Moon and Mercury

• ‹ Previous program
• 298 of 1997
• Next program ›

To most of us, the surfaces of the Moon and the planet Mercury would be hard to tell apart. Both worlds are marked by big impact craters, rugged mountains, deep canyons, and vast plains of volcanic rock. To geologists, though, there are some differences. The differences mean that the two worlds had different histories.

One of the main differences is that giant cliffs criss-cross the surface of Mercury. They can be thousands of feet tall, and run for hundreds of miles. To a scientist, that means that Mercury has probably shrunk a little over the past few billion years.

The culprit is the planet's iron core. Originally, it was hot and mostly melted. Over the eons, though, it's cooled and shrunk. As a result, the solid crust around it cracked and buckled. Some parts of the crust ran over the tops of other parts, creating the tall cliffs.

This process appears to be continuing today. Recent research suggests that solid bits of iron fall like snowflakes from the outer core, which is still partially melted, toward the solid inner core. So the crust may continue to crack and move as Mercury continues to shrink.

Mercury and the Moon team up low in the eastern sky before sunrise tomorrow. Mercury looks like a fairly bright star to the lower left of the crescent Moon. It's so low in the sky, though, that you'll need a clear horizon to find it. Binoculars will enhance the view of this incredible shrinking planet.

Script by Damond Benningfield, Copyright 2008

For more skywatching tips, astronomy news, and much more, read StarDate magazine.