The Solar System's Innermost Planet Welcomes a Visitor
Mysterious Mercury, the closest planet to the sun and smallest planet in the solar system, is now being orbited by a robotic satellite for the first time in history. NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) spacecraft finally arrived after a "near perfect" (according to mission managers) orbital insertion on March 17, 2011. This was the culmination of an epic six year, 4.9 billion mile, journey that took messenger on a gravitational roller coaster ride through the planets of the inner solar system and three flybys of Mercury, all in an effort to slow the spacecraft down as it fell deep into the sun's gravitational well. Now that MESSENGER is comfortably in orbit, it has opened its eyes and started to photograph the surface of Mercury in glorious detail. Here are the first photographs from Mercury orbit...
A Colorful View
MESSENGER beamed the first ever black and white photograph of Mercury's cratered surface back to Earth on March 29, but that wasn't the only shot. It was actually part of an eight-image sequence taken through MESSENGER's Wide Angle Camera (WAC). In this image of the same view, the red, green and blue filtered images have been superimposed to generate a color photograph. The use of color helps the science team discern variations in composition of Mercury's surface. The large crater in shot is called "Debussy," and to give a sense of scale, the crater measures approximately 50 miles across.
Craters of Interest
An annotated photograph of the region surrounding Debussy crater. Another crater named Matabei can be seen. Strangely, Matabei exhibits dark impact ejecta, or "rays." This is in sharp contrast to the bright rays emanating from the larger Debussy crater. The reasons for these contrasting rays will be explored by the MESSENGER mission.
Crater Close Up
Using its Narrow Angle Camera (NAC), MESSENGER has captured a close up of the bright impact ejecta and secondary craters radiating from the primary impact crater of Debussy. This level of detail of Debussy has never been seen before. As noted by MESSENGER during its second flyby of Mercury in 2008, Debussy's rays extend for hundreds of miles.
The spacecraft also carries the Mercury Laser Altimeter (MLA) that measures the elevation of Mercury surface features. In this plot, the altitude profile of the planet has been plotted for two MESSENGER orbits. As can be seen, surface features vary by around 6 kilometers (3.7 miles) from lowest trough to highest peak. A variety of features have been plotted, including several craters. Both orbital passes have been offset by 3 kilometers for clarity.
One of the biggest puzzles about Mercury is that of its planet-wide magnetic field. Why would such a small planet have such a strong magnetic field when the larger planets Mars and Venus do not? This plot represents the measurements taken by MESSENGER's magnetometer over 10 successive orbits.
North Polar Region
As it made a low pass over Mercury's north polar region, MESSENGER managed to pivot to capture this image of smooth plains punctuated by craters with its Wide Angle Camera (WAC). These deep craters will be of increasing interest to scientists as they investigate the possibility of water ice in their shaded, frozen interiors.
Never Before Seen Terrain
A never before seen region near Mercury's north pole. The image was captured from an altitude of approximately 450 km (280 miles).
The Wide Angle Camera (WAC) can capture images of Mercury's surface through 11 colors. By doing this, the surface composition can be analyzed. For example, some minerals such as olivine and pyroxene often absorb more light at longer wavelengths than at shorter ones, so the MESSENGER team will be keeping a lookout for the absorption signature for minerals such as these.
Sometimes, when not looking directly below, images of Mercury's limb (or horizon) will be captured by MESSENGER's Wide Angle Camera (WAC), reminding us that this little spacecraft is orbiting this little world.