Bad Astronomy

Majestic mountains of the Moon

The Lunar Reconnaissance Orbiter takes amazing pictures of the Moon; I’ve posted dozens over the past couple of years. One of my favorite things is when the spacecraft snaps features I know: craters, mountains, winding valleys that I’ve seen myself behind the eyepiece. When I was younger I spent countless hours scouring the lunar surface with my telescope, and it’s still a fun target when I haul my ‘scope out to the end of the driveway.

And among the best of the best is the crater Tycho. You probably know it already; when the Moon is full the crater is bright, and the rays extending from it – plumes of material ejected radially during the impact that formed the crater – are extremely obvious. At 86 km (50 miles) across, it’s a decent-sized hole in the surface, with a beautifully-defined system of central mountain peaks 15 km (8 miles) across. So when LRO sets its sights on Tycho’s peaks, well… you get a gorgeous panorama like this:

You must click that to enlunenate it and see it in incredible detail. It’s truly spectacular!

That peak rises about 2 km (1.2 miles) about the crater floor. Look how steep it is! I was mentally comparing it to the local foothills of the Rockies near where I live in Boulder, and realized it’s not a bad analogy as far as size and shape go. In one way, hiking to the top of Tycho’s peak would be easier, since the gravity is only 1/6th of Earth’s… but while the air is thin here in Boulder, it’s literally nonexistent on the Moon. So I’m thinking hiking Tycho would be somewhat more taxing.

But what a sight when you reached the top! Sitting smack dab on that largest peak is a boulder I’d very much like to see up close:

The view on the left is a closeup from the image above (here’s a higher-res version), and the one on the right is from May 2010. The big difference is lighting and viewing angle; the left shot is lit obliquely and seen from the side, while on the right it’s seen with a higher angle of sunlight and the view is nearly straight down.

That rock is huge, 120 meters across. An American football field would fit right on top of it, goalpost to goalpost. The smooth terrain around it is what’s called impact melt, rock that was melted when the impactor hit over one hundred million years ago. That’s why it looks smooth; it was probably molten material that flew off the surface and then rained back down.

But how did that rock get there? I have a hard time picturing something that big getting ripped off the surface by the impact and landing softly enough to stay intact. It looks out of place there, unlike the craggy peaks around it. I had thought perhaps it was fractured off one of those peaks and rolled to the local depression between them. I’m not so sure, and a web search didn’t turn anything up on it. The top of the boulder appears to have some of the melt on it, too, so perhaps it really was blown out intact from the impact, fell back down after the peak formed, and then suffered through the rain of molten rock after.

I bet lunar geologists have lots of ideas about it… and I also bet we won’t know for sure until one of them (or more likely, a colleague who is as yet in grade school right now) stands there with a pick and a bag, ready to take some samples back to the lab. Will her equipment be at Clavius base, I wonder?

Image credit: NASA/GSFC/Arizona State University

Related posts:

- I think the Moon watched Bad Universe
- A flower bloom on the Moon
- What, no monolith?
- Watermelon planet (great pic of Mercury showing rays like Tycho’s