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Carlton Allen - Water on Mars
Introduction
Mars today is a polar desert. Orbiting 1.5 times farther from the Sun than Earth, Mars intercepts only 44% of the solar energy received by our planet. Martian gravity, about 1/3 of Earth's, can hold only a very thin
atmosphere, which traps little solar heat. The temperature rarely rises above freezing anywhere on the planet. The low atmospheric pressure prevents liquid water from remaining on the surface. It seems that a discussion of
water on Mars might be short and not very interesting.
But … things are not always as they seem, nor are things today the same as they have always been. Mars does
have water - albeit frozen - in the air, on the surface, and buried deep underground. Also, strong evidence points to an earlier time when great floods swept the planet, rivers ran, and lakes and possibly oceans existed.
Water in all of its forms has strongly affected the history of Mars.
Water is the one absolute prerequisite for life on Earth, and the presence of water makes a good starting point
in the search for life anywhere else. The continuing search for life on Mars is closely tied to the search for past and present water.
Water will also be an invaluable resource for humanity's future expansion into space. Humans obviously need water, as a liquid and in our food, to survive. We also need oxygen, which can be easily made from the H2O
molecule. Less obviously, almost any space transportation system will use huge quantities of oxygen to burn fuel for propulsion.
NASA has constructed a long-term Mars exploration strategy around the theme of water - recognizing its importance in planetary history, in the search for life, and in future human missions. Let's see what we know
about water on present-day Mars, what that implies for the past, and what we hope to learn in the future. Water on present-day Mars
Let's look again at a picture of Earth from space, which was used in a previous presentation.
http://campus.coexploration.org/~caucus/LIB/nasa_main_hall/wsslowes/GLLearth.gif
The clouds, the oceans, and the Antarctic ice strikingly illustrate that water exists in three physical states - solid, liquid, and vapor - on our planet. Earth's range of temperatures, combined with our atmospheric pressure,
allows water to exist in all three states and pass among them in a continuous cycle.
Now let's look at Mars, courtesy of the Hubble Space Telescope.
http://oposite.stsci.edu/pubinfo/jpeg/Mars95.jpg
This picture shows white clouds and white caps near both poles, but no evidence of oceans. In fact, liquid water cannot exist on the surface of Mars. The atmospheric pressure is so low (less that 1% of Earth's
pressure) that the liquid phase is not stable. Ice passes directly to vapor and vice versa. This lack of liquid water at the surface has very serious consequences for present-day geologic processes and possible life on Mars.
So, is there water in any form on Mars? Spacecraft measurements tell us that there is. The white clouds are composed of water ice, as are clouds on Earth. Both polar caps contain frozen carbon dioxide (dry ice), but at
least the north cap also includes a large deposit of frozen water.
Water on Mars cycles between vapor and solid depending on the local temperature. Icy fogs form at night in
some of the deep canyons, only to disappear with the coming of day. In polar latitudes, frost also coats rocks at night - the Viking 2 lander photographed frosted rocks during the Martian winter.
We have also found small amounts of water in Martian soil. The two Viking spacecraft heated soil samples and measured the gases that they released. Even at the desiccated Martian surface the soils contain around 1% water by weight.
Even the rocks of Mars contain small amounts of water. While none of our Mars spacecraft have yet returned samples, we do have Martian rocks in our labs. These rocks were knocked off of Mars by large meteorite
impacts, went into orbit around the Sun, and eventually landed on Earth. To date we have identified 14 of these
Martian meteorites, and they have provided some of the most precise data we have about their home planet.
Several years ago a team of scientists ground up part of a Martian meteorite, heated it carefully, and condensed a single drop of water from Mars. Here is a picture of that precious drop of Martian water:
This water was probably contained in minerals, which decomposed at low temperature, releasing water vapor.
Such hydrous minerals, now identified in several Martian meteorites, tell us that small amounts of liquid water once moved through cracks in rocks near the planet's surface.
Water in Mars' past
Mars was not always a desert. Look at this picture, taken from orbit by the Viking spacecraft.
http://cass.jsc.nasa.gov/expmars/channels.html
The left-hand picture spans about 200 km, so the channels are the size of the largest rivers on Earth. Most scientists believe that the only substance that could have made such massive channels is flowing water. What do
big river channels imply about a planet where liquid water cannot presently exist?
The answer, as we said in the Introduction, is that things today are not the same as they have always been. At
some time in the past Mars did have water on the surface – lots of it!
Look at this picture again, paying attention to the big circular features. These are impact craters, formed when
meteorites smashed into the Martian surface. Most of the degraded craters are large. Several of these craters seem to be sources for channels. Most of the fresh-looking craters are smaller. Some of these craters lie on top
of channels and have not been eroded by later channel flow.
What is the history implied by this picture? Sometime in the past big meteorites were hitting Mars, making big
craters. Some of these craters are cut by channels, and some channels start at breaks in the crater rims. Later, when only small craters were being made, flow in the channels had stopped. What might make a really big
river start flowing and later stop? Obviously the history of Mars has included major changes in climate or in the availability of water.
(Notice that the web site above contains a complete lesson based on these ideas.)
