* Astronomy

The smallest planet in the Solar System has become even smaller, studies by the Messenger spacecraft have shown.
Data from a flyby of Mercury in January 2008 show the planet has contracted by more than one mile (1.5km) in diameter over its history.

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Credit: Courtesy of Science/AAAS

Colour image of the Caloris basin and adjacent regions. Orange hues just inside the Caloris basin rim mark the locations of the kidney-shaped depression and related features. The craters on the floor of the basin show several different colours on their rims, indicating different depths of excavation, and some have been embayed, suggesting post-impact volcanic activity. Statistical methods were used to isolate and enhance subtle colour differences using data from the multi-band multispectral images obtained by the MDIS (Mercury Dual Imaging System) instrument.

MESSENGER Settles old Debates and Makes new Discoveries at Mercury
Scientists have argued about the origins of Mercurys smooth plains and the source of its magnetic field for over 30 years. Now, analyses of data from the January 2008 flyby of the planet by the MESSENGER spacecraft have shown that volcanoes were involved in plains formation and suggest that its magnetic field is actively produced in the planets core and is not a frozen relic. Scientists additionally took their first look at the chemical composition the planets surface material. The tiny craft probed the composition of Mercurys thin atmosphere, sampled charged particles (ions) near the planet, and demonstrated new links between both sets of observations and materials on Mercurys surface. The results are reported in a series of 11 papers published in a special section of the July 4 issue of Science(italic) magazine.
The controversy over the origin of Mercurys smooth plains began with the 1972 Apollo 16 Moon mission, which suggested that some lunar plains came from material that was ejected by large impacts and then formed smooth ponds. When Mariner 10 imaged similar formations on Mercury in 1975, some scientists believed that the same processes were at work. Others thought that Mercurys plains material came from erupted lavas, but the absence of volcanic vents or other volcanic features in images from that mission prevented a consensus.
Six of the papers in Science report on analyses of the planets surface through its reflectance and color variation, surface chemistry, high-resolution imaging at different wavelengths, and altitude measurements. The researchers found evidence of volcanic vents along the margins of the Caloris basin, one of the Solar Systems largest and youngest impact basins. They also found that Caloris has a much more complicated geologic history than previously believed.

By combining Mariner 10 and MESSENGER data, the science team was able to reconstruct a comprehensive geologic history of the entire basin interior. The Caloris basin was formed from an impact by an asteroid or comet during the heavy bombardment period in the first billion years of Solar System history. As with the lunar maria, a period of volcanic activity produced lava flows that filled the basin interior. This volcanism produced the comparatively light, red material of the interior plains intermingled with impact crater deposits. Subsidence caused the surface of the Caloris floor to shorten, producing what we call wrinkle-ridges. The large troughs, or graben, then formed as a result of later uplift, and more recent impacts yielded newer craters - James Head of Brown University, the lead author of one of the Science reports.

The first altitude measurements from any spacecraft at Mercury also found that craters on that planet are about a factor of two shallower than those on the Moon and they, too, show a complex geologic history.

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Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The spider now has an official name: Pantheon Fossae.
The word fossa is Latin and means trench.

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