Fluid Mystery: How Did Mars Ever Have Liquid Water?
Trying to understand the ancient climate of our own planet is hard enough, but to understand Mars' climatic history, planetary scientists have had to turn to a rather inventive method of climate forensics. Continue reading →
Trying to understand the ancient climate of our own planet is hard enough, but to understand Mars' climatic history, planetary scientists have had to turn to a rather inventive method of climate forensics.
In case you didn't get the memo, Mars used to be a lot wetter than it is now; water flowed across its surface and vast lakes - or even seas - used to cover huge swathes of land. But as the red planet's atmosphere was stripped away by the solar wind, global air pressure plummeted, leaving Mars to freeze-dry. The liquid water froze into the crust and sublimated while any atmospheric moisture was lost to space.
However, the biggest puzzle for scientists isn't necessarily why Mars is now so dry now, but how it was able to sustain liquid water on its surface at all.
In a new study published in the journal Nature Geoscience, Edwin Kite, a planetary geologist of the California Institute of Technology (Caltech), tackled the problem by first devising a novel means of measuring the thickness of the Martian atmosphere in the planet's past.
By measuring impact craters on the Martian surface, Kite was able to gauge how thick the atmosphere was in Mars' ancient past. Kite's team focused on the 3.6-billion-year-old Aeolis Dorsa region, measuring 319 craters.
As a meteorite blasts through a planetary atmosphere, the thicker the atmosphere, the greater the drag. Therefore, the impact energy of a falling space rock should relate to the thickness of the atmosphere - and therefore its atmospheric pressure.
Fascinatingly, the team found that when the impact craters were excavated, the Martian atmosphere must have had a pressure of 0.9 bar - 150 times higher that Mars' current atmospheric pressure and approximately equivalent to Earth's current sea level pressure of 1 bar. With an atmospheric pressure so high, suddenly it doesn't seem like too much of a stretch to think liquid water could have existed for extended periods of time on the surface.
But there's a problem. Mars is located 50 percent further away from the sun than Earth is, so the amount of solar energy it receives is far too low to keep any water on its surface in a liquid state. To add to the puzzling nature of Mars' wet past, the young sun was radiating even less energy in the past.
As a consequence, according to Kite, Mars would have needed to have far higher atmospheric pressures to make liquid water exist on the surface - a pressure of around 5 bar, or 5 times the Earth's atmospheric pressure at sea level.
"If Mars did not have a stable multi-bar atmosphere at the time that the rivers were flowing - as suggested by our results-then a warm and wet CO2/H2O greenhouse is ruled out, and long-term average temperatures were most likely below freezing," writes Kite and co. in their study.
If Mars was so cold and atmospheric pressures had to have been so high to keep water in a liquid state, how could Mars have accommodated liquid water at all?
In a separate paper published in the same journal, Sanjoy Som of NASA Ames Research Center outlined some possible mechanisms that may have allowed Mars to maintain its liquid reservoir of water.
Perhaps the Mars water is heavily laced in salts that lower the freezing point of water, allowing water to flow at temperatures that would have otherwise caused it to freeze. This theory has been bandied around as a possible explanation for pools of water that may be accumulating near the Martian surface. The Martian regolith is packed with perchlorates, a highly toxic oxidizing agent that could create briny pockets of liquid water.
Alternatively, periods of intense volcanic activity may have released vast quantities of greenhouse gases, incubating any surface water in a liquid state.
Som also points to "transient intervals" where cyclical changes in Mars' tilt created atmospheric conditions favorable for a thicker atmosphere. Every 120,000 years, the red planet's tilt undergoes precession, which would have influenced the quantity of sunlight hitting the poles. This cycle may have caused episodic freezing and thawing of the Martian surface water.
Although this is a puzzle, the facts are laid out in front of the Mars rovers working on the surface and orbiters that survey the planet from hundreds of miles overhead: Mars used to be a lot wetter than it is now. But how could the small world have sustained liquid water for any period of time? That's for planetary scientists to try to work out.
A high resolution digital terrain model (DTM) of an ancient river and tributaries on Mars as observed by the HiRISE camera on NASA's Mars Reconnaissance Orbiter (MRO).
The High Resolution Imaging Science Experiment (HiRISE) camera is the most powerful imager in orbit around Mars. Capable of resolving objects less than a meter wide on the surface of the Red Planet while attached to NASA's Mars Reconnaissance Orbiