On August 5, 2011, Science Magazine published a paper, announcing that astronomers had observed unusual features on the northward slopes of cliffs in Mars’ southern hemisphere. Strangely, these narrow, dark features are seasonal; they appear in spring and summer and disappear as fall approaches. So far, the explanation that best fits the evidence is that briny water sometimes flows on Mars.

Fourth Planet From The Sun

Mars, the fourth planet from the Sun, is just over half as big across as the Earth and almost one-ninth as massive. Mars takes almost two Earth years (687 days to be exact) to complete an orbit which, on average, is half again as big as ours.  However, Mars’ orbit is over five and a half times as eccentric, or out-of-round, as ours.  Unlike on Earth, then, the variation in Mars’ distance from the Sun is significant enough to influence its climate.

Similarly to Earth, Mars rotates on its axis once every 24.6 hours.  This axis is tilted by about 24.5 degrees, giving Mars seasons similar to those on Earth, whose axis is tilted by 23.5 degrees.  In its interior, Mars has no liquid outer core and therefore lacks a global magnetic field to deflect the solar wind away from its atmosphere.

photo credit: Bluedharma

These factors contribute to a climate where pure liquid water is highly unstable and cannot persist for long.

Due to its greater distance from the Sun, Mars is much colder than Earth.  Mars Global Surveyor measured temperatures ranging from zero to -113 degrees Celsius (32 to -170 ^oF).  Temperatures at the surface can be above freezing in summer, particularly in Mars’ southern hemisphere, because summertime there coincides with perihelion.  Even so, temperatures just one meter above the surface can be cooler than on the ground, as measured by Mars Pathfinder.  Nighttime temperatures at the poles can approach -200 ^oF, colder than the coldest temperature ever recorded in Antarctica (-129 ^oF).

Further, lack of a magnetic field means that Mars was unable to retain much of an atmosphere. 

Earth’s atmosphere is over 200 times as massive as that of Mars.  Although it is over 95% composed of carbon dioxide, such a tenuous atmosphere produces no significant greenhouse effect to raise temperatures on Mars.  Except at the very lowest elevations, the very thin Martian atmosphere exerts a pressure lower than the triple point of water; even on rare occasions when the temperature might be above freezing, ice sublimates rather than melting.

Why Salt Water Might Exist On Mars

Salt water, however, freezes at a lower temperature than pure water, and thus might remain liquid for brief periods on Mars.  Salt water (or brines), then, might explain observations made by NASA’s Mars Reconnaissance Orbiter, which has orbited Mars since November 2006.  Images from this orbiter’s High Resolution Imaging Science Experiment show features called Recurring Slope Lineae (RSL).  They are in Mars’ southern hemisphere, on the north (equator facing) slopes such as crater walls.  The RSLs appear in local springtime and persist until local autumn, when they vanish.  Only 0.5 meters to 5 meters wide, they can become hundreds of meters long during Mars’ southern summer.

Life Requires Water

We have always been interested in finding signs of water on planets other than Earth because life as we know it requires water to exist.  As the only other planet in our solar system with conditions even approaching ours, Mars is among the first places we looked for life beyond Earth.  The search for water, and thus possible life, has been a major goal of our robotic missions to Mars, including the Viking program in the 1970′s.

We have established that H2O exists on Mars in other phases. 

Mars’ polar caps are cold enough for dry ice (frozen carbon dioxide), but they contain water ice as well.  In 2005, the European Space Agency’s Mars Express satellite snapped a picture of an ‘ice lake’ in the bottom of a crater near Mars’ north pole. In 2008, the Phoenix lander revealed water ice in trenches dug by its robotic arm.  In 2004, the rover Opportunity took photos of water ice clouds and was also at times covered in frost, indicating water vapor had frozen onto the rover.

For liquid water, however, scientists had looked into Mars’ past.  Vastitas Borealis, a huge region between four and five kilometers lower than the mean elevation of Mars, fills much of Mars’ northern hemisphere.  It is also flatter, with craters in the Vastitas Borealis much rarer than on the rest of Mars.  Many scientists subscribe to the Mars Ocean Hypothesis, which posits that about 3.8 million years ago, Vastitas Borealis was the site of a vast ocean covering about one-third of Mars’ surface, which then either evaporated or froze into the ground as Mars’ environment ceased to support large bodies of liquid water.  For one thing, the size and shape of craters in Vastitas Borealis suggest that sublimation of water ice played a role in weathering them.  Also, there are networks of valleys resembling river systems on Earth, as if they once flowed into the ancient ocean.

A large water ocean could have persisted on early Mars

To support this ancient ocean, there are indications that Mars once had enough carbon dioxide to exert up to one bar of atmospheric pressure.  Under this higher pressure, combined with higher temperatures (due to the greenhouse effect) a large water ocean could have persisted on early Mars.  However, Mars lacks a global magnetic field; its atmosphere interacts directly with the solar wind.  This interaction would have gradually dispersed Mars’ atmosphere into space, a process we can observe today.  As the atmosphere went away, Mars would have lost its ocean due to the end of the greenhouse effect and the lower air pressure.

The existence of this ancient water ocean on Mars is not yet fully established; competing explanations such as wind erosion or liquid methane are not yet excluded.  Still, a former ocean 3.8 milllion years in Mars’ past has been our best bet for liquid water on the Red Planet.

Liquid Water on Mars Today

Until now, that is.  If confirmed by future observations, August 2011 would mark the first published evidence of liquid water on Mars today.  Yet many questions remain.  We know neither the source of the salty water, nor the precise mechanism that brings it to the surface, not to mention whether or not such water might contain germs.  So, the science we’re doing  at Mars continues, as the new questions raised make Mars an even more fascinating world to study.