Views from the Viking Orbiters: centered on the Schiaparelli Crater (left) and centered on Valles Marineris (right).
Mars is about half the diameter of the Earth and has 1/10th the Earth's mass. Mars' thin atmosphere (just 1/100th the Earth's) does not trap much heat at all even though it is 95% carbon dioxide (CO2). The other 3% is nitrogen (N2). Because the atmosphere is so thin, the greenhouse effect is insignificant and Mars has rapid cooling between night and day. When night comes the temperature can drop by over 100 K (180° F)! The large temperature differences create strong winds. The strong winds whip up dust and within a few weeks time, they can make dust storms that cover the entire planet for a few months. Two "before-after" image sets are shown below. The first pair is from the Mars Global Surveyor of the Tharsis bulge side of the planet. The "before-after" images are about 1.5 months apart. The second pair is from the Hubble Space Telescope (HST) of the other side of the planet. The "before-after" images are about 2.5 months apart and the truly global dust storm was still going on. You can see the dust storm beginning in the HST image in the left image in the Hellas Basin at about the 4 o'clock position.
Color view of river drainage system from Viking orbiter on left and black-and-white zoom-in of Nanedi Vallis canyon using Viking (wide-field view) and Mars Global Surveyor (magnified view) images on right. |
 |
In Eberswalde Crater is a "fossil delta" that shows that Mars experienced ongoing, persistent flow of water over an extended period of time. It also shows that some sedimentary rocks on Mars were deposited in a water environment. Select the image to find out more details leading to these deductions. |
Fossil delta in Jezero Crater. Color-enhancing shows the clay-like minerals (green here) that were carried into the lake, forming the delta. The clays would be a good place to look for signs of ancient life. |
What makes Mars so intriguing is that there is evidence for sustained running liquid water in its past. Some geologic features look very much like the river drainage systems on Earth and other features points to huge floods. The Mars Pathfinder studied martian rocks in the summer of 1997 and found some rocks are conglomerates (rocks made of pebbles cemented together in sand) that require flowing water to form. Abundant sand also points to widespread water long ago. More recently, the larger and more advanced Mars Exploration Rovers (one called "Spirit" and the other called "Opportunity") have further strengthened the conclusion that there was liquid water on Mars in the past. Highly magnified images of the rocks examined by Opportunity (see image below) show a particular type of rippling patterns on the rocks that are formed under a gentle current of water instead of wind. In the image below the green stripes show the sedimentary layers laid down in flowing water and the blue lines show the boundaries between the layers. Furthermore, detailed chemical analysis of the compositions of the rocks by Opportunity show that they formed in mineral-rich water when the water got very concentrated with the minerals as the water evaporated (spectrum 1, spectrum 2, spectrum 3, spectrum 4, spectrum 5, spectrum 6 links).
There is evidence for liquid water flowing relatively recently. The Mars Global Surveyor found places where gullies are etched into the sides of craters that themselves have very few smaller impacts inside of them. That means the crater walls are geologically young, so the gullies have to be even younger still. The orbiter also found gullies where bright new deposits were seen in images taken just four years apart from each other that might be the result of water carrying sediments down the sides of the craters for a short time. More recently, the team of scientists working with the Mars Reconnaissance Orbiter (MRO) released images of seven craters where thin (0.5 to 5 meters wide) dark streaks are seen to flow down steep slopes repeatedly during the summer seasons when temperatures on the surface there can reach -23º to +27ºC (-10º to +80ºF). In an effort to not bias our interpretation of them, the team has called the features "recurring slope lineae". There are 12 to 20 other sites the team is keeping a close eye on to see if they too have the repeated dark flows.
Select image for other craters with time-lapse images from the MRO press release
Mars' surface air pressure is much too low for pure liquid water to exist now. At very low pressure, water can exist as either frozen ice or as a gas but not in the intermediate liquid phase. If you have ever cooked food at high elevations using boiling water, you know that it takes longer because water boils at a lower temperature than at sea level. That is because the air pressure at high elevations is less. If you were several miles above the Earth's surface, you would find that water would boil (turn into steam) at even room temperature! However, if the liquid is very salty water, then it may be able to exist long enough to flow partway down the crater walls before freezing or evaporating. The dark streaks that grow during the warm summer time could be the result of liquid brines near the surface of Mars breaking through to the surface. Because of the widespread presence of salts on the surface of Mars, even pure water from below would get mixed with the salts. Unfortunately, the spectrometer on board MRO does not have sufficient spatial resolution to analyze the very narrow dark streaks. To strengthen the argument for these being the result of salty water flowing downhill, research teams on Earth will simulate the conditions (soil composition, low air pressure, and temperature) in the lab to see what all could create the dark streaks.
