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Tundra plants and desert plants only grow roots a few centimetres beneath the soil. Even wheat normally grows 0.7 to 1.0 metre root depth, although it can grow as deep as 2.8 metres if soil nutrients near the surface are scarce. Water avilablility is the most dramatic, plants will quickly grow roots down to where the water is. If permafrost keeps liquid water near the surface, there won't be a need to grow roots deep. There certainly isn't a need to grow 100+ metres deep.
References:
http://www.cropscience.org.au/icsc2004/ … altaja.htm
http://www.grdc.com.au/director/events/ … geNumber=3The goal is to grow oxygen generating plants on the surface, and to grow crops for settlers. Does permafrost deep beneath the soil matter?
I completely agree and understand that you don't need a very deep active layer to get vegetation to grow on the surface. The only reason I mentioned thaw time for deep soil was to answer noosfractal's question about how long it could take for the permafrost to thaw down to the bedrock.
I do think that permafrost deep beneath the soil matters because it can determine how much water you will 'loose' into the ground before you create a water table high enough for plants roots to reach. At first things will be nice beause the premafrost will be shallow and you needn't have much water, but later as the permafrost becomes very deep you will need lots more water to create a root-reachable watertable. By the time things thaw to the point where that becomes an issue Mars may very well have been warmed enough that there is plenty of liquid water to fill in the ground with. I just think it's something to keep in mind.
It'll definitely take a good long time for Mars' crust to completely thaw out. It's hard to estimate that sort of thing since we don't know how deep the bedrock is for much of Mars or how the regolith parameters change, but based on extrapolation of that formula in my last post and what we know of how Earth responds to global climate change, it will probably at least a millenia before the soil +100 meters deep begins to thaw.
I do think that terraformers will ultimately have to fill down to bedrock for the areas that are above the groundwater freezing point on average and thus unable to support permafrost. I also think that's a water sink that many people don't consider. People talk about the caps "creating an ocean X meters deep" when evenly spread out across the surface, but that water would slip into the parched, porous regolith pretty fast!
Hey nickname,
You've got some good points. Taking only the chlorine found in the Martian regolith into account and assuming that it is all bound as salts (probably not a bad assumption) the initial salinity of the groundwater and shallow overlying bodies of water would be around 2.6%, which is brackish and close to what you'd find in a salt marsh. Of course, I imagine there'd be sulfate and phosphate salts as well, which could bump it up to being saline water, or perhaps even brine in some areas.
I think you're absolutely correct when you say we need to start with bacteria. My gut feeling on terraformation is that we climb up the evolutionary ladder:
Bacteria -> Fungus/Moss -> Simple Plants -> Higher Plants/Insects -> etc.
You could perhaps also write it like this:
Bacterial Salt Marshes -> Peat Bogs -> Moorlands + Briny Lakes -> Meadows + Saline Lakes -> etc.
The diversification into regions of 'land' and 'water' would occur once the Martian surface had enough water that it could pool into large scale watershed structures. Small scale lakes and ponds could exist in low lying areas very early on, which is probably where your "bacterial salt marshes" would be. I call them that because at the cold temperatures, lower pressures, and higher UV we're talking about you wouldn't really have anything like a terrestrial "salt marsh"; they'd be more like primordial icy hypersaline bacteria filled puddles with none of the abundant plant life typically associated with salt marshes. Nevertheless, bioterraformation would definitely start in such puddles.
You suggest that the transport of salts would be extremely slow, and that differentiation of salty bodies of water and less salty land would take millenia. I suppose this might be the case for global scale terraformation, but I don't see why local systems should take nearly so long. A small stream flowing at a very modest 10 liter/second could drain much of the salt from a 1 km^2 area in less than 20 years. It thus seems to me that it is on a decade, or at most century, time scale that peat bogs could begin to be established in some areas.
In the end, I'm not sure just how much say terraformers would have in an icy salt marsh versus peat bog debate. The two environments would naturally form in low lying and slightly elevated areas respectively and the biology that suited each best would be grown. As water levels advanced the bacterial salt marshes would become saline lakes, the lower elevation peat bogs would become inundated by these (terraformers would just harvest these areas before that happened), and higher elevations would become more hospitable waterwise and give rise to moorlands with bogs and eventually higher plants as terraformation advanced.
By the way, here's the answer to your question about water's freezing point. 350 mb = 35,000 Pa. You can see that above 611 Pa the freezing point of water is essentially constant until you hit extreme pressures. Since 600 Pa is more or less the current pressure in the low lying areas of Mars that would be initially terraformed, it's safe to assume a freezing point of 0C for pure water there.
