How scientists concluded there is water on Mars

As scientist looked at photos of seasonally occurring streaks on Mars' surface, they ruled out a number of options before concluding the most likely cause of the streaks was salty water.
Written by Laura Shin, Contributor

In case you haven't heard the news, new photos of Mars taken by NASA's Mars Reconnaissance Orbiter (MRO) have led scientists to believe there is salty water on Mars.

How did scientists, looking at some photos, conclude that dark streaks that occur seasonally on Mars are most likely caused by briny water?

The photos show dark, narrow clusters of streaks on slopes appearing during spring across several midlatitude locations in the southern hemisphere. They grow in length down slopes during the warmer months and then disappear during colder weather. They also tend to form on slopes that face Mars' equator.

They can be hundreds of feet long, but they are only about a foot-and-a-half to 16 feet wide.

Scientists considered that they could be dry phenomena such as rock slides, but the timing of their appearance and the way they grow seem incompatible with that theory.

They also considered that the streaks were made by volatile chemicals, such as freshwater or carbon dioxide, that boil at low temperatures. (Compared to Earth, Mars has a weak atmospheric pressure -- less than 1% that of Earth's -- which means that water at these sites on Mars would actually boil at these temperatures.) But the temperatures at these locations were too high for carbon dioxide to be in the form of frost, as well as too low for freshwater ice to melt.

However, the conditions could be right for salt water, because it remains liquid at the low temperatures of summer on Mars.

"Mars is just a very salty place," lead study author Alfred McEwen, a planetary scientist at the University of Arizona in Tucson, told Scientific American. "Any water that flows at the surface or subsurface gets salty."

As briny water flows, it could rearrange grains or change the surface in a way makes it appear darker, although the darkness would not necessarily indicate that the streaks are wet.

Brines that could exist at the temperatures in these spots on Mars include magnesium chloride, sodium chloride, calcium chloride or iron sulfates.

Calling these lines the noncommittal phrase "recurring slope lineae," or RSL, scientists announced their findings in the August 5 issue of Science.

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via: Scientific American, CBS News, Los Angeles Times

Photo: NASA/JPL-Caltech/Univ. of Arizona

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