Neither Venus nor Mercury have any moons to call their own. As a matter of fact, with a diameter of 3,030 miles (4,878 kilometers), Mercury isn't much bigger than Earth's one and only moon.
Easily, Mercury is the smallest planet in our sun's orbit. Yet the evidence tells us it used to be larger.
Mercury is interesting to look at from a topographic standpoint. The thin atmosphere offers very little protection from asteroids, so impact craters are abundant. A single photo taken by the Messenger probe in 2008 shows 763 identifiable craters within a region of the planet's surface that's just 172 miles (276 kilometers) wide.
One of the better-known craters is the Carolis Basin; this depression is 950 miles wide (1,525 kilometers), making it larger than the state of Texas.
The planet's face is also marked by fault scarps; cliff ridges that were first sighted on Mercury in the 1970s. Some of the smaller ones appear to be around 50 million years old, making them geologically young. (For context, the last non-avian dinosaurs died out 65.5 million years ago.)
Mercury itself formed about 4.5 billion years ago. Since then, its metallic core has been cooling down at a rapid pace — and as a result, the whole planet is currently shrinking.
Indeed, the diameter of Mercury may have contracted by as much as 8.6 miles (14 kilometers) over the eons. This has had a tremendous effect on the planet's rocky outer surface. Those previously mentioned fault scarps are born when crust materials break apart and press into each other, forcing some terrain upward.
Being tectonically active (in its own peculiar way), Mercury — like our own planet — may experience surface-level earthquakes. "Mercuryquakes" will definitely be a topic worth investigating as we plot out future probe missions to our curious little neighbor.
Originally Published: Sep 9, 2005