This is a big mystery. Nobody quite understands it yet. First, all the terrestrial planets from Mercury to Mars are thought to have formed originally without water (or not much), was just too hot so close to the sun when they formed.
The outer solar system objects are beyond the "frost_line" the distance where it is cold enough for volatiles like water, ammonia etc to condense into grains. But the terrestrial ones are within it, so if they just formed naturally with no water brought to them, they would have no water.
So then the question is, what brought water to some of them? Earth for instance, how did our planet get its oceans?
So you'd think asteroids or comets. But then why did Mars get hardly any water? I know that Mars did have more water in the early solar system, and lost it all, at least that's the general consensus. And it's a smaller planet also. But still, if we are right that it did have an ocean, its ocean was tiny compared to Earth, with a depth of a mile and 20 million cubic kilometers of water Nasa finds evidence of a vast ancient ocean on Mars
By comparison Earth's oceans have 332,519,000 cubic miles of water, or 1,386 million cubic kilometers, and Challenger Deep is nearly eleven kilometers deep. How much water is in the ocean?
How come Earth got roughly 70 times as much water as Mars? The estimate for the amount of water on early Mars is very approximate, but still, it's quite well founded based on tracing the shore lines of the ancient ocean, and deltas feeding into it. If there is skepticism about this result, it is usually skepticism that it had as much as this, not a suggestion it could have had more than this.
You'd expect Mars to get more than Earth because it is further from the inner solar system, so the water would condense out of the forming nebula more easily - also is right next to the asteroid belt and at present gets hit by ten times as many asteroids as Earth. And what's more, it didn't have a massive Mars sized object hit it in the early stages forming our Moon.
So why does Earth have so much water and not Mars? I don't think anyone knows.
One other possibility is that perhaps Earth did form with a lot of water after all: Mystery of Earth's Water Origin Solved
Now for Venus, we just don't really know, it's lost its oceans long ago. Still has some water in its atmosphere and I think most think it did have extensive oceans early on. But it became so hot, with a runaway greenhouse effect, that it lost its water first to water vapour then some to space.
Ceres and some of the other asteroids have ice - it is too cold for water which isn't stable anyway in vacuum conditions. Some think Ceres may have a subsurface "muddy ocean" to this day. Surely had vast ice covered oceans in the early solar system.
When you get out further, to Jupiter - then water is abundant but in the form of ice on the surface.
Europa's subsurface oceans may have more water even than the Earth, although this moon of Jupiter is smaller than our Moon.
One estimate makes it three trillion cubic kilometers of water in the subsurface ocean, compared with Earth's 1.386 trillion cubic kilometers. And that's just the liquid part, kept warm by tidal heating. It also probably has a ten to thirty kilometers depth of ice on top of that.
There are many other suggested oceans now in the outer solar system, liquid water, in Ganymede, even Callisto might have one, Titan might, Enceladus certainly with water actually erupting into space as frozen droplets of ice, perhaps Dione, and several others. There's some speculation that even Pluto could have a subsurface ocean.
But Earth's ocean is (to best of our knowledge) by far the most habitable, the only one where photosynthetic lifeforms can live in the upper layers. It's possible to have photosynthesis in Europa's ocean also, and these others, but that would be photosynthesis of the long radiation light from hydrothermal vents. That's a low energy source, so not likely to be able to sustain a large population. Hydrothermal vents have "marginally sufficient light for photosynthesis". See Sun free photosynthesis (An obligately photosynthetic bacterial anaerobe from a deep-sea hydrothermal vent)
If there is life there, most of it probably depends on chemical energy. Still Europa particularly has the advantage that the radiation from Jupiter should split the hydrogen away from the oxygen in its water, which would then get subducted into its interior and most now think it will have an oxygen rich ocean. Possibly enough oxygen to support a megafauna, although probably not nearly as many as on Earth.
