Since it seems we are going to Mars fairly soon, and it's good to consider impossible things, here's a speculative piece I originally wrote back in 2001. Hopefully you'll enjoy it.
Where the Martian Water Went TerraHertz Originally written 20011211
An article in the Sydney Morning Herald, 11/12/01, page 3, titled: 'Hope for the world-weary: Mars travel by 2016'. (sic)
One quote from the article:
"We know [Mars] had a body of water that basically covered the whole northern hemisphere of the planet." Mr Lavery said.
"We want to know where that water went, to understand if there are any parallels between that and the ancient history of Mars."
This prompted me to give the matter some thought, and resulted in this little essay. My appologies to geologists, and especially to those who study the geology of Mars. I expect this is a stupid idea, just the sort of dumb-arsed thing a retired electronics engineer (me) would be expected to say about rocks. Only an idle moment's speculation, not meant to be a serious suggestion.
I wondered:
Does Mars still have a liquid magma core, or has it cooled enough for all (or significant portions) of its interior to solidify?
I seem to recall reading somewhere that Mars is thought to be a tectonicly dead planet, ie there is no longer any plate movement going on. Presumably, this means the crust is at least much thicker than Earths, or there is no active molten core left at all. Something about Mars lacking sufficient radioactives to maintain core heating?
This is significant to the 'whither the water' question, because if the planet has predominantly solidified, rather than remaining a thin solid crust on a liquid core like Earth, then it is obvious where the water has gone.
Consider what happens when glass is 'tempered', ie cooled rapidly from a near molten state. The outside goes solid, then the zone of solidification progresses inward. As each volume of glass solidifies, it contracts. The surfae has no problem contracting - it has space on one
side and can pull away from that. But the inside volumes are now surrounded by a rigid glass shell. They cannot contract - without creating cavities that is. And in glass tempering there are insufficient cavitation nucleation sites, and the process is too fast for cavities to form in the highly viscous glass.
So in hardened glass, the inside volume is in 'material tension' (of quite large magnitude) and the glass surface is under a resulting longitudinal compression, also of very great magnitude. Strong enough to resist considerable point impact loading. But if the surface compression does give way, and a crack is produced in the surface, the instant it propogates through to the interior zone of tension, the whole body of tempered glass is doomed. The crack branches and propogates through all of the glass, supplying the 'cavities' the glass needs to relieve its internal stress. The glass explodes into a shower of small pieces, each one with insufficient remaining internal stress to produce further internal fracturing. You see this as Safety Glass shatters into small pieces, and even more dramatically in a 'Prince Rupert's Drop.'
Now, a planet in its 'Earthlike' ie mostly molten state, is like a body of glass. If cooling and solidification progresses inwards from the surface, especially with smaller, low gravity planets, at some point the solid crustal layer may become sufficiently thick to resist all further 'wrinkling' effects that allow the crust to contract to match the shrinking volume of the internal mass as it cools.
Anyone who has poured molten metal into molds will be familiar with this effect. The metal shrinks as it cools, and so the still molten central volume of metal pulls an open cavity down at the center of the pouring inlet channel. You have to keep pouring in more molten metal to fill this, or you will have a casting with an internal void. The amount you have to add can be surprising.
With a cooling and solidifying planet there will be plenty of opportunities for internal cavities to open, to relieve the accumulating stresses. No shortage of either time, or sources of cavitation. There will be gasses, able to form 'bubbles' within the volume tension areas below the rigid crust. There will be local stress zones and fractures in the crust, able to propogate down into the more-tensioned deeper areas.
Keeping in mind, that at the depths below the surface that these effects will be occuring 'tension' is a relative term, and more likely to mean 'locally reduced pressure'. Areas into which mobile substances, such as gasses and liquids from nearby regions still under greater compressive forces, could migrate.
Now, supposing that a pathway developed between some body of surface water, and a deep zone of relative 'tension'. In this case, 'tension' would mean any pressure less than the natural pressure of water at that depth, given a head of water reaching to the surface. Inevitably, the surface water would infiltrate the deep zone, expanding it until the residual tension in surrounding rocks equalled the available head of water.
Also note that the temperature to which the deep zone of rock would have to 'cool' before water could infiltrate, would still be quite high, given that the boiling point of water rises with the pressure. And here we are talking about hundreds, even thousands of kilometers of depth, not just hundreds of meters.
