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Making a Splash on MarsOn a planet that's colder than Antarctica and where water boils at ten degrees above freezing, how could liquid water ever exist? Scientists say a dash of salt might help. |
"The air pressure is so low on Mars that even in the most favorable spots, where the pressure is higher than average, liquid water is restricted to the range 0 to +10 °C," says Bob Haberle of the NASA/Ames Research Center. "Fresh water on Mars begins to boil at 10 °C. Here on Earth we can have water anywhere between 0 and 100 °C -- that range is reduced by a factor of ten on Mars." If the thought of boiling water at 10 degrees °C seems bizarre, simply consult a high-altitude cookbook for a reality check. On mountaintops where the air pressure is low, water boils at a lower temperature than it does at sea level. (At 9000 ft a 'three-minute' boiled egg takes about five minutes to fully cook!) Mars simply takes the principles of high-altitude cooking to an extreme. Above: Water on Mars. A: A 3D view of the Martian
north pole created from Mars Global Surveyor laser altimeter
data. The cap is composed mainly of solid water ice. [more
from GSFC] B:
Wispy clouds of water ice hover over the Kasai Vallis region
of Mars. [more
from GSFC] C:
Ground frost (or snow) consisting of water ice at the Viking
2 landing site on Utopia Planitia.
So, the big question is not whether water exists on Mars --
it does -- but rather is there liquid water despite the planet
being so cold? The prospects for life on Mars, both human and
martian, hinge on the answer.
"I used the model to look for regions that meet the minimum requirements for liquid water -- above the triple point and below the boiling point," explained Haberle. "According to the model, the highest surface pressure, 12.4 millibars, occurs at the bottom of the Hellas Basin (a low-lying area created by an ancient asteroid strike). The problem is that the boiling temperature there is only +10 °C. It can't get very hot or the water will boil away." Evaporation of water in contact with Mars' dry atmosphere is also a problem, says Haberle. "Liquid water can be stable against freezing and stable against boiling, but unstable with respect to evaporation. The situation is analogous to Earth's oceans. Liquid water on the surface does not freeze ... or boil, yet it can evaporate if the atmosphere is not saturated with water vapor. [more information] "There are 5 five distinct regions where we might sometimes find surface water: in the Amazonis, Chryse and Elysium Planitia, in the Hellas Basin and the Argyre Basin. Together they comprise about 30% of the planet's surface. That's not to say that liquid water really does exist in those places, just that it could." ![]() Left: The massive Hellas impact basin in the southern hemisphere of Mars is nearly 9 kilometers deep and 2,100 kilometers across. The air pressure at the bottom of the basin is about twice the global average. In this false-color image based on measurements from the Mars Global Surveyor laser altimeter, red and white colors denote high elevations and blue denotes low. Follow the Salt... "One thing we have to be careful of is our everyday experience that water always freezes at zero degrees," noted Hoover. "It doesn't. Water containing dissolved salts freezes at a significantly lower temperature. Don Juan Pond in Antarctica is a good example. It's a high salinity pond with liquid water at temperatures as low as -24 °C." "Salts have the potential to significantly lower the freezing point of water," agrees Steve Clifford of the Lunar and Planetary Institute. "Indeed, there are some combinations of salts that can lower the freezing point by as much as 60 °C. However, thermodynamic and chemical stability arguments (arising from work by Benton Clark) suggest that, on Mars, the most potent freezing point-depressing brines are likely to be based on NaCl (common table salt)." ![]() Above: This cartoon, which is based on a figure presented by Dr. Ken Edgett of Malin Space Science Systems at the June 22nd NASA press conference, shows one way that gullies might form on Mars. Underground liquid water behind a barrier of ice erupts for a short-lived flash flood, creating the characteristic channels and aprons of martian gullies. The ice plugs are formed on the shadowed slopes of craters and ravines. Salts dissolved in the water behind the plug could help it stay liquid. [more from JPL] ... and go with the flow Another way to help keep water liquid -- on Mars or Earth -- is to keep it moving. "If you know a hard freeze is coming where you live, what's the first thing you do?" asks Hoover. "You turn your faucets on a little to let water trickle out. This way your pipes won't freeze." The same principle applies on Mars where salty water could be moving through subterranean aquifers. "Ice is a crystal," explains Hoover, "and it's harder to form crystals when the water is flowing." ![]() "I chose the Matanuska Glacier to visit because it's accessible and has dark rock in contact with ice," says Hoover. "The sun shining on the rock causes the ice to melt. There are pools of liquid water where microorganisms grow in abundance. There is something very interesting and exciting about this picture of me taking samples from the edge of a moulin (a water-carved crevasse). Most of what we see is ice and the air temperature is below freezing, yet there is liquid water pouring out of the glacier. How is that possible? The water had broken free further back up the glacier where sunlit rocks melted the ice. Then it flowed beneath the ice until it broke through a hole in the wall of the ice. Everything the liquid water came in contact with was freezing, yet the moving water did not freeze. "I have also seen liquid water running from snow melting on dark rocks heated by sunlight in Antarctica, even though the air temperature was below -20 °C." Above: Sampling ice from a moulin in the tongue of Alaska's Matanuska glacier. Orange moss can be seen growing on broken rock debris on ice ledge [larger image]. (Photos Courtesy Richard B. Hoover) There are many places on Earth where liquid water and ice
co-exist in sub-zero conditions, says Hoover. The most famous
example is Lake Vostok, an expanse of water roughly the size
of lake Ontario lying 4 km beneath the Antarctic ice sheet. The
ice sheet acts as a blanket, shielding the lake from Mars-like
temperatures at the surface. JPL manages the Mars Global Surveyor Mission for NASA's Office of Space Science, Washington, DC. Malin Space Science Systems built and operates the camera system. JPL is a division of the California Institute of Technology, Pasadena, CA. JPL's industrial partner is Lockheed Martin Astronautics, Denver, CO, which developed and operates the spacecraft. |
Web Links |
Mars Global Surveyor Home Page - from NASA/JPL Malin
Space Science Systems
-- MSSS operates and processes data from instruments on planetary
missions under contract to the National Aeronautics and Space
Administration (NASA). Martian Swiss Cheese -- March 9, 2000. New pictures
from NASA's Mars Global Surveyor spacecraft show exotic terrain
made of dry ice near the Red Planet's south pole. Search for Life on Mars will Start in Siberia -- May 27, 1999. NASA funds permafrost study to support astrobiology research. |
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