Venus surface isn't older than about 300 million years. The only explanation I've come across is that some kind of global volcanism resurfaced the planet. But couldn't it have been an impact event? A huge comet hitting it by chance, maybe making Venus tip over and stop spinning too.

Or could Venus have had a moon like Phobos which spiraled inwards, disintegrated and rained down during a few million years? Phobos will do so in only 50 million years, maybe it is a common feature for terrestrial planets to form with such moons that hit them billions of years later. Maybe Earth is an exception because of the large Moon and how it formed.

Would an impact event leave visible traces like impact basins, or could the entire surface melt and reform as it is today, as I suppose Earth did when the Moon formed? Could Venus have been a very different planet up until 0.3 billion years ago? How could one find out, what kind of investigation would be needed?

Pluto's mass is about 1/50th of Mars (and Theia), so I suppose that it would be enough for a Pluto sized comet to have had about 7 times the impact speed compared to the pre-Earth/Theia collision. Comets could have up to 70+35 km/s speed relative to Venus. Someone maybe can make a rough estimate of what's needed to resurface Venus and how likely such an impact event would be?

And could reformation and redifferentiation after an impact have eliminated a previous magnetic field?

The diversity of the planets and moons, I suggest, depend more on their vast history of random events, than on where and how they once formed. I suspect that planetary scientists generally want to discount a unique planet reforming event only 0.3 billion years ago, just to make their job a bit easier ;-)

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    $\begingroup$ I like the Venus/moon idea. I'd never considered that. Venus slow rotation and the solar tidal effect would both draw a moon into Venus, assuming Venus once had a moon. It would need to be bigger than Phobos to do a full re-surfacing I think. A fair bit bigger. My guess, I see no reason why it couldn't have been a big impact. An Impact (I think) is likely the cause of Venus' reverse rotation, but that could have happened any time and impacts of that size were more common when the solar system was young. $\endgroup$
    – userLTK
    Sep 25, 2015 at 5:52
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    $\begingroup$ impacts large enough to twist it's axis of rotation 180' are less common 300 million years ago, as it would be a 100ds kilometer diameter object. The tectonic plates are a lot thicker on Venus, whereas on earth, the lighter crust has become the moon which complexifies the convection and tectonics and encourages constant smaller earthquakes. When a 20km asteroid hits venus it's enough to trigger all the volcanos of venus at the same time and all it's supervolcano magma chambers which are under pressure. $\endgroup$ Nov 30, 2016 at 12:55
  • $\begingroup$ Just had to check on this. Velikovsky does not cover this scenario: en.wikipedia.org/wiki/Worlds_in_Collision $\endgroup$ Oct 25, 2017 at 15:08

2 Answers 2


Would an impact event leave visible traces like impact basins, or could the entire surface melt and reform as it is today, as I suppose Earth did when the Moon formed? Could Venus have been a very different planet up until 0.3 billion years ago? How could one find out, what kind of investigation would be needed?

Certainly giant impacts were fairly common, though I don't think they were common any more as recent as 300-500 million years ago. That would have been a kind of unusual event. Mostly they occurred when the solar-system was young.

Mercury probably had one

Venus may have too - and I recommend this article as very loosely related.

The Earth had one - as you know.

and Mars too, but not quite as "giant", though still very big and it left a visible mark that spans nearly 1/2 of Mars' surface. See Here and Here.

The Moon even had one and there is evidence in the thickness of the Moon's crust and location of it's volcanic magma pools/dark spots. See Here and Here.

The point I'm trying to make with all these examples is that unless the Impact is very very large, it would leave a impact footprint of sorts. The impact has to be large enough to not only liquefy the surface but to also allow the surface enough time to reset in mostly uniform layers so you don't get 1/2 of the planet with a thicker crust than the other half. Impacts of that size are statistically improbable, within the last 300 million years. Most of the big impacts happened long before then.

Wikipedia suggests that there's other reasons for believing the global resurfacing though interior heat idea. Venus Global resurfacing event - Wikipedia - Venus lack of a magnetic field, it's influx and loss of water and D to H ratio and it's Sulfur content. Do these have had to happened through internal heat resurfacing? Now, the article says there's no proof, only some pretty good evidence, so it it possible Venus resurfacing was due to an impact? I see no reason why it wouldn't be possible.

Given Venus' Heat trapping atmosphere, higher temperature and greater gravity, which also means, greater impact velocity, at least escape velocity of about 10 km/s where as the Mars impact that covers a hemisphere was thought to be about 5 km/s and the impacting object into Mars, at least 1,600 km in radius which is nearly the size of the Moon. You might not need that big to impact Venus and create a complete resurfacing, but I wouldn't think you could go too much smaller.