How much volume would be required, to make up for the thermal shrinkage of the core of a planet the size of Mars, assuming some depth of hardened crust that might be structurally stable against further contraction under its own weight? (Remember gravity is only 1/3 of Earth's, but the rocks will be of similar strength to Earth rocks.)
We should allow for some 'mobile' fluid that could infiltrate at least some of the cavities forming below the rigid crust, at least in early stages of the 'separation' of the rigid crust from the still shrinking core. This fluid would provide some support for the overhead 'arch' of stony crust, and reduce the mechanical strength that would be required of the crust, to resist simple collapse into a rubble pile.
That thought suggests yet another possibility - that the solid crust has at least partially converted into a 'rubble pile', and in that form is able to shrink to match the contraction of the core. But this implies that there would still be vast volumes of space beneath the surface, just distributed as a massive network of small cavities and fractures between the 'rubble'.
Again, this might well have more than enough volume to have swallowed the Martian oceans.
Possibly a great deal of the atmosphere also, depending on the ability of the subsurface materials to sustain cavities filled only with gas, not liquid, against the weak compressive forces of Martian gravity.
In any case, the water had to get from the surface, down to the deep cavities. Perhaps it merely seeped down over millenia, vanishing with barely a whimper. Or perhaps, rather more dramaticly, there was at least some sizable body of surface water left, at a point when some channel
developed to an internal area of stress sufficiently extensive to drain the lot. That would lead to an event rather more spectacular than the shattering of hardened glass, the removal of a bath plug, or that amusing case in the USA where a drilling rig accidentally connected a lake with a very extensive saltmine beneath it. The entire lake (and the drill rig) vanished.
The Martian gurgler(s) would have been on a rather larger scale.
Possibly, such an event might have left some evidence on the surface of its occurence. Something like a rather deep, eroded looking hole or pit. Not necessarily round - it might have eroded along some preexisting faultline perhaps.
You don't suppose... that bloody great canyon, Valles Marineris?
Maybe, if there were other bodies of open water on the surface, separated from the 'sinkhole', then the planet's surface water might have taken some time to all drain away. Only that which fell in the catchment area of the sinkhole as rain would be permanently lost to the surface. If rain was infrequent, this could take quite a while. Also, with the loss of surface water, and maybe atmosphere, the temperature would fall and remaining surface water would freeze. From then on, water loss to the sinkhole (or holes) would occur only during rare thaws, or via sublimation and frosts.
There's another rather spectacular possibility too. During the process of Mars cooling, internal contraction and infiltration of the Martian oceans into subteranean voids, the planet's core heat and thermal outflow would have been enough to heat deep water volumes enough to overcome the pressure at great depth, and flash boil the water. The standard process by which geysers on Earth operate. Cyclic inflow of water into deep, hot chambers, water heating to the temperature at which it will flash boil and explode upward to the surface in a spectacular steam geyser.
Except on Mars, we're talking about whole oceans running down to vast spaces, potentially hundreds of miles below the surface. Geysers on a planetary scale. Possibly erupting with cycle times of hundreds or thousands of years. Entire oceans being boiled and blown out into the atmosphere, falling to form new oceans, then perhaps taking very long intervals to once again find a drainage path to deep cavities in the planet.
What would the surface signs of such events look like? Apart from 'river erosion channels' that seem to lead nowhere, and giant sink holes, what about the vast but very odd looking 'volcanoes' Olympus Mons and it's nearby sisters? The ones that look more like enormous symmetrical mud ejections, than lava flows. With the fringing cliffs that look like an ocean erosion coastline. (Because they probably are.)
One other interesting consequence of this scenario, is that there might remain cavities around the deep 'water table', where temperatures and pressures suitable to maintain liquid water, and a gaseous atmosphere supportive of life could exist. Though such a zone could have little in the way of thermodynamic gradients vital for the operation of higher chemical metabolisms. The only heat flow would be the slow leakage of remaining core heat upwards to the surface. There might be concentrations though, in forms similar to Earth's deep ocean 'smokers'.
Perhaps deep within the Red Planet, we may one day find the Martians: clams, worms, crustaceans, and Martian muscles?
I suspect when we get to Mars and start exploring, there will be a lot of real surprises. More than just 'some unusual rocks.'
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