StarDate: Evening Mercury

FROM THE UNIVERSITY OF TEXAS McDONALD OBSERVATORY, AS HEARD ON KCBS RADIO WEEKDAYS @ 9:52 A.M., 7:35 P.M. & 2:52 A.M

STARDATE 5/15/2014: Mercury has been shrink-wrapped. Observations by an orbiting spacecraft suggest that the Sun's closest planet has probably shrunk by several miles since it was born. That's wrinkled and stretched its surface like shrink wrapping on a basketball.

Mercury consists of a large core made of iron and nickel surrounded by thin layers of rock. That structure allowed the heat from the planet's formation to escape into space quickly.

As Mercury cooled, it shrank. That cracked and wrinkled its surface, forming cliffs that can be more than a mile high and run across the surface for hundreds of miles.

The orbiting Messenger spacecraft has photographed many of these features. Their numbers and sizes led a team of scientists to conclude that Mercury has shrunk by about six or seven miles since its birth.

Earth's core isn't that much bigger than Mercury's. But it's wrapped in much thicker layers of rock, so Earth hasn't lost as much heat from its interior. In fact, heat from inside our planet drives changes at the surface, as the thin plates that make up the crust float atop a thick mantle of molten rock.

Look for Mercury just popping into view in the early evening sky over the next couple of weeks. The little planet looks like a fairly bright star quite low in the west-northwest. It's so low in the sky that you need a clear horizon to see it. And the view is best from southern latitudes, where Mercury stands a little higher in the sky.

Script by Damond Benningfield Copyright ©2014 The University of Texas McDonald Observatory