I love the Venus had a moon that crashed into it idea cause that seems to work on a few levels. A giant impact that created Venus could have happened 4 and change billion years ago and created a Venusian Moon, which over time, could both slow Venus' rotation and work it's way towards Venus in the process, eventually crashing into it. It seems at least, possible, though I'm not sure what evidence could be gathered to prove it. I like your idea a lot and I think it's much more likely than this one: Earth stole the Moon from Venus.

As to Venus being very different prior to it's resurfacing. The sun was only about 3%-5% less luminous when Venus resurfaced and given Venus' proximity to the sun, I have a very hard time seeing Venus as not already in a run-away greenhouse effect even then. I love the Venus once had oceans and maybe life hypothesis, but with a CO2 rich atmosphere, it's hard for me to see it as likely. My guess is that Venus was hot and virtually without water 300 million years ago too. Unless it somehow stored a huge amount of it's CO2 underground, I have a hard time not seeing it as too hot for oceans 300-500 million years ago. Maybe, but I'm skeptical. It would be very neat to have a picture of what Venus was like prior to it's resurfacing and over the hundreds of millions and billions of years before that.


In this article it mentions that it's improbable that Venus once had a Moon based on angular momentum. A planet-Moon system maintains it's angular momentum and if the moon spirals in towards the planet, it should carry significant angular momentum with it. Such an impact should give Venus a pretty rapid rotation which would be difficult to explain given that it rotates so slowly now. Unless perhaps, the Moon was in a retrograde orbit, Moon going in one direction the planet in another and the combined angular momentum largely cancelled out, but adding more specific conditions tends to decrease the probability. I still like the hypothesis, but the article above says it's unlikely.

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    $\begingroup$ Kilometres are usually denoted by km not KM $\endgroup$ Sep 25, 2015 at 9:49
  • $\begingroup$ Thank you & fixed. I'm unfortunately spelling and grammar challenged though Firefox helps a lot with the spelling. Dyslexia. $\endgroup$
    – userLTK
    Sep 25, 2015 at 10:09
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    $\begingroup$ @userLTK But a comet rich in ice and organics could maybe explain the atmosphere too, along with resurfacing and rotation. $\endgroup$
    – LocalFluff
    Sep 29, 2015 at 8:42
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    $\begingroup$ @LocalFluff it depends when. Venus's atmosphere is 96% carbon dioxide with a healthy amount of SO2 and other Sulfur compounds (which probably comes from Volcanism) and surprisingly little water. Comets rarely have compositions with ratios like that. Also, Venus Deuterium to Hydrogen ratio is much higher than (as far as I know) any other observed in the solar-system. This suggests that lighter elements in it's atmosphere have been stripped over time by the solar wind. Our atmosphere and probably Venus' did come from comets but most of that, around 4 billion years ago plus/minus a bit $\endgroup$
    – userLTK
    Oct 4, 2015 at 12:44
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    $\begingroup$ @LocalFluff old one, but some new info. This article says that giant impacts expel all gases equally. Unlike jeans escape or photochemical dissociation which affect upper atmosphere molecules in specific ways. Lots of good info ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140017487.pdf I've also updated the once had a moon answer. $\endgroup$
    – userLTK
    Oct 25, 2017 at 10:33

What about the possibility that Venus (and its former moon) went through a much more rapid evolution then occurs normally? Typically a planet-moon system will undergo the following sequence of events:

  1. A large object hits the planet and a moon is formed.
  2. The planet rapidly tidally locks its moon
  3. The moon gradually slows down the rotation of the planet until the planet is tidally locked to the moon (this also causes the moon to receede)
  4. The sun then robs energy from both and the moon then heads back towards the planet
  5. The two collide!

I have read that for the Earth and Moon this whole evolution will take roughly 65 billion years! But if the original object struck the planet more slowly or at a different angle, is it possible the process could evolve much quicker (ie in less then the age of the solar system). If Venus was then tidally locked to its moon it would mean a very slow rotation rate and when the moon finally did collide, there might not be much if any kinetic energy left to cause a rapid speed up of Venus's rotation after the event?

  • $\begingroup$ Can you add a reference to where you've read " this whole evolution will take roughly 65 billion years"? The age of the Earth is believed to be "only" about 4.5 billion years, this is 14x longer. If you can add any other supporting links or references as well, that will be great. Welcome to Stack Exchange, and Thanks! $\endgroup$
    – uhoh
    Sep 19, 2020 at 8:29
  • $\begingroup$ I think it would work, but I think it is unlikely that the end angular velocity would be more than $\approx$ 2 times more. It is because the original angular momentum was divided $\approx$ 50%-50% at the first collision event. $\endgroup$
    – peterh
    Sep 19, 2020 at 14:18
  • $\begingroup$ Btw, possibly there would be other significant signatures of such an event $\approx$ 300million years ago, what would be visible on the radar mapping of the Venus. But I don't know if anybody had considered it until now. So it is an open question, imho. $\endgroup$
    – peterh
    Sep 19, 2020 at 14:19

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