One of the results from study of Comet 67p is that the water there has the wrong deuterium to hydrogen ratio to be the source of water for Earth. So it seems that probably it came from icy asteroids instead.
But this must be one of the least understood aspects of planet formation - the origin of Earth's water and why we have so much and Mars so litte.
The outer solar system objects are beyond the "frost_line" the distance where it is cold enough for volatiles like water, ammonia etc to condense into grains. But the terrestrial ones are within it, so if they just formed naturally with no water brought to them, they would have no water.
So then the question is, what brought water to some of them? Earth for instance, how did our planet get its oceans?
So you'd think asteroids or comets. But then why did Mars get hardly any water? I know that Mars did have more water in the early solar system, and lost it all, at least that's the general consensus. And it's a smaller planet also. But still, if we are right that it did have an ocean, its ocean was tiny compared to Earth, with a depth of a mile and 20 million cubic kilometers of water Nasa finds evidence of a vast ancient ocean on Mars
By comparison Earth's oceans have 332,519,000 cubic miles of water, or 1,386 million cubic kilometers, and Challenger Deep is nearly eleven kilometers deep. How much water is in the ocean?
How come Earth got roughly 70 times as much water as Mars? The estimate for the amount of water on early Mars is very approximate, but still, it's quite well founded based on tracing the shore lines of the ancient ocean, and deltas feeding into it. If there is skepticism about this result, it is usually skepticism that it had as much as this, not a suggestion it could have had more than this.
You'd expect Mars to get more than Earth because it is further from the inner solar system, so the water would condense out of the forming nebula more easily - also is right next to the asteroid belt and at present gets hit by ten times as many asteroids as Earth. And what's more, it didn't have a massive Mars sized object hit it in the early stages forming our Moon.
So why does Earth have so much water and not Mars? I don't think anyone knows.
One other possibility is that perhaps Earth did form with a lot of water after all: Mystery of Earth's Water Origin Solved
Now for Venus, we just don't really know, it's lost its oceans long ago. Still has some water in its atmosphere and I think most think it did have extensive oceans early on. But it became so hot, with a runaway greenhouse effect, that it lost its water first to water vapour then some to space.
Ceres and some of the other asteroids have ice - it is too cold for water which isn't stable anyway in vacuum conditions. Some think Ceres may have a subsurface "muddy ocean" to this day. Surely had vast ice covered oceans in the early solar system.
When you get out further, to Jupiter - then water is abundant but in the form of ice on the surface.
Europa's subsurface oceans may have more water even than the Earth, although this moon of Jupiter is smaller than our Moon.
There are many other suggested oceans now in the outer solar system, liquid water, in Ganymede, even Callisto might have one, Titan might, Enceladus certainly with water actually erupting into space as frozen droplets of ice, perhaps Dione, and several others. There's some speculation that even Pluto could have a subsurface ocean.
But Earth's ocean is (to best of our knowledge) by far the most habitable, the only one where photosynthetic lifeforms can live in the upper layers. It's possible to have photosynthesis in Europa's ocean also, and these others, but that would be photosynthesis of the long radiation light from hydrothermal vents. That's a low energy source, so not likely to be able to sustain a large population. Hydrothermal vents have "marginally sufficient light for photosynthesis". See Sun free photosynthesis (An obligately photosynthetic bacterial anaerobe from a deep-sea hydrothermal vent)
If there is life there, most of it probably depends on chemical energy. Still Europa particularly has the advantage that the radiation from Jupiter should split the hydrogen away from the oxygen in its water, which would then get subducted into its interior and most now think it will have an oxygen rich ocean. Possibly enough oxygen to support a megafauna, although probably not nearly as many as on Earth.
One of the results from study of Comet 67p is that the water there has the wrong deuterium to hydrogen ratio to be the source of water for Earth. So it seems that probably it came from icy asteroids instead.
But this must be one of the least understood aspects of planet formation - the origin of Earth's water and why we have so much and Mars so litte.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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