There are actually many large channels on Mars, pointing to several episodes of massive flooding. Some large, ancient craters are the sources of channels, and some are partly filled by layers of easily eroded material,
suggesting that the craters once contained lakes. Take a look at these pictures of Gusev crater and the channel
that enters it:
http://www.msss.com/mars_images/moc/7_17_98_gusev_rel/index.html
There are other, much smaller channels on Mars that give evidence of sustained, long-term water flow rather than massive floods. Nanedi valley, with a relatively small channel on its floor, was recently photographed by the Mars Global Surveyor spacecraft:
http://www.msss.com/mars_images/moc/top102_Dec98_rel/nanedi/index.html
Most of the large channels start in the Martian highlands and end in low plains that may once have held an ocean several kilometers deep. Altitude measurements from orbit show that these northern plains are incredibly
flat – flatter than any terrain on Earth except deep ocean basins. There is also a narrow line of steep slopes that surrounds the northern plains, everywhere at the same altitude. Some investigators have suggested that this line
was eroded by the waves of an ancient Martian ocean.
Where did the water go? Did it escape into space, evaporate into the atmosphere, freeze out in the polar caps,
become locked into the soil and rocks, or disappear underground? The answer seems to be "all of the above".
Early in Mars' history the atmosphere was much more substantial than it is today. The mix of isotopes in the Martian atmosphere indicates that a substantial fraction, perhaps as much as 95%, was lost to space. Certainly
some of the planet's water was lost along with other gases. Much of the water flow, however, probably occurred well after this atmospheric loss.
The thin Martian atmosphere, mostly carbon dioxide, contains water vapor that sometime freezes to form ice clouds, fogs, or frost. The atmosphere, though, contains only a tiny fraction of the planet's water.
The water ice cap at the north pole, and the matching ice cap presumed to exist at the south pole, store a portion of the planet's water. However, they are simply not large enough to account for the water volumes
indicated by the large flood channels.
As discussed above, the soil of Mars contains around 1% water, and some of the Martian meteorites contain
small amounts of water-bearing minerals. A substantial fraction of Mars' water could be in these surface materials, depending on the thickness of the planet-wide soil layer.
Most of the water that once flowed on Mars, though, is probably trapped underground. The temperature just below the surface is well below freezing, so any water there would be in the form of ice, or permafrost. Some
parts of the northern plains are covered by giant polygonal cracks, reminiscent of cracks formed in frozen
ground on Earth.
Heat from the decay of radioactive elements deep within the planet may raise the temperature above the freezing point several kilometers into the ground. If so, a layer of rock containing liquid water may exist below
the near-surface permafrost. The water would be under considerable pressure, due to the weight of rock and
soil above it. This pressurized water, possibly released by volcanic heat or meteorite impacts, has been cited as
a possible source for the huge floods.
Life on Mars?
The surface of Mars is extremely inhospitable to life. It is very cold, very dry, and bombarded by intense solar
radiation. The soil may even be highly reactive. Measurements by the Viking landers showed no evidence of life and no detectable organic chemicals in the surface soil.
Several years ago, a team of scientists announced signs of possible ancient life in one of the Martian meteorites. The team presented evidence of mineralized bacteria-like objects, crystals that may have been formed by
bacteria, and organic chemicals that could be bacterial decay products. This finding is still extremely controversial, and is the subject of intensive continuing research. We should note, though, that all of the
evidence for ancient Martian life was found in minerals formed when water moved through the rock billions of years ago.
If life ever existed on Mars it almost certainly required liquid water, as does all life on Earth. Water did flow at some times in Martian history, and may still exist kilometers below the ground. The search for signs of Martian
life will be guided by the search for water. Sites that have been suggested for this search include dry lake beds, ancient springs, and the deep subsurface.
Water as a Resource
Water, and the oxygen derived from it, will be required to support future human missions on Mars. Initially water and oxygen will be brought from Earth, as we did when we explored the Moon. Carbon dioxide in the
Martian atmosphere can be decomposed to produce oxygen. A reliable source of water, though, would be an invaluable resource.
One source of water is the Martian atmosphere. Researchers are developing equipment to pass large volumes of atmospheric gas through materials called zeolites. These zeolites can extract the water vapor at low
temperatures and release it for use when the temperature is raised.
An alternative is to mine ice, and possibly water, below the ground. Scientists and engineers are developing
radar and seismic tools to locate subsurface ice. They are also working on methods to drill deep into the Martian crust in search of liquid water.
What's Next?
NASA's Mars exploration strategy is built around water. Spacecraft currently at Mars, on their way to the planet, and in development promise to greatly increase our understanding of this very special substance.
Mars Global Surveyor is orbiting the planet now, collecting huge amounts of data. The spacecraft carries a high-resolution camera, an infrared spectrometer, and a laser altimeter, all of which are continually updating the
water story. You can share in many of the latest results online at:
The Mars Polar Lander is due to reach Mars on December 3, 1999. If all goes well it will land near the south pole, on a huge deposit thought to be a mixture of soil and ice. The lander will dig into the ground and, hopefully, provide our first direct evidence of ice in the Martian soil. Follow the landing and investigations at :http://mars.jpl.nasa.gov/msp98/index.html
The next opportunity to send spacecraft to Mars will be in 2001. An orbiter is being built that will carry instruments specifically designed to detect near-surface water. Instruments on the lander will measure water and a variety of other chemicals in the soil. The landing point has not been chosen, but one possible site is a region containing hematite, a mineral that is only formed in the presence of water. You can learn about the 2001 spacecraft at:
http://mars.jpl.nasa.gov/2001/index.html
Finally, in 2003 and 2005, spacecraft are scheduled to collect Martian soil and rock for return to Earth. These samples, due back in 2008, will provide our best information about water on Mars and its role in geologic history, possible life, and future human exploration.