By Sabine Stanley, Ph.D., John Hopkins University
Mars is a red desert with no liquid water today, but there is water ice at the poles. The ice, the icy soil under the surface called permafrost, the large crater-like places with evidence of past water, and some other phenomena suggest that Mars used to look very different some billion years ago. What happened then?
Liquid Water on Mars
Mars is a dry and cold planet with no large bodies of liquid water. The temperature and the surface pressure are too low to keep liquid water on the surface, but water ice has never been difficult to find.
Mars has ice caps at the poles. Besides, there is probably a significant amount of permafrost–icy soils–a little below the surface. If the permafrost is brought to the surface, it can cover the whole planet in a 35-meter layer! The Mars Phoenix lander even got a sample from the frost. Despite all the difficulties in finding liquid water, some temporary water has been spotted on the surface of Mars.
The Mars Reconnaissance Orbiter has sent many images from Mars at different seasons. They show that dark streaks appear on some crater rims when the walls of the crater are warmed in Martian spring and summer. These dark streaks are periodical and fade when the season changes.
These recurring slope lineae may be the result of liquid water running down the craters when the ice just below the surface melts. How does liquid water run on Mars under such a low temperature and pressure? The running water must be too salty to freeze quickly in the warmer seasons.
Stable liquid water is required for life to form, and this might be found under the polar ice caps. There are some hints to the existence of a lake, 20 kilometers across and buried one and a half kilometers below the surface, under the southern pole ice cap. If Mars lacks so many elements for life to form, why do scientists still look for it?
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Dried-Up Rivers and Valleys
Mars used to be much wetter, warmer, and had a thicker atmosphere. There are shapes on the surface indicating past lakes on Mars and even waterfalls, showing that surface water was a common trait billions of years ago. This type of evidence is called geomorphic evidence and shows more than 40,000 dried-up river valleys.
In Kasei Valles, there are dried riverbeds of about 2400 kilometers long and giant dried-up waterfalls, ten times higher and 100 times wider than Niagara Falls. Dendritic, branching networks, and tributaries are commonly seen in other dry river valleys.
There are also signs of streambeds, where round and angled pebbles and gravel indicate erosion by fast-moving liquids. The water in the Gale crater must have been between ankle and hip deep, flowing at about one meter per second.
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Past Lakes on Mars
Besides rivers and waterfalls, Mars also used to have lakes. The structure found in some craters, such as Eberswalde and Jezero, can form only in deep water lakes. The fan-shaped structure in the Jezero crater is sediment transported from a fast-moving river into the bottom of the slow-moving lake water. Similar structures on Earth are found in the Mississippi Delta, where the Mississippi River meets the Gulf of Mexico, or the Ganges Delta in India and Bangladesh.
Curiosity rover saw layered structures at these dry lakebeds. These layers have also been formed in the presence of water as the sediments sank to the bottom at different times. Did the past lakes on Mars and the dried-up rivers and waterfalls lead to a big ocean?
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The Giant Ocean in the North
Mars’s northern hemisphere was once fully covered under a huge ocean. The Mars Global Surveyor mission sent data required for a topographic map of Mars. The northern hemisphere is mostly two to three miles lower than the average planetary elevation and is smoother.
On Earth, the only place with such criteria is the abyssal plains at the bottom of the oceans. Another evidence is the mismatch of gravity data and images. There must be circular structures according to the gravity data, but none is seen in the northern hemisphere. Perhaps, water has changed the structure through the years.
Mars is not only filled with impact craters, but it also has a significant number of past lakes, waterfalls, rivers, and a huge ocean that might have left some evidence of life behind.
Common Questions about Lakes on Mars
Mars has no liquid water today, but there is evidence of past lakes on Mars. The dry lake beds hold evidence, such as hydrated minerals, that Mars most probably used to have liquid water like lakes, oceans, and even waterfalls.
Mars has frozen water ice at its poles and icy soil, called permafrost, under its surface.
There is evidence of past waterfalls and lakes on Mars, but not liquid water anymore. The remaining water ice can be an indicator of past liquid water in different forms.
Mars has no liquid water on its surface, but there is water ice at its poles and water frost under its surface. There was water above and under the surface, as the evidence of past lakes shows, but now all that remains in this red desert is some ice and icy soil, and maybe some water under the caps.