Where there is liquid water, there is the possibility for life to arise. Tiny structures in a meteorite that was blasted from Mars in a huge impact of an asteroid look like they were formed by ancient simple lifeforms. However, there is still a lot of debate among scientists on that but strong evidence of contamination by terrestrial organic molecules has probably killed the possibility of conclusive proof of martian life in the meteorite. The dissenters are not wanting to be party poopers. They just want greater certainty that the tiny structures could not be formed by ordinary geologic processes. The great importance of discovering life on another world warrants great skeptism---``extraordinary claims require extraordinary evidence''. The search for martian life will need to be done with a sample-return mission or experiments done right on Mars.
The Mars Science Laboratory, "Curiosity", scheduled to launch in November 2011 and begin driving around Gale Crater near the equator of Mars in August 2012 will have a suite of instruments for identifying organic compounds such as proteins, amino acids, and other acids and bases that attach themselves to carbon backbones and are essential to life as we know it. It will also be able to detect gases that could be the result of biological activity. It will identify the best possible sites of biological activity (past or present) for a follow-up mission to more definitely confirm. Gale Crater is 155 kilometers (96 miles) across with a large mountain inside it that appears to be the remnant of an extensive series of deposits. The layers at the base of the mountain contain clay and sulfates that very likely formed in liquid water.
The fact that Mars had sustained liquid water in the past tells us that the early Martian atmosphere was thicker and the surface was warmer from the greenhouse effect a few billion years ago. Some of the topics for follow-up research are: how long was there liquid water present on the surface; when did the liquid water disappear from the surface; how widespread was the liquid water; how much liquid water was there; and were there repeated episodes of liquid water appearing and then disappearing.
Life may have started there so current explorations of Mars are focussing on finding signs of ancient, long-dead life. Any lifeforms living now would have to be living below the surface to prevent exposure from the harsh ultraviolet light of the Sun. Mars has no protective ozone layer, so all of the ultraviolet light reaching Mars can make it to the surface. The Viking landers that landed in 1976 conducted experiments looking for biological activity, past or present, in the soil but found the soil to be sterile with no organic matter (in the top several centimeters at least). The soil is more chemically reactive than terrestrial soil from the action of the harsh ultraviolet light. More recently, the Phoenix mission described below in the "Ice on Mars" section may have found another reason for the lack of organic matter in the soil: perchlorate in the soil would broken down any organic compounds that would have been in the soil when the soil was heated up during the Viking experiments. In any case, Mars appears to have undergone significant global change. What changed Mars into the cold desert of today?
 |
 |
Three views of Mars from three generations of Mars explorers: Viking 2 lander
on top
left,
Mars Pathfinder on top right,
and Opportunity on bottom (much less rocky terrain---long picture!).
Mars Exploration Rover Sites movie (select the link to view a Flash movie showing where the Mars Explorations Rovers landed)
The runaway refrigerator is described in a flowchart on the Earth-Venus-Mars page. The flowchart up to the last dashed arrow occurred a LONG time ago. The box at the end describes the current state: frozen water and carbon dioxide below the surface and a very thin atmosphere.
Human explorers will need to use spacesuits on Mars' surface. The low pressure would kill them in a fraction of a second without something to provide an inward pressure on their bodies (far faster than movies like Total Recall would have you believe, but moviemakers don't want to kill off the bad guy too quickly and Arnie must survive!). Explorers will also need to contend with temperatures that are way below the freezing point of water even during the day and have enough shielding to block the abundant ultraviolet light from the Sun.
One of the predictions of the runaway refrigerator is that there should be water ice below the surface. Mars does have polar ice caps made of frozen carbon dioxide ("dry ice") and frozen water, but is there frozen water below the surface away from from the polar ice caps. Yes.
