Big and Little
 A cliff that is almost a mile high slices across the right side of this image of Mercury snapped by the Messenger spacecraft in January. Similar cliffs were detected by the only other Mercury mission, in the 1970s, but this one may be the longest yet, stretching several hundred miles across the volcanic landscape. [NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution]
The big one is Jupiter, the largest planet in the solar system. It's low in the southwest at first light. But it outshines everything else in the sky at that hour, so it's easy to see. In fact, Jupiter is putting in its best appearance of the year; more about that tomorrow.
The little guy is Mercury, the smallest planet. About 30 to 45 minutes before sunrise, it's quite low in the east-northeast. Even though it's fairly bright, you might need binoculars to find it -- especially from the northern half of the country, where it lurks deep within the glow of sunrise.
Mercury's surface resembles the Moon's, with lots of craters and other scars from eons of impacts by asteroids and comets. But its most interesting features are cliffs that can be hundreds of miles long and close to a mile high.
They probably formed when Mercury was young. As its molten surface began to cool, it shrank. The crust buckled and cracked, with some sections lifted high above the others.
The biggest one known to date is about 350 miles long. But when the Messenger spacecraft flew past Mercury in January of this year, it discovered another one that could be longer. Since the other cliffs were named after ships of exploration, scientists named the new one for HMS Beagle -- the ship that carried Charles Darwin around our own little world almost two centuries ago.
Script by Damond Benningfield, Copyright 2008
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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