Thanks RickSmith and noosfractal! I appreciate your welcome. :3
So, I guess I should start with some background theory on how shallow subsurface temperatures work. Couple of assumptions here..
#1) That we're close enough to the surface that the radiation from the Sun absorbed by the surface dominates the subsurface temperature and that heating by the planet's internal sources is negligible. This is a pretty good approximation down to a couple hundred meters on Mars in places without geothermal hotspots.
#2) That the density and thermodynamic nature of the regolith is the same underground as it is on the surface. This one really depends on the local geography, since you might have bedrock a few centimeters beneath the surface in some places and perhaps almost a kilometer in others. Based on analysis of craters, there's evidence that the regolith goes down a few tens of meters on much of Mars (inside a crater I'm not so sure).
#3) We're talking about a one dimensional model here. Just temperature versus depth. Pretty good assumption if the regolith is fairly homogenous horizontally, which it is in most open places. Areas near the edges of crater walls or at the interface of a sandy and rocky area would be exceptions.
Okay, now that those are out of the way, it's time to introduce the idea of 'thermal skin depth'. This is basically a first order approximation to how deep a change in temperature on the surface effects the subsurface temperature. It turns out that relatively fast changes (like daily temperature cycles) don't penetrate very deep into the ground; that is, they have a shallow skin depth. Annual changes, on the other hand, penetrate much deeper, and long term trends on a geological scale can effect the temperature of the ground very deep indeed.
And so, given the assumptions above, the skin depth of a temperature change occuring over a period of time t effects down to a depth:
d = I/(p*C)*sqrt(t/pi)
where I is the thermal inertia of the ground, p is the density of the ground, and C is the heat capacity of the ground. There are many ways to express this equation, but I find this the most convenient for this application since we have maps of Mars' thermal inertia, as well as measurements of regolith density from several surface probes and decent estimates of heat capacity based on our knowledge of the regolith composition.
Okay, on to the numbers I promised! Pretend, if you will, that we have jacked up the average annual surface temperature of some part of the Martian surface to 0C. It turns out that the underground temperature at the skin depth is equal to the integrated average of the surface temperature over the period of time t. This means that the temperature at the annual skin depth will be 0C. Since (fairly pure) water freezes at 0C, the annual skin depth will give us a first order estimate to the active layer depth, that is, how far down the ground will ever thaw and allow liquid water to persist in the summer. For various 'surface' parameters:
pure liquid water @ 0C: 2.8 meters
pure water ice@ 0C: 8.4 meters
high thermal inertia regolith: 2.6 meters
low thermal inertia regolith: 0.11 meters
One thing I have to say right away is that the water and ice calculations do NOT in any way take into account the heat of fusion, that is, the energy it takes to melt ice into water. To do that would require more complex modeling involving a function describing the surface temperature at any point in the year...even including day/night cycles if they were violent enough to cross the 0C point routinely! While this wouldn't be incredibly hard to do, we can extract a lot of info without going to all that trouble. Suffice it to say that 8.4 meters is an upper limit and 2.8 meters is something of a more realistic estimate for a thin water 'crust'. Don't take this too seriously though. It's not like we're accouting for convection or anything, since we're really talking about regolith saturated with water where convection is retarded to the point of negligibility.
It's interesting how close the liquid water and high inertia regolith are, and without doing explicit calculations for groundwater saturated regolith we can guess that the skin depth is going to be ~3 meters for much of Mars. The ice and low inertia regolith represent the extremes of the Martian skin depths, varying by nearly an order of magnitude! Note that like with electrical conductivity, the characteristics of the material presenting the 'path of least resistance' dominates when present in significant amounts. Thus, low inertia regolith (60% porosity) saturated with pore ice would have a skin depth much closer to that of pure ice than pure regolith.
So what good does all this do us? Well, most importantly we can make some estimates of the maximum amount of water needed to saturate Mars' regolith on a global scale. Assuming the 'worse case scenario' of having to fill down to 8.4 meters with liquid water over the entire globe at 60% porosity, we get an equivalent ice volume of 1.3 million cubic kilometers, which is slightly less than the amount estimated to be in a Martian single ice cap (1.6 million for each, as I understand). This is very comforting, and it gets better. Assuming a slightly more realistic warming by filling only up to ±30° lat and estimating a skin depth of 3 meters, we obtain a required ice volume only 150,000 cubic kilometers, or less than 5% of the total estimated cap inventory. Note that this does NOT account for the mass of the water required to be in the atmosphere for rain to fall on Mars...I've made other models for that. ;3 Suffice it to say that the amount (at least for the atmospheric parameters I've been using lately) is close to negligible compared to the total amount of groundwater needed for regolith saturation between the surface and permafrost limits.
