Why does Venus have an atmosphere about 100X thicker than the Earth’s when it has slightly less gravity than Earth?
Partly, there’s no reason it shouldn’t, if conditions had been different.
It’s not really gravity but chemistry that’s the big difference.
Earth’s early atmosphere was likely similar in many ways to Venus. But where Venus has a ton of CO2, on earth, a lot of that CO2 got bound up in rocks or in the ocean on Earth. The current thinking is early life is responsible for much of the required chemistry to make that happen, i.e. early bacteria or whatever free up oxygen from CO2, leading to oxidizing reactions and basically converting sunlight and CO2 to rocks.
And for life to exist, it needed to be warm enough to support liquid water but not so warm that liquid water couldn’t exist. Venus would be too hot for this.
There’s also some possibility that chunks of the atmosphere were blown off, too, by the solar wind or by meteor impacts. As it is, the Earth could easily support an atmosphere much denser than it currently does. So could Mars, really.
I don’t think scientists actually know exactly why Venus is the way that it is.
Part of Venus’s problem is that it’s too close to the sun (by Earth standards, anyway). So, as you’d expect, it gets hotter there.
Another problem is that Venus seems to be in a bit of a runaway greenhouse effect. As the planet got hotter, whatever oceans it had boiled away, further increasing the dense atmosphere and increasing the greenhouse effect. Once the oceans were gone, things like sulfur dioxide and carbon dioxide from volcanic activity, which would normally get dissolved into the water, end up just staying in the atmosphere, again furthering the greenhouse effect. A lot of other materials that are trapped on the earth’s surface also boiled away on Venus, again adding more stuff to its already thick atmosphere.
Earth, being farther away from the sun and therefore colder, didn’t boil away its oceans, which allowed a lot of the other greenhouse gases (especially carbon dioxide) to settle out of the atmosphere, preventing the same kind of runaway greenhouse effect from occurring here. The evolution of life further changed the atmosphere, as plants absorbed carbon dioxide and released oxygen into the air, which further changed our atmosphere away from the type that Venus has.
In planetary science, this is known as a damn good question. (Why Mars is so different from Earth is another good question) This kind of question has probably launched a thousand careers in planetary science.
There are four fairly obvious differences between Venus and Earth:
- Venus is closer to the Sun
- Venus has no magnetic field, Earth does
- Venus has no plate tectonics, Earth does
- Venus has no moon, Earth has one that we think was formed by a huge impact early in the history of the solar system
One or more of those probably contribute to making Venus and Earth so different, but it’s debatable which one or which ones make the most difference. One or more of them might just be a coincidence.
Venus’ atmosphere is made up mostly of carbon dioxide. One theory says that plate tectonics on Earth recycles carbon dioxide, so it doesn’t build up here like it does on Venus. Why Venus does not have plate tectonics is also an interesting question in planetary science. There’s a theory that you need liquid water for plate tectonics to not grind to a halt. Venus may have lost its liquid water from a runaway greenhouse effect early in the history of the solar system.
Of course, the question of how far from its star a planet like Earth would have to be to be habitable is of interest to people who are looking for planets outside our solar system, as well as to anyone who is interested in looking for life on other planets.
Or Venus (or Earth, for that matter) could be the way it is because of some freak random event that happened early in the history of the solar system.
We’d like to look at a lot of planets at about Venus’ distance from a sunlike star and see if they all look like Venus, some with plate tectonics, some with large moons, some with a magnetic field, some without those things. That would tell us how much each factor contributes (or doesn’t) to conditions on Venus. But we don’t have many such planets to look at (we’re finding more, but seeing what conditions are like on a planet outside our solar system is harder than finding the planet in the first place).
Bear in mind that we only found out what conditions were like on Venus in the middle of the 20th century. You might have read science fiction where Venus has more Earthlike conditions, that’s because we didn’t know until relatively recently what it’s actually like on Venus.
It’s always chancy to say why a planet or solar system is the way it is based on a small sample. I’m old enough to remember pre-1995 theories about why there can’t be giant planets near a star. Obviously, those turned out to be wrong. It could be that some of our theories about why Venus is the way it is will meet a similar fate.
Really, the better question is why Earth and Mars have such thin atmospheres.