Science Questions

Is there evidence of Earth meteor impacts on the moon?

Sat, 6th Aug 2011

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Question

Matthew Lewis asked:

Dear the Naked scientists,

My question is really quite a simple one. I understand that in some of the larger historical meteorite strikes on Earth that they were sufficiently energetic that some matter achieved escape velocity and if that's true, did the moon capture any of this matter and is there any detectable evidence of these impacts on the moon?

 

Cheers,

Matt Lewis (UK).

 

P.S. Great show! Love it! :)

Answer

Dominic -  You're absolutely right that most impacts of bodies onto a planet will give a lot of material escape velocity, and that material will be lost as debris to space.  And so, there's a lot of debris out there in the solar system and the chances are that it will, within a few million years, collide with one of the planets.  Now, if you've got bodies colliding with the Earth then statistically, that material is quite likely to end up on the moon simply because the moon is the closest body to us.  Itís only 400,000 kilometres away, about a hundred times closer than the next nearest object which would be Mars.

The problem would be actually identifying individual features on the Moon as being from meteor impacts from material coming from the Earth.  If you just look at a crater, all you can say is that material of such and such a mass has impacted the Moon, you have to actually find the meteorite and recover it to start doing chemical analysis on that to work out where it come from.  For example, if you can find trapped gases in there, you can try and match it against the atmosphere of one of the planets.

The Moon has no atmosphere and that means these meteorites will get quite a hard impact onto the surface and they will probably be totally vaporized in the impact - unlike, for example, a meteor impact on the Earth or Mars which is cushioned by the atmosphere and so, those objects can survive and be found.

Also of course, we havenít explored very much of the Moonís surface in comparison to Mars where weíve had rovers roving the surface for the last 6 years or so, finding, say, dozens of meteorites on the surface of Mars.  So we havenít really explored enough to find meteorites from the Moon even if they were there.  So I think you'd be lucky to find them.

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Hi Matthew. This is not my field but I will have a go at answering as nobody else has yet. The K-T boundary defines the distinct change between the Cretaceous and Tertiary periods around 65 million years ago. These are notable changes to the earth that are reflected by the type of species of plants and animals and are characterised by apparant mass extinctions, most notably of dinosaurs, over a very short period of time.

The discovery of a high concentration of Iridium associated with the K-T boundary supports the theory of the cause of the extinctions was one or more massive meterorite strikes (meteors tend to have a high Iridium content).

To answer your question, it may be difficult to find any such analogue to the K-T boundary on the moon as a result of a correlated meteorite impact because there would be no specific dramatic change on the moon to mark such an event. The moon is full of craters formed since its solidification around 3.5 billion years ago. It is also subject to many smaller impacts from which the earth is shielded by its atmoshere. The best way to find something that may correlate with the K-T period would be if the ejected material (assuming the TV programme you saw was correct) comprised material peculiar to the earth at that time which were effectively then deposited on the moon. I think if 10% of material was ejected into space then it is quite likely that some of this would be captured by the moon.

Note though that it is not sufficient to just achieve escape velocity to get ejected into space because this neglects the considerable effect of atmospheric friction. The velocity of any ejected material would have to be considerably higher and there would be some filtering effects of what size/mass of objects would make it out. Small pieces would not get out, I suspect, because their surface area to mass ratio would be too high and air friction would dominate. Very large objects may not be structurally strong enough to remain in one piece, given the huge forces involved during impact as well as the thermal stresses that would be created. It would be interesting to have a second opinion on the TV programme's assertions although it is widely accepted that some stuff we find on earth may have originated on Mars and got here by similar means, so it seems plausible that their 10% number is right. graham.d, Fri, 3rd Jun 2011

Matthew Lewis asked the Naked Scientists: Dear the Naked scientists, I saw on TV (so i tmust be true) that up to 10% of the matter ejected by the Chicxulub Meteorite impact achieved escape velocity and was ejected into space. http://en.wikipedia.org/wiki/Chicxulub_crater If the impact left detectable traces in the KT boundary, even if most of that is deposits from indirect effects, my question is: Is there an analouge of the KT boundary on Earth, found on the moon made from ejected impact matter captured by the moon's gravity? Cheers, Matt Lewis (UK). P.S. Great show! Love it! :) What do you think? Matthew Lewis , Tue, 9th Aug 2011

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