These dark streaks on Horowitz Crater may be caused by salty water. Credit: Science/AAAS

The story of water on Mars seems to be getting wetter, saltier and altogether more juicy.

NASA scientists have found evidence for liquid brines near the planet's surface that might provide a habitable zone for microbes today. If the discovery holds, it should guide future missions to Mars in the quest to find out whether the red planet can support life.

Images from the High Resolution Imaging Science Experiment (HiRISE), a camera aboard the Mars Reconnaissance Orbiter (MRO), have revealed thousands of narrow, dark streaks that appear on some of Mars's steep slopes during warmer seasons.

The streaks appear on equator-facing slopes in the mid-latitudes of the southern hemisphere, and can grow by as much as 20 metres a day. Temperatures in this part of the planet can rise as high as 27ºC during warm seasons. That's easily warm enough for water to exist, especially if it contains salts, which lower the melting temperature of ice. By winter, the streaks have faded or vanished.

HiRISE principal investigator Alfred McEwen, a planetary scientist at the University of Arizona in Tucson, thinks that the most likely explanation is that the streaks are formed by briny water oozing downhill in small, steep channels.

He says that the streaks are the strongest evidence yet for the existence of liquid water on Mars today. "This can focus the search for extant life on Mars," he says. He and his colleagues report their findings today in Science1.

Liquid presence

The public has grown accustomed to hearing about water-related discoveries on Mars, but few researchers have suggested that liquid water currently exists at the planet's surface. In the ancient past, Mars was much warmer and wetter, and scientists suspect that rivers once flowed freely into lakes and even oceans. Today, most of the water that remains on the planet is locked up as ice, in the visible polar ice caps and in sub-surface glaciers2.

There is also some water vapour in the thin atmosphere, but freezing temperatures and low atmospheric pressures mean that water is barely stable at the planet's surface.

McEwen is quick to describe his group's results as indirect; images obtained from orbit have fooled planetary scientists before. In 2006, for example, scientists thought that the change in colour of a few gullies suggested that they were being actively carved by water3. The phenomenon is now generally thought to result from the cyclical freezing and sublimation of carbon dioxide.

Another reason for caution is that a second instrument on the MRO found no spectroscopic signal associated with water. However, the instrument's resolution was much larger than the streaks, and the streaks' southern location, in spots too warm for carbon dioxide to freeze, along with their seasonality — too regular for wind action to be responsible — still make water the most likely cause.

McEwen says that the water could sit just below the planet's surface. As it evaporates, it might leave pits and pockets in the soil, creating the dark colour seen from orbit by subtly altering the streaks' surface texture.

A closer look

McEwen hopes that follow-up observations will confirm his finding, but where these might come from is unclear. Curiosity, the car-sized rover due for launch later this year, will head to Gale Crater. The site does not contain any such streaks, and if it did Curiosity would be forbidden from landing close by, says John Grotzinger, project scientist for the mission at the California Institute of Technology in Pasadena. The rover is not designed to be sterile, so planetary-protection rules forbid it from landing where ice might lie near the surface, in case the rover is contaminated with microbes from Earth.

The Trace Gas Orbiter (TGO), a joint NASA–European Space Agency mission planned for launch in 2016, might not help either, says project scientist Mark Allen of the Jet Propulsion Laboratory in Pasadena. The satellite will carry spectrometers capable of detecting atmospheric gases at very low levels, but it would struggle to find such a faint, seasonal evaporation of water in an atmosphere that already contains water vapour.

However, if microbes or geological processes associated with the water were producing more exotic gases, such as methane or sulphur dioxide, then the TGO would be the mission to detect those, Allen says.

Further study of the streaks may have to wait for a visit from a rover planned for launch in 2018, which will meet strict requirements for sterility. To cut costs, the European Space Agency and NASA are trying to combine the capabilities of two rovers into one vehicle: the European ExoMars would drill below the surface and test samples for life; meanwhile NASA had begun designing a rover to cache samples as a first step towards bringing them back to Earth.

Many of the streaks sit on the sheer walls of crater rims — challenging places for a rover to access, says McEwen. But he has identified a few spots, such as Horowitz Crater, where a rover could reach the base of some streaks — and hopefully the bottom of the mystery.

Credit: Science/AAAS