Yuty Crater is a type of crater called a rampart crater (or "splash crater") because of the distinctive ridges along the edge of the "fluidized" ejecta. This image from the Viking 1 orbiter as well as others it took of craters in the surrounding area show features formed when frozen (or liquid?) water was melted and mixed with the dirt and rocks to flow like mud upon impact (see also the image of Yuty Crater from the Mars Global Surveyor and a high-resolution image of the fluidized ejecta flow or link 2).
The Mars Odyssey spacecraft orbiting Mars found that the highest concentrations of the sub-surface ice are near the poles from about latitudes 60 degrees and higher. The Phoenix Mars Lander that landed at the end of May 2008 scooped up ice and soil at its landing spot near the martian north pole south of the north polar cap. It uncovered water ice just a few centimeters below the surface. Phoenix also ran tests to see if the soil & ice could be habitable for microbial life. Phoenix did not have the capability to detect biological activity; it could only determine if life could exist in the soil at its landing spot. The answer: maybe. Phoenix found perchlorate that is toxic to most Earth life but it can be food for some types of microbes. Water vapor from the atmosphere could attach to the perchlorate to form a thin film layer of water for biological activity. In addition perchlorate in water can lower its freezing temperature enough to keep it liquid even in Mars' cold temperatures.
"Holy cow!" image from the robotic arm camera of the Phoenix Mars Lander shows what looks like ice uncovered by the rocket exhaust upon landing. |
Phoenix uncovered ice in this trench called "Dodo-Goldilocks" that sublimated (from ice directly to water vapor) over the course of four Mars days. The ice lumps seen in the lower left corner on day 20 are no longer there on day 24. If the ice was carbon dioxide, they would have sublimated away in less than a day at the temperatures of Phoenix's location when the trench was dug. |
The image above and others from the Mars Reconnaissance Orbiter show that frozen water is just below the surface at latitudes closer to the equator than thought possible before. The bright areas in the upper panels are are sub-surface water ice freshly exposed by meteorite impacts. They fade over 15 weeks as the water ice sublimates (bottom panels). See the MRO video archive for a nice video about this particular discovery (choose the September 24, 2009 video). These craters were near the location of the Viking 2 lander and the scientists figured out that if Viking 2 had been able to dig just 15 cm (6 in) deeper, it would have found the ice (33 years before Phoenix).
The runaway refrigerator theory recently received further support when the Mars Reconnaisance Orbiter found places where deposits of iron and calcium carbonates had been uncovered at large impact sites great distances apart from each other. These types of carbonates form most easily in the presence of large quantities of liquid water and fit the runaway refrigerator idea of atmospheric carbon dioxide dissolving in bodies of liquid water. The carbonate layers are buried under a few miles (about 5 kilometers) of younger rocks, including volcanic flows, similar to what Spirit found when exploring Gusev crater. At Gusev Crater, Spirit had to climb the hills near where it landed to find the older minerals that formed in the presence of liquid water sticking above the surrounding crater floor covered in lava flows (see "Panel 1" of the "Follow the Water" forum). Large impacts are able to uncover the deeper carbonate layers, so MRO will explore other large impact craters closely to see how widespread the buried carbonate layers are.
And just for fun: Zoom in to the "Face on Mars". The "Face on Mars" is a remnant massif that attracted a lot of attention in the 1990s when a Viking 1 Orbiter picture of it, that made it appear like a face had been carved onto the martian surface, circulated around the web. Those with an over-active imagination thought the feature was artificial (made by ancient martian astronauts) and conspiracy theories were created of a NASA cover-up. The Viking image has poor resolution, poor lighting, and a number of "bit errors" that create black speckles, a couple of which were located at the feature's "nostril" and a "dimple" on its chin. Later much higher resolution images from the Mars Global Surveyor and Mars Express spacecrafts show it to be a naturally-occuring geologic formation. The "`Face on Mars' Zoom In" page shows where the "Face" is on Mars and increasingly higher-resolution views of the feature.
Finally, my notes from the January 13, 2011 update on the Mars Program.
Go back to previous section --
Go to next section
last updated: October 22, 2011
