Evening Mercury 
Rugged little Mercury, the smallest planet in the solar system, is putting in a quick but decent showing in the early evening sky right now. It’s especially easy to find tonight because it’s close to the upper left of the crescent Moon.
Mercury’s surface resembles the Moon’s — a desolate landscape marked by countless impact craters. But observations by the orbiting MESSENGER spacecraft show a few differences.
The main difference is the volcanic history of the two worlds. The Moon has been geologically “dead” for close to four billion years, so there’s been little volcanic activity. But Mercury stayed active for much longer. Relatively young volcanic plains fill much of the space between old craters — some of them formed just a couple of billion years ago.
By then, Mercury’s interior had cooled quite a bit, causing the entire planet to shrink. Mercury’s crust got thicker and stronger, shutting off the flow of molten rock to the surface and ending the planet’s volcanic activity.
The shrinking planet also created giant “wrinkles” in Mercury’s surface. These rounded cliffs can be up to a mile high and hundreds of miles long — features that are not found on the Moon.
And Mercury and the Moon are in good view shortly after sunset this evening. The planet looks like a bright star close to the Moon. But they’re so low in the sky that you need a clear horizon to see them. They drop from sight not much more than an hour after sunset.
Script by Damond Benningfield, Copyright 2013