NASA Spacecraft Data Suggest Water Flowing on Mars
NASA | JPL-Caltech |
Mars Reconnaissance Orbiter | 2011 Aug 04
Observations from NASA's Mars Reconnaissance Orbiter have revealed possible flowing water during the warmest months on Mars.
"NASA's Mars Exploration Program keeps bringing us closer to determining whether the Red Planet could harbor life in some form," NASA Administrator Charles Bolden said, "and it reaffirms Mars as an important future destination for human exploration."
Dark, finger-like features appear and extend down some Martian slopes during late spring through summer, fade in winter, and return during the next spring. Repeated observations have tracked the seasonal changes in these recurring features on several steep slopes in the middle latitudes of Mars' southern hemisphere.
"The best explanation for these observations so far is the flow of briny water," said Alfred McEwen of the University of Arizona, Tucson. McEwen is the principal investigator for the orbiter's High Resolution Imaging Science Experiment (HiRISE) and lead author of a report about the recurring flows published in Thursday's edition of the journal Science.
Some aspects of the observations still puzzle researchers, but flows of liquid brine fit the features' characteristics better than alternate hypotheses. Saltiness lowers the freezing temperature of water. Sites with active flows get warm enough, even in the shallow subsurface, to sustain liquid water that is about as salty as Earth's oceans, while pure water would freeze at the observed temperatures.
"These dark lineations are different from other types of features on Martian slopes," said Mars Reconnaissance Orbiter Project Scientist Richard Zurek of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Repeated observations show they extend ever farther downhill with time during the warm season."
The features imaged are only about 0.5 to 5 yards or meters wide, with lengths up to hundreds of yards. The width is much narrower than previously reported gullies on Martian slopes. However, some of those locations display more than 1,000 individual flows. Also, while gullies are abundant on cold, pole-facing slopes, these dark flows are on warmer, equator-facing slopes.
The images show flows lengthen and darken on rocky equator-facing slopes from late spring to early fall. The seasonality, latitude distribution and brightness changes suggest a volatile material is involved, but there is no direct detection of one. The settings are too warm for carbon-dioxide frost and, at some sites, too cold for pure water. This suggests the action of brines, which have lower freezing points. Salt deposits over much of Mars indicate brines were abundant in Mars' past. These recent observations suggest brines still may form near the surface today in limited times and places.
When researchers checked flow-marked slopes with the orbiter's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), no sign of water appeared. The features may quickly dry on the surface or could be shallow subsurface flows.
"The flows are not dark because of being wet," McEwen said. "They are dark for some other reason."
A flow initiated by briny water could rearrange grains or change surface roughness in a way that darkens the appearance. How the features brighten again when temperatures drop is harder to explain.
"It's a mystery now, but I think it's a solvable mystery with further observations and laboratory experiments," McEwen said.
These results are the closest scientists have come to finding evidence of liquid water on the planet's surface today. Frozen water, however has been detected near the surface in many middle to high-latitude regions. Fresh-looking gullies suggest slope movements in geologically recent times, perhaps aided by water. Purported droplets of brine also appeared on struts of the Phoenix Mars Lander. If further study of the recurring dark flows supports evidence of brines, these could be the first known Martian locations with liquid water.
Dark streaks guide search for life on Mars
Nature News | Eric Hand | 2011 Aug 04
Seasonal features could be seeping brines.
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.
Is Mars Weeping Salty Tears?
Science NOW | Richard A Kerr | 2011 Aug 04
"I'm going to hear from my colleagues: 'So, you've discovered water on Mars for the thousandth time?' " says planetary scientist Alfred McEwen of Arizona State University in Tucson. Actually, it's much better than that. Using the most powerful camera ever to orbit Mars, McEwen and his colleagues are reporting the strongest evidence yet for water on Mars that's flowing, not
frozen—and the water is flowing today,
not a millennium or an eon ago.
At a few spots, the meager warmth of martian summer seems able to coax enough liquid water out of the ground to darken the soil in streaks. The marks, which sometimes number in the hundreds, grow downhill hundreds of meters only to fade with the winter cold. And where there is liquid water, as they say, there could be life.
The newly recognized seasonal streaks are like nothing else on Mars
1, McEwen and his colleagues
reported last March at the Lunar and Planetary Science Conference—a finding that they also publish online today in Science. That's because nothing else—no other streaks or gullies—behaves as if flowing water is forming it today, they say. Seasonal streaks act as if elevated temperatures around the melting point of water ice unleash liquid water. By monitoring the same areas using the HiRISE camera aboard Mars Reconnaissance Orbiter, the team watched the shapes cycle with the seasons. Dark streaks a few meters wide grew from rocky outcrops down steep, equator-facing slopes beginning in the martian spring and continuing until the early fall. In the colder seasons, the streaks faded away.
Measurements made by another orbiter show that areas marked by seasonal streaks are basking in solar warmth like sunbathers trying to catch the maximum rays on a cool spring day. As a result, surface temperatures rise to within 20°C to 30°C of the melting point of ice when seasonal streaks are forming. Given that the Phoenix lander found abundant salts in martian soil that can lower the freezing point of any water present, the observed near-freezing temperatures look sufficient to allow flowing, briny water on or just beneath the martian surface.
"This may be our best evidence yet of liquid water emerging on the surface of Mars," says planetary scientist Oded Aharonson of the California Institute of Technology in Pasadena. Flowing water is not yet proven, he says, but "I would bet my bike on this, and probably my car, but I wouldn't bet my house on it."
"It's exciting," says planetary scientist Joseph Levy of Portland State University in Oregon. "Mars looks more like the Dry Valleys of Antarctica every day." That's the largely ice-free part of the continent where trickles of summertime meltwater can dampen slopes and support a hardscrabble microbial population, including algae, just beneath the surface. A damp subsurface is where life is most likely to be found today on Mars, notes McEwen. "Now we see perhaps some groundwater is coming to the surface. It gives us a location to focus on" as NASA continues to "follow the water" in search of martian life, past or present.
- Seasonal Flows on Warm Martian Slopes - AS McEwen et al
- Radar Sounding Evidence for Buried Glaciers in the Southern Mid-Latitudes of Mars - JW Holt et al
- Present-Day Impact Cratering Rate and Contemporary Gully Activity on Mars - MC Malin et al
Near-tropical subsurface ice on Mars - M Vincendon et al
Water ice at low to midlatitudes on Mars - M Vincendon et al
Briny Water May be at Work in Seasonal Flows on Mars
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