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SYFY WIRE Bad Astronomy

The central peaks of Tycho

By Phil Plait
Central peaks of Tycho crater.

One my favorite things about living in Colorado is the view of the mountains. Even in late spring, the Rockies nearby are tall enough to have snow on them, and a decent rainfall where I live means more snow on the loftier peaks. If it’s deep enough the mountains lose all contrast, just appearing as eye-achingly white figures thrusting up into the sky.

And that may be why I love the above image so much. Truthfully, if I didn’t tell you, would you have guessed that those are mountains on the Moon?

But they are. That cluster of peaks sits right in the center of the large crater Tycho, in the Moon’s southern hemisphere on the near side. Check out the peaks in context:

Wide view of the central peaks of Tycho crater.

Ah, get it now? In this oblique view taken by the Lunar Reconnaissance Orbiter when it was a mere 59 kilometers above the Moon’s surface, Tycho’s far crater rim wall can be seen above the mountains, and the near rim below them. The Sun was high in the sky when this was taken, so shadows are short, giving the landscape its luminous quality.

Usually LRO observes the area of the Moon directly below it, but it is sometimes commanded to look off to the side to get a wider view of the moonscape. In this case, it helps get context for these mountains, instead of just seeing them from straight above.

Contrast this with how they appear when the Sun is low:

Lunar Reconnaissance Orbiter image of the central peaks of Tycho crater.

I know, right? That was also taken by LRO, back in 2011. I love the long shadow of the cluster stretching behind them, and the shadow of the crater rim encroaching on the lower right.

As much as they might look like the mountains I can see out my window, they formed very differently. The Rockies were pushed up by tectonic forces under the Earth’s surface, taking millions of years (the current mountains formed near the end of the Cretaceous period).

The Tycho peaks formed in a few minutes. Yes, minutes.

About 100 million years ago, an asteroid something like 5-10 km across slammed into the Moon’s surface. The huge energy released was far, far larger than what you’d get out of every nuke on Earth today if you detonated them all simultaneously. The explosion created a colossal shock wave in the Moon’s surface, carving out many cubic kilometers of material, creating the crater in just a few minutes. Material ejected from the center flew upward and outward for hundreds of kilometers, and when the plumes collapsed onto the surface they formed bright rays, all pointing back toward the impact center.

As for the central peaks ... take a glass, fill it with water, and then let a drop fall into it from a height. It will form a temporary crater in the water surface that quickly collapses. That’s due to gravity; the displaced water is in a wave that is above the surface of the rest of the water, so it falls down and flows back inward. This creates a wave rushing toward the center from all sides. When it reaches the center, that water all crashes into itself, sending a column of water up into the air.

That’s what happens when craters form, too! The rock flows outward after the impact, but then once its momentum dies it starts to flow back toward the impact point. That shrinking circle of material meets at the center, then flies upward. It solidifies like that, forming those mountains.

Mind you, the peaks in Tycho are 2000 meters high! That must have been a helluva flow. Incredible.

full Moon

Over the years, I’ve seen Tycho through telescopes hundreds of times. It’s best at full Moon when the rays are bright, and it’s beautiful. But its formation was an event so colossal that had that rock hit Earth instead, the dinosaurs would’ve been wiped out much earlier.

By the fortunes of trajectory and velocity they got an extra 35 million years to rule the Earth, but then were wiped out by a similar event anyway. We know no asteroid that big is headed our way anytime soon, but it doesn’t take one 10 km across to give us a bad day. Hopefully, when we do see one coming our way, we’ll be able to do better than just watch it come in.

Image Credit: NASA/GSFC/Arizona State University