subsystems continue. The solar-powered spacecraft continues to fly with its sunshade away from the sun, allowing it to keep its key systems warm without using power for heaters. Since launch last August 3, MESSENGER's computers have executed more than 15,000 commands from mission control.
"We're not like Cassini, going out where it's nice and cool," said Robert Strom, MESSENGER co-Investigator from the University of Arizona in Tucson, . "Orbiters of Mars and Jupiter and Saturn, they're kind of orbiting paradise compared to MESSENGER, which is going to orbit hell. But it's a very interesting hell."
Because Mercury is so close to the sun, its surface temperature can exceed 450 degrees Celsius (840 degrees Fahrenheit). But Mercury is not uniformly hot, because the thin atmosphere does not transfer heat from the equator to the poles. Temperatures on the dark side of the planet can drop to -185 degrees C (-300 degrees F).
MESSENGER carries seven scientific instruments that will provide images of the entire planet, as well as information on the composition of Mercury's crust, core, and polar materials, its geologic history, and the nature of its thin atmosphere and active magnetosphere.
Mariner 10 images of Mercury, the innermost planet, revealed a heavily cratered landscape similar to the Moon, but also revealed prominent topographic scarps like the skin of a dried apple. Credit: NASA
"The mystery is why has tiny Mercury retained a magnetic field, when larger planets in the inner solar system -- Mars and Venus -- do not have a global magnetic field today," says Solomon.
Mercury is about the size of our moon, and extremely dense. The planet's density is so high that two-thirds of the planet is believed to be iron metal. For this reason, Mercury's surface gravity is about the same as the surface gravity of Mars.
Scientists aren't sure why Mercury is so dense. Perhaps there was a gradient in chemistry of the solar disk of gas and dust that formed the planets, with more metal closer to the sun. Solomon says that if Mercury's high iron composition is due to a solar nebula gradient, then the silicate material at Mercury's surface should have the major elements in approximately solar proportions.
Another possibility is that Mercury's composition was Earth-like to start, but the heat from the sun caused Mercury to lose its rocky material. If so, then the surface will have relatively less of the more easily vaporized materials.
When Mariner 10 flew by Mercury in 1974 and 1975, it took pictures of less than half of the planet's surface. Still, these images allowed scientists to see Mercury's surface close-up, revealing a moon-like landscape dotted with craters.
Mariner images also showed large-scale faulting on the planet's surface. Scarps scattered randomly over the hemisphere are a result of the crust buckling, and suggest that the planet has shrunk from its original size.
"(That) seems a little crazy at first, but when you think about it, as the core cooled, when it goes from a liquid to a solid there's going to be a volume change," says Mark Robinson, MESSENGER investigator from Northwestern University. "If the crust had already formed when that happened, as the planet shrank, that tension had to be taken up somewhere, and buckling of the crust made these large scarps."