From: Johns Hopkins University
Posted: Sunday, January 20, 2008
MESSENGER Views an Intriguing Crater
MESSENGER's Narrow Angle Camera (NAC) on the Mercury Dual Imaging System (MDIS) acquired this view of Mercury's surface illuminated obliquely from the right by the Sun. The unnamed crater (52 kilometers, or 31 miles, in diameter) in the center of the image displays a telephone-shaped collapse feature on its floor. Such a collapse feature, not seen on the floors of other craters in this image, could reflect past volcanic activity at and just below the surface of this particular crater.
MESSENGER team members are examining closely the more than 1,200 images returned from this flyby for other surface features that can provide clues to the geological history of the innermost planet.
The crater is located in the southern hemisphere of Mercury, on the side that was not viewed by Mariner 10 during any of its three flybys in 1974 and 1975. This scene was imaged while MESSENGER was departing from Mercury from a distance of about 19,300 kilometers (12,000 miles), about one hour after the spacecraft's closest encounter with Mercury. The image is of a region approximately 236 kilometers (147 miles) across, and craters as small as 1.6 kilometers (1 mile) can be seen.
Ridges and Cliffs on Mercury's Surface
A complex history of geological evolution is recorded in this frame from the Narrow Angle Camera (NAC), part of the MDIS instrument, taken during MESSENGER's close flyby of Mercury on January 14, 2008. Part of an old, large crater occupies most of the lower left portion of the frame. An arrangement of ridges and cliffs in the shape of a "Y" crosses the crater's floor. The shadows defining the ridges are cast on the floor of the crater by the Sun shining from the right, indicating a descending stair-step of plains.
The main, right-hand branch of the "Y" crosses the crater floor, the crater rim, and continues off the top edge of the picture; it appears to be a classic "lobate scarp" (irregularly shaped cliff) common in all areas of Mercury imaged so far. These lobate scarps were formed during a period when Mercury's crust was contracting as the planet cooled. In contrast, the branch of the Y to the left ends at the crater rim and is restricted to the floor of the crater. Both it and the lighter-colored ridge that extends downward from it resemble "wrinkle ridges" that are common on the large volcanic plains, or "maria," on the Moon.
The MESSENGER science team is studying what features like these reveal about the interior cooling history of Mercury.
Ghostly remnants of a few craters are seen on the right side of this image, possibly indicating that once-pristine, bowl-shaped craters (like those on the large crater's floor) have been subsequently flooded by volcanism or some other plains-forming process.
This image was taken 18 minutes after close approach, when MESSENGER was about 5,000 kilometers (about 3,000 miles) away from Mercury. The image is about 200 kilometers (about 125 miles) across, and features as small as about 400 meters (about 400 yards) can be resolved.
Additional information and features from MESSENGER's first flyby of Mercury are online at http://messenger.jhuapl.edu/mer_flyby1.html.
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