Well there are only two ways really - down or up.
First, Mars has much more radiation than Earth because of the lack of magnetic field. So the water might have just been dissociated into hydrogen and oxygen, and the hydrogen lost to space. The surface is, obviously, very oxygenated with its rusty appearance and salts are in the form of chlorates, perchlorates and sulfates, so that might be what happened to at least some of the oxygen from the lost water.
Then - Mars has no continental drift, maybe the water just sunk into it. It is thought to have a "hydrosphere" - water deep underground, trapped by the surface layers.
There is a lot of ice trapped at the poles, but only enough to cover the surface to a depth of 11 meters or so with water, not enough to fill the Northern ocean. Especially when you realize that the equatorial regions are dry to a considerable depth below the surface, drier than the Sahara desert. Also when Mars had oceans - it still also had polar ice caps, probably as extensive as they are today, or quite likely, more extensive. The current ice caps have nothing like the amount of ice there is in Antarctica for instance, even allowing for Mars's smaller size.
So - though there may well be a lot of ice trapped underground in the southern uplands, doesn't seem too likely that there is enough to account for the water in the original Mars ocean.
It's also likely that Earth had much more water than Mars originally. But Mars seems likely to have had extensive oceans in its early days, at least the evidence for that is pretty strong now though not totally conclusively proved. So there is still a mystery of what happened to all that water.
Chances are it is a bit of all of those. Anyway we should find out more as we explore Mars. One of its many mysteries.
First, Mars has much more radiation than Earth because of the lack of magnetic field. So the water might have just been dissociated into hydrogen and oxygen, and the hydrogen lost to space. The surface is, obviously, very oxygenated with its rusty appearance and salts are in the form of chlorates, perchlorates and sulfates, so that might be what happened to at least some of the oxygen from the lost water.
Then - Mars has no continental drift, maybe the water just sunk into it. It is thought to have a "hydrosphere" - water deep underground, trapped by the surface layers.
There is a lot of ice trapped at the poles, but only enough to cover the surface to a depth of 11 meters or so with water, not enough to fill the Northern ocean. Especially when you realize that the equatorial regions are dry to a considerable depth below the surface, drier than the Sahara desert. Also when Mars had oceans - it still also had polar ice caps, probably as extensive as they are today, or quite likely, more extensive. The current ice caps have nothing like the amount of ice there is in Antarctica for instance, even allowing for Mars's smaller size.
So - though there may well be a lot of ice trapped underground in the southern uplands, doesn't seem too likely that there is enough to account for the water in the original Mars ocean.
It's also likely that Earth had much more water than Mars originally. But Mars seems likely to have had extensive oceans in its early days, at least the evidence for that is pretty strong now though not totally conclusively proved. So there is still a mystery of what happened to all that water.
Chances are it is a bit of all of those. Anyway we should find out more as we explore Mars. One of its many mysteries.
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
4.8m answer views110.4k this month
Top Writer2017, 2016, and 2015
Published WriterHuffPost, Slate, and 4 moreHuffPost, Slate, Forbes, Newsweek, Business Insider, and Science 2.0