While these calculations give some idea of how much water would be needed to create a significant bog-like environment on Mars, they've got some issues. Notice, for example, that I didn't take salinity into account. The easy way to account for this is to just say the average surface temperature is equal to the freezing point of the salty water, e.g. if an area of Mars with groundwater salinity equal to that of the ocean (too high for moss...?) and an average annual surface temperature of -2°C (the freezing point of said water), the skin depth results would be pretty much the same as the ones derived for pure water and an average temperature of 0C. A really complete analysis would include the fact that the first water that rained down would pull salts underground with it...making the upper levels less salty (as has happened on Earth) and the deeper levels more so. This would mean that there could be a layer of liquid salty groundwater above salty ice but below fresher permafrost...and it'd mean you'd need slightly more water to fully saturate the ground.
Another (probably more obvious) thing I didn't account for was the topography of the Martian surface and the tendency for the groundwater to pool and form rivers and lakes (possibly seas) in depressions. In fact, bogs could probably form in isolated craters long before widespread surface bogs became possible. Another benefit of craters (some moreso than others) is that you are dug in closer to the bedrock, meaning that once average annual temperatures break the freezing point of your groundwater it won't take much extra water (if any) to fill in down to the bedrock than it did to fill down to the permafrost. In these locations noosfractal's domed craters could possibly begin terraformation before outside pressure levels would otherwise allow, thus accelerating the oxygenation process and ozone formation (which I also have a model of).
Well this is really cool for me. First post on New Mars, by the way. Short story is that I'm a fresh physics bachelor's graduate with a taste for hard sci-fi.
I happened to come up with the same idea of using a peat bog to terraform Mars a week or two ago. Lo and behold, when I poke around the internet I find a recent thread here on New Mars that confirms my hunches and really make a plausible case for the method. Awesome!
Unlike RobertDyck, I didn't come up with the idea through trying to figure out how to transform the soil (which was exceedingly clever, by the way). My thoughts were more along the line of characterizing what would happen once the temperature and pressure of Mars were raised and rain could develop (I've thought about how to do that too, but nevermind for the moment). I realized that the >611 Pa, >0 C conditions that would allow the first rains to fall (during some distant Martian summer, most likely) the average annual temperature of the regolith would still be far below 0C. [Note: Everything I talk about here refers to local environment parameters] The rain would permeate the ground until it hit the isotherm at which it froze (the depth of which would depend on local temperature, regolith thermal diffusivity, and the salinity of the water as it passed through the regolith) and would basically start forming permafrost. As the planet warmed more and more, the bottom of the permafrost layer would sink deeper and deeper, and whenever the temperature of the top was warm enough during the summers it would form an unfrozen active layer above the permafrost. This would continue until the average annual surface temperature was above the groundwater's freezing point, at which point you'd have no more permafrost caused by the annual cycle. You would still find it if you went down far enough that the ground hadn't yet felt the full effects of terraformation. But this level too would slowly recede deeper into the ground if average temperatures above freezing point were maintained at the surface. I'm interested mostly in the sub-freezing point surface average period, during which the permafrost layer would be near the surface.
What does this have to do with bogs? Well, it's not hard to realize that the permafrost would cap the depth to which rain could seep, just like in the arctic on Earth. As mentioned before, this limit varies according to the local average surface temperature, the water's salinity and the thermal diffusivity of the regolith, but would be on the order of centimeters to meters using typical regolith parameters and freezing points of fairly dilute salt solutions, meaning that a relatively small amount of groundwater would actually be needed to saturate the regolith up to the surface (I have numbers for much of this, if anyone is interested...). Once the soil was saturated, the temperatures and soil type (as so beautifully explicated by RobertDyck) would combine to produce an environment very similar to a tundra bog here on Earth. With vegetation this type of system is capable of producing peat, which could be used not only for fertilizer but as an economic crude fuel for colonists as well.
Things might get a bit tricky once you increase the average surface temperature above the water's freezing point. In some places bedrock would be close enough to the surface that it would sustain the bogs, in others you might have pre-existing ancient permafrost that would provide ready-made groundwater as things warmed up. Unlucky areas would simply drain out and become cool deserts with an ever deepening watertable. I've been looking at some radiograms of the subsurface by MARSIS to get an idea of where you'd want to avoid farming because of excessively deep bedrock or permafrost, but not much has been released and the images aren't very illuminating. I'd like to add that information to a map I'm constructing of suitible settlement locations, so if anyone's knows of any data/images on that sort of thing I'd be much obliged.
My apologies if that was excessively long for a first post.