Over the weekend, the terminator sweeps most of the way across the mare’s 1,300-kilometre expanse. As you view Imbrium, notice that its surface is not perfectly smooth—there are conspicuous undulations and wrinkles. You’ll also find a few large craters near its eastern shore, including Archimedes (82-km diameter), Aristillus (55 km) and Autolycus (39 km). (Remember that east and west on the lunar surface are the opposite of sky directions.) What is really interesting is the lack of smaller craters. This is because the mare is relatively young—it isn’t old enough to have accumulated as many craters as the surrounding regions. On the Moon, mind you, there’s young and then there’s young.
Like other lunar maria, Imbrium’s formation was a two-stage process. First, about 3.85 billion years ago, an unimaginably powerful impact excavated a large, circular depression—like a giant crater. The mountain ranges we see ringing Imbrium today are the remains of the impact basin’s rim. During stage two of Imbrium’s formation, the basin gradually filled with lavas seeping up from under the lunar crust. This process took hundreds of millions of years and resulted in the mare surface we see today. The last of the lava likely seeped to the surface some three billion years ago. So while Mare Imbrium is young by lunar standards, it certainly isn’t young in the conventional sense of the word!
To read more about what’s happening in the current sky, be sure to check out my regular This Week’s Sky column at SkyNews.ca.