There's a strikingly visible boulder sitting right at the top of the peak complex in the center of Tycho Crater on the Moon.

Is it just a coincidence that this gigantic, bright rock is sitting dead center of the peak complex or is it a known piece of the original impactor that caused the wider crater? Does this remarkable rock have a name?

Image source

On 10 June 2011 the LRO spacecraft slewed 65° to the west, allowing the LROC NACs to capture this dramatic sunrise view of Tycho crater. A very popular target with amateur astronomers, Tycho is located at 43.37°S, 348.68°E, and is ~82 km (51 miles) in diameter. The summit of the central peak is 2 km (6562 ft) above the crater floor, and the crater floor is about 4700 m (15,420 ft) below the rim. Many "clasts" ranging in size from 10 meters to 100s of meters are exposed in the central peak slopes. Were these distinctive outcrops formed as a result of crushing and deformation of the target rock as the peak grew? Or do they represent preexisting rock layers that were brought intact to the surface? Imagine future geologists carefully making their way across these steep slopes, sampling a diversity of rocks brought up from depth. Tycho's features are so steep and sharp because the crater is young by lunar standards, only about 110 million years old. Over time, micrometeorites, and not so micro meteorites, will grind and erode these steep slopes into smooth mountains. For a preview of what Tycho's central peak may look like in a few billion years, visit Bhabha crater.

This is a version with about one fourth the linear pixel density of the original NASA full size image.

Original sources 1, 2 Credit: NASA / GSFC / Arizona State University

Tycho crater central peak

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    $\begingroup$ Cool! Can you credit and add a link to the source of the image? Thanks! $\endgroup$
    – uhoh
    May 26, 2019 at 14:52
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    $\begingroup$ @uhoh commons.wikimedia.org/wiki/File:LRO_Tycho_Central_Peak_0.25.jpg $\endgroup$
    – Rag
    May 28, 2019 at 1:46
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    $\begingroup$ Thank you! I've added the information back into your question. In Stack Exchange comments are considered temporary and can be cleaned up at any time, so it's always good to include any relevant information back into the original post. $\endgroup$
    – uhoh
    May 28, 2019 at 3:03

2 Answers 2


It is just a rock.

A complex crater like Tycho is formed in several stages as the rock behaves like a fluid. The initial impact completely destroys the impactor and excavates a large cavity in the moon, and ejecta is shot out to all sides.

Powerful shock waves push the rock to the side building large walls. As these push down on the edges of the crater, the centre is pushed up forming a central peak. Further faulting and cracking will continue to modify the crater and the peak.

It is possible that this is part of the breccia that falls back to the surface having been shot upwards in the initial impact, but that seems very unlikely. It is more probable that this is part of the ridges of the central peak that cracked and fell into the shallow valley on the top of the central peak, which formed during the faulting as the peak was pushed up from the crater floor. In all likelyhood, this is a piece of moon rock. As it hasn't been exposed to solar radiation for billions of years, it is comparatively light. It can't be a piece of the impactor, as that would have been completely destroyed

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    $\begingroup$ "It is possible that this is part of the breccia that falls back to the surface having been shot into the air in the initial impact..." I will be surprised! For the impact that produced Tycho wouldn't this area still have been pretty darn hot and soft? A boulder of this size being shot almost straight up into the sky and staying there high and long enough for the rock to cool, then landing dead-center sounds pretty improbable to me. Are there any other examples of where this is thought to happen, or sources that describe this possibility, or is it just a guess? $\endgroup$
    – uhoh
    May 28, 2019 at 3:02
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    $\begingroup$ Possibly ejected material from another crater? $\endgroup$
    – MiguelH
    Sep 16, 2019 at 15:53
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    $\begingroup$ I'm annoyed by the expression "shot into the air", since there is no air on the moon. Maybe "shot upwards"? $\endgroup$
    – tutizeri
    Aug 22, 2020 at 8:00
  • $\begingroup$ Good point..... $\endgroup$
    – James K
    Aug 22, 2020 at 8:05
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    $\begingroup$ @uhoh It's possible this happened, but it should have formed a crater if it did. The question of self-secondary craters is based on this premise, that ejecta blocks go almost straight up and come almost straight down, forming craters on top of the one that just formed, but it is still very controversial in the field. It would have too much speed to NOT form a crater in this case, so I don't think it could be part of the ejecta at all. Hydrocode simulations do NOT predict secondary craters at all (the ejecta should not go straight up), so this makes things more questionable. $\endgroup$ Aug 26, 2020 at 6:56

The photo shows a mix of dark basalt, light anortosite from under the basalt, and a thin layer of impact melt (shiny and rippled) that the Boulder is sitting in. On the slope of the peak towards the lower right, there’s a huge block that has slid down the slope from the rim. You can see blocks of light anorthosite in it, and you can see grooves running down the side like the gouges you see where a car has grazed a concrete wall.

I think that:

  1. Initial impact melted surface basalt and flung it up.
  2. Impact mobilized the entire thickness of dark basalt and a thick layer of underlying anorthosite that formed the central peak including the big block that slid off a few minutes later
  3. There was a pause and a rain of molten rock covered the central peak (if you peek into corners around the peak you see it in a lot of low spots).
  4. The big block wobbles, the Boulder rolls down into the hollow and sits on top of the impact melt, and a big hunk of the rim then slides down the outer slope where it still sits on top of impact melt leaving the rim shorter as you see it now.

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