by Sarah Scoles
The idea of H20 hanging out on planets and moons that we don't live on is a pretty exciting one. After all, it's water that makes us who we are, water that keeps us alive to be who we are, and water that lets us sit on Funnoodles in our neighbors' pools in July.
|The caption on the page from which I borrowed
this image says, "Monster funnoodles with
a glued wooden handle make excellent training tools
for your next duel with Darth Andy.
So the idea of water outside our atmosphere leads us logically to think of life out there, whether that life be currently sliming around or merely sliming around in the past (and, here, I say "sliming" because life, at least within our solar system, was likely only have been microbial if it existed at all, aside from on Earth).
So it's been firmly established, based not only on theory and Earth-based observations, but on actual physical samples, that H20 exists on Mars. Currently, it takes the form of ice, often (though not always) frozen into the dirt to make a kind of cement. But this was not always the case.
What was the state of water on Mars in the past?
Planetary scientists divide the history of Mars up into three geological epochs: the Noachian (oldest), the Hesperian, the Amazonian (the latest). During the Noachian era, liquid surface water was abundant, and river valleys just like the ones on the Mississippi formed all over the place.
|An image taken by the Mars Reconnaisance Orbiter in 2008.
Check out the delta-esque shape of this river delta.
In the Hesperian era, which is distinguished from the Noachian by the decrease in cratering, the water was compressed and pushed downward, causing periodic outbursts and catastrophic flooding.
The geology of the Amazonian era was largely influenced by the Martian winds, and the water stopped being liquid and started being ice.
Where is this ice?
When I think of ice on Mars, I think mostly of the polar caps, but there is actually a lot of ice below the surface soil at latitudes above 50 degrees North and South. The ice table is the boundary between the dry soil and the ice-cemented soil.
About the ice table, planetary scientists (specifically Hanna Sizemore, who gave a colloquium here at NRAO last week) want to know "How far down is it?" And the answer is not something simple, like "one cubit." Its depth is affected by the shape and composition of the surface above it, and consequently varies from crater to crater, rock to rock, dune to dune.
So how do scientists determine the depth of the ice table, if it's different all over the place?
Well, given how long it takes just to command a Mars lander's arm to scoop up a little bit of dirt, the answer is not to dig up the whole globe until we hit the ice at all locations.
The correct answer is "What are simulations?" Or at least that was the answer. Until an indirect but empirical method led to results that contradicted the simulations.
The original simulations were 1D and assumed a homogeneous, flat surface.
|Insert spherical cow joke here.
The empirical method relied on cosmic rays, which hit the surface of Mars (and Earth) all the time. When they hit, they react with present atoms, and the atoms release neutrons and characteristic radiation. In other words, if a cosmic ray hits hydrogen, the result is fundamentally different from when it hits carbon. An instrument on the Mars Odyssey called the Gamma Ray Spectrometer (GRS) was able to use this behavior to learn more about the hydrogen content (and, by inference, the water content) below the Martian surface.
The point is, the results from the GRS disagreed with the simulations. Which meant that something about the simulations was wrong. Because experiments are like spades: trump cards.
So if the experiment and the simulation based on theory do not agree, the theory must be wrong, right?
Or simply too simple. Sizemore decided to write some surely complicated code to make a more realistic model--one with 3D and rocks. Perhaps the same physical principles, but with a few stones thrown in, would match the evidence.
And it did, actually, which is great. Turns out, where there are rocks, the ice table is deeper. It curves around them, because of how they hold and distribute heat, in the same way that they do on Earth (which you'll know if you've ever sat on a sunny rock like an iguana). And the presence of rocks means that more neutrons are displaced when cosmic rays hit, which affects the readings of the GRS. In short, the 3D modeling explained the discrepancies between the data and the initial 1D model.
Okay, so there is ice-cement, which is basically frozen soil, but is there any pure ice?
Funny you should ask, as there actually is. The landers on Mars found strange "lenses" of pure ice sticking up out of the soil, and their purity was confirmed (around 99% dihydrogen monoxide).
How did this ice get itself alone, and how did it get to the surface?
Through a very Earth-like process called "frost heave," which happens quite frequently in crazy places like Alaska. In this process, the ice that exists at the ice table boundary grows in the direction out of which heat is being lost (toward the surface). It also sucks up the water below the ice, through capillary action (like the action that makes pen ink bleed), which allows the ice to grow. This growing ice is called a "lens," and it can eventually push itself out of the ground, causing damage like that shown in the picture below. The resulting ice is almost totally pure, as the soil is not "drawn" to grow upward like the water is. Scientists were initially dubious that frost heave and ice lenses were the causes of the pure ice on Mars, but conditions were more favorable for their formation than initially thought.
|This picture not taken on Mars. Source.|
It's good to know that the three-legged, one-eyed people on Mars can lick ice lenses to get their water, instead of having to dig down to the ice table.
Sizemore, H., Mellon, M., Searls, M., Lemmon, M., Zent, A., Heet, T., Arvidson, R., Blaney, D., & Keller, H. (2010). In situ analysis of ice table depth variations in the vicinity of small rocks at the Phoenix landing site Journal of Geophysical Research, 115 DOI: 10.1029/2009JE003414
Sizemore, H. G., Zent, A. P., & Rempel, A. W. (2012). Ice Lens Formation and Unfrozen Water at the Phoenix Landing Site 43rd Lunar and Planetary Science Conference DOI: 2012LPI....43.2397S