Where did the water go on Mars? Maybe it's right under our noses: Up to 99% may still be in planet's crust Up to 99 per cent of the water once in the ancient lakes and oceans on Mars is trapped in the planet’s crust and was not lost to space, according to a study in Science this week. Scientists at the California Institute of Technology (Caltech) and NASA’s Jet Propulsion Laboratory reckon the water has been sucked into minerals in …
If Sam's right this should get pretty exciting!
"...
Day after day, day after day / We stuck, nor breath nor motion/ As idle as a painted ship / Upon a painted ocean.
Water, water, every where / And all the boards did shrink;/ Water, water, every where, / Nor any drop to drink.
The very deep did rot: O Christ! / That ever this should be! / Yea, slimy things did crawl with legs / Upon the slimy sea.
..."
Samuel Taylor Coleridge https://www.poetryfoundation.org/poems/43997/the-rime-of-the-ancient-mariner-text-of-1834
As the article states, water on Earth is recycled by volcanic activity and would otherwise not be found in any great quantity on the surface. This has been known long, so it is not really a surprise that the lack of volcanic activity on Mars has contributed to its loss of liquid water.
What is new is that measurements of H20 vs. d2O can give a (very rough) estimate of how much is lost underground compared to lost to space.
In any case, for those who dream of terraforming Mars, its low gravity and lack of volcanic activity will make it hard to sustain a viable biosphere without having to replenish it forever. In spite of its current unfriendly environment, I think Venus is a better long-term option for terraforming: Blow away most of the current atmosphere and add water. Redirecting comets from the Kuiper belt to hit Venus will contribute to both. Sure, we are a long way from being able to do that, but in the long run, it will make more sense.
Or perhaps:
https://arxiv.org/abs/2011.07487
Terraforming the dwarf planet: Interconnected and growable Ceres megasatellite world
Pekka Janhunen
We analyse a megasatellite settlement built from Ceres materials in high Ceres orbit. Ceres is selected because it has nitrogen, which is necessary for an earthlike atmosphere. To have $1 g$ artificial gravity, spinning habitats are attached to a disk-shaped megasatellite frame by passively safe magnetic bearings. The habitats are illuminated by concentrated sunlight produced by planar and parabolic mirrors. The motivation is to have a settlement with artificial gravity that allows growth beyond Earth's living area, while also providing easy intra-settlement travel for the inhabitants and reasonably low population density of 500 /km$^2$. To enable gardens and trees, a 1.5 m thick soil is used. The soil is upgradable to 4 m if more energy is expended in the manufacturing phase. The mass per person is $10^7$ kg, most of which is lightly processed radiation shield and soil. The goal is a long-term sustainable world where all atoms circulate. Because intra-settlement travel can be propellantless, achieving this goal is possible at least in principle. Lifting the materials from Ceres is energetically cheap compared to processing them into habitats, if a space elevator is used. Because Ceres has low gravity and rotates relatively fast, the space elevator is feasible.
The trick is in the time scale. A Mars atmosphere will eventually escape, true, but you don't need it to last forever any more than you need to be able to build a house that lasts forever. It just needs to be reasonably easy to maintain.
Most claims that Mars had an atmosphere and subsequently lost it to space seem to imply that the process took many, many millions of years.
This means that, if you have the ability to terraform Mars in any time frame that makes sense for a human civilization, then maintaining the atmosphere afterwards would be an utterly trivial effort by comparison.
I can't run the actual math, but I wouldn't be surprised if you just needed to fling an ice-rich asteroid at it every hundred years or so. Any civilization capable of terraforming Mars would find such maintenance to be an irrelevant amount of effort.
I find it surprising that there is so little discussion of features on Mars that look like dust-covered glaciers, or the other immense elephant in the room, the elevation of the Martian south polar regions. There are many satellite shots of the surface that contain what look like glacial features, but they are dust colored. Yet, no one thinks back to the "melting" of Himalayan glaciers that turned out not to have happened. The glacier surfaces were covered in rock and soil and were not visible as ice. The whole Himalayan ice loss issue had to be scaled back a very long way. There are also photos of events where it appears that an entire stratum is jetting some type of vapour out of a cliff face, accompanied by a land slide. The color, form and spacing indicates a level feature or stratum in the cliff is discharging some form of pressurized gas or vapour, and that discharge triggered the slides. Then, there is the Martian south pole. The planetary surface is higher at the south pole. That should lead to morphologically driven adiabatic cooling and precipitation. If the planet entered an ice age, that pole would grow and rise, the ice itself adding to the adiabatic effect. The long term result could be the fixing of water at the pole, and as the planet chilled, that would be further insulated by CO2 ice.
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