# Asteroid making a hole through the crust

How plausible would it be for an asteroid impact to create a hole through the Earth's crust into the mantle? What sort of traces would such an impact leave behind? Would it be survivable by at least some animals and plants?

Let's assume that we are dealing with Earth, or the third planet in a solar system roughly identical to our own, although the specific asteroid responsible doesn't have to really exist. Let's also assume the asteroid itself ends up embedded in the mantle, none of it remains on the crust.

# It is plausible

Impact depth is generally modelled by $$D\approx L\frac{A}{B}$$ where $L$ is the length of the object, $A$ is the density of the object, and $B$ is the density of the reciever of impact.

The Earth's crust has density between 2200 and 2900 kg/m$^3$. 16 Psyche is a metallic asteroid with a density of 3300 kg/m$^3$. Assuming a metallic asteroid of this density hit earth, and the desired impact depth is the 30 km of the Earth's crust, then the length of the asteroid would be

$$L\approx D\frac{B}{A} = 30 \text{ km} \frac{2900}{3300} = 26 \text{ km}.$$

So 16 Psyche would certainly punch into the mantle if it hit Earth, and an asteroid of metallic composition, more than twice the diameter of the Chicxulub impactor would reach also the mantle. Surely this is a rare event, but its not as if it hasn't happened before...

As for survival....no. We'd lose half our atmosphere, a good portion of our oceans, and much of the crust would be liquified. Not a good day to be alive.

# It is not plausible

Obligatory xkcd, what-if reference: Hockey Puck

If you’re like me, when you first saw this question, you might’ve imagined the puck leaving a cartoon-style hockey-puck-shaped hole.

But that’s because our intuitions are shaky about how materials react at very high speeds. Instead, a different mental picture might be more accurate: Imagine throwing a ripe tomato — as hard as you can — at a cake.

The thing is that even rocks and metals behave like soft putty when impacting things at that speed, especially if the target is not yielding.

So your asteroid will not punch through the crust and end up in the mantle. It will splat against the crust.

How deep will it go? Newton made a rough approximation of that. The Impact Depth is roughly the length of the impactor, multiplied by the ratio between the density of the impactor, and the density of the target.

Assuming both the asteroid and the Earth are rock, then the ratio is 1, and the impact depth is then the length of the asteroid, that is to say it will leave a crater, and the top of the asteroid will be in level with the surface of the Earth.

Assuming a metal asteroid, let us say one that has twice the density of rock, then if could dip down under the surface. But the asteroid would have to be huge in impact terms, at least 10 kilometers, if it is to break through 10 km of crust. An impactor of that size will make for one hell of a bang.

For example: The Chicxulub impactor is estimated to have been about 10 km.That left a lasting impression...

Also the asteroid will break apart into little pieces.

• I have to disagree. From this site impact.ese.ic.ac.uk/ImpactEffects/Chicxulub.html the transient crater depth is estimated at 30.3 km/18.8 miles. Ocean crust runs 5-10 km/3-6 miles, continental crust 30-50 km/20-30 miles. So a Chicxulub-sized impact in the ocean could make a hole into the mantle, though it would collapse/reflow almost immediately. And of course the impactor would probably be vaporized. – jamesqf Mar 24 '17 at 23:43
• @jamesqf So where was the disagreement? – MichaelK Mar 25 '17 at 0:38
• That it is not plausible for an impactor to make a hole into the mantle. Arguably the Chicxulub impactor could have done so (and per impact modelling, would have if the impact was close to vertical), though the hole was short-lived. As any such hole would be - see e.g. the large lunar impact basins that refilled with molten rock. – jamesqf Mar 25 '17 at 4:59

No way. The mass that's in the way of the asteroid and gets compressed, vaporized and moved to the sites as a shockwave is too much. That's what killed the dinosaurs. Yours would need much more energy to get through there. Even if the asteroid was of a super heavy not yet discovered and somehow stable element and therefore super small for its mass, the energy needed to shoot through so much earth would be really bad.

• But exactly how bad is "really bad"? The mass extinction that killed the dinosaurs was pretty small as mass extinctions go, Earth has faced much worse with life still going on. – TheBlackCat Mar 23 '17 at 16:02
• Tbh I can't find the math behind it, but normal asteroids (with an assumed density of 2600kg/m^3 see en.wikipedia.org/wiki/Impact_event#Impacts_and_the_Earth have to be huge to generate enough force to go even a little deep. Normal impact craters wikipedia en.wikipedia.org/wiki/Impact_crater#Excavation are wider than deep. You can more or less guess hoe dense the asteroid would have to be to be small enough to go through instead of forming a big crater. Very high velocity and very high density. I stand by impossible, but prove me wrong if you can ;) – DonQuiKong Mar 23 '17 at 16:13

Crust is much thinner under the ocean. Density of water is low compared to rock.

Some of the meteors recovered are close to pure nickel iron with a density of about 8,000/kg/m3

So a 10 km diameter ocean strike, not on the continental shelves may well get through the crust.

There are the usual variables, but most discussions forget the major question of hardness of the materials. At high speeds, items of approximately the same hardness act like equal liquids. You always get a circular crater, usually shallow, and not steep enough to penetrate. The total energy involved may cause secondary fractures along faults as the earth 'rings like a bell'. Where you hit in relationship to the edges of plates and existing pressures matters.

Now drop an intentional asteroid of tempered steel. It makes a much steeper crater and is more likely to smash through. Then what? A hole doesn't make the whole plate just sink because the plate is also held at the edges and magma is viscous. A mass driver using hard bullets might be able to stitch out a plate's edges and drop it, or it might not.

You should probably look at Lucifer's Hammer where two different earth destroying methods are used: drop a big bomb through the crust and plant small bombs along the tectonic boundaries.