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Over four and a half billion years ago, Theia, a rogue planet the size of Mars, slammed into the infant Earth. This impact is believed to have resulted in the birth of many things, from the moon to plate tectonics.

But let's say that Theia ditched Earth and instead set a collision course for Earth's sister, Venus.

This is no identical twin. Important to this question is the atmosphere of this planet. It's not just a blanket of excessive carbon dioxide, it's primarily a Trojan wall--92 bars of pressure. That's 92 times higher than Earth's atmosphere!

So if Theia did hit Venus, even glancingly, would it ever hit the surface? Or would the high pressure of the atmosphere destroy it midair?

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    $\begingroup$ You're slamming Mars into Venus. It will vaporize Venus. Of course it'll touch the surface! They'll merge! You're going to have a giant molten ball of rock! And a sizable halo of debris. $\endgroup$
    – Xandar The Zenon
    Commented Feb 2, 2016 at 4:46
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    $\begingroup$ tl;dr: Even though there may be some meteorites that would be destroyed in Venus's atmosphere that would reach Earth's surface, you can hit anything with a big enough object. Theia was... definitely big enough. $\endgroup$
    – Random832
    Commented Feb 2, 2016 at 15:51

2 Answers 2

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Thickness and density matter more than pressure. As does the velocity and mass of your impactor.

The portion of Venus's atmosphere that has a pressure of > 1 bar is approximately 50km thick. For the sake of argument, let's say that the entire 50km column is pressurized at 92 bar (which isn't the case; you only experience 92 bar at/near surface level and then the pressure gradually declines with increasing altitude, as it does on Earth).

Now what we want to know is the density of that atmosphere. On Earth at STP, air has a density of 1.2754 kg/m3. The atmosphere on Venus, however, is predominately CO2, which is a bit more dense; approximately 2 kg/m3. Scale that up 92 times, and you've got 184 kg/m3 (note that this is actually about 3x the value reported here, so I'm being quite generous, probably because I've not taken Venus's extreme temperatures into account). For comparison, water has a density of 1,000 kg/m3 and rock can be up around 3,000 kg/m3. So while the atmosphere on Venus is quite dense in atmospheric terms, it's still not very dense at all in terms of 'things a meteorite might encounter'.

But the question is, will interfacing with that atmosphere make an appreciable effect on an impactor the size of Mars? We can use some math to work out how much of the impactor's surface encounters the atmosphere before it reaches the surface of Venus. This is on the order of 100,000 square kilometers (rounding up to give the atmosphere the best possible chance of affecting the outcome). Let's say that all 50km of atmosphere within that entire area must be moved out of the way for the impact to occur (not actually the case, but again we want to give the atmosphere the best possible chance). So that's 5 million cubic kilometers of atmosphere that needs to be pushed out of the way, with a mass of 9.2 × 10^17 kg.

So how massive is Mars/Theia? Approximately 6.39 × 10^23 kg. Or in other words orders of magnitude more massive than the atmosphere that it needs to push out of the way. And how fast is the relative impact speed? 50km/second is within the ballpark, and works nicely with our 50km thick atmosphere.

Anyways, for an impact to happen we have to move 9.2 × 10^17 kg of atmosphere 177km (assuming the worst possible case mathematically, with the entirety of the atmosphere sitting right at the point of impact) in 1 second. That takes about 5.765 × 10^28 joules away from our impactor's total kinetic energy.

Problem is, at a 50km/sec approach speed our impactor has 7.988 × 10^32 joules of kinetic energy. Which is still orders of magnitude more than it will expend plowing through that atmosphere. Even if we drop the impact speed to 10 km/sec, we're still talking about 3.195 × 10^31 joules.

In other words, even in an unrealistic best-case scenario, Venus's atmosphere is basically transparent to an object the size of Mars. It doesn't prevent the impact or even appreciably decelerate the incoming projectile immediately before impact. At best it reduces the final impact energy by ~0.2%. Which is more than I thought it would do when I started this exercise, but still not nearly enough to save Venus from disaster.

The shockwave that gets created when that dense atmosphere is brutally shoved out of the way would probably be incredibly destructive by Earth standards, however.

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  • $\begingroup$ Great answer! It could perhaps be even better if you also include how you arrived at the various figures quoted (show us the math!). That way, the answer will be applicable to anyone doing anything similar, rather than just this particular case. $\endgroup$
    – user
    Commented Feb 2, 2016 at 9:49
  • $\begingroup$ The proto-Earth that existed before the Theia impact would have been smaller. Basically its mass would have been the current Earth mass minus the mass of Theia and the mass of the moon. Proto-Earth formed about 100 million years before the Theia impact. Assuming that Venus formed about the same time that proto-Earth formed, Venus probably had a much thinner atmosphere than it does today. $\endgroup$
    – Howard Miller
    Commented Feb 2, 2016 at 14:21
  • $\begingroup$ @HowardMiller The real assumption is right here: worldbuilding.stackexchange.com/questions/33466/… $\endgroup$
    – JohnWDailey
    Commented Feb 2, 2016 at 15:29
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    $\begingroup$ @HowardMiller Minus the mass of Theia, plus the mass of the moon. $\endgroup$
    – Random832
    Commented Feb 2, 2016 at 15:52
  • $\begingroup$ @Random832 I stand corrected. Further, I think that the accretion of the Moon from ejecta from the collision probably means that the collision was relatively low speed, and very few bits would have achieved escape velocity. Maybe we'll find some pieces floating around with some kind of orbital relation to Earth. Cruithne, maybe. $\endgroup$
    – Howard Miller
    Commented Feb 2, 2016 at 21:58
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I just used this video for a recent answer. Time step 1:00 shows this large asteroid (about 1/1000 the mass of Theia) impacting the atmosphere.

Note that other than some pretty pyrotechnics, the atmosphere (and ocean) don't have any noticeable affect upon the incoming object. In fact, think of the atmosphere and hydrosphere of the Earth as not affecting the impact at all. Think of the solid components of Earth and the impacting object as liquids. What happens when two droplets collide?

That's about what you'll see for the collision of these objects.

Essentially the thickness and mass of the Venusian atmosphere has no measurable effect upon the collision.

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  • $\begingroup$ Not Venetian but Venereal it is unfortunate but the one you used means from Venice. $\endgroup$
    – King-Ink
    Commented Feb 2, 2016 at 5:16
  • $\begingroup$ @King-Ink I certainly hope not, considering the definition of venereal. I believe what you mean is Venusian. Slight difference. $\endgroup$
    – Frostfyre
    Commented Feb 2, 2016 at 5:21
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    $\begingroup$ @Frostfyre Both are correct. STDs are Diseases of Venus (Love). You can also use it to refer to copper. Cytherean can also be used. en.wikipedia.org/wiki/Cytherean $\endgroup$
    – King-Ink
    Commented Feb 2, 2016 at 5:30
  • $\begingroup$ Venerean is probably most correct and what he typed it was most likely auto-corrected. $\endgroup$
    – King-Ink
    Commented Feb 2, 2016 at 5:36
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    $\begingroup$ I always said Venusian, i think that if we (as a society) started talking about Venus a lot then we'd reserve "venereal" for STD's, whether it's "technically correct" or not. $\endgroup$
    – Max Williams
    Commented Feb 2, 2016 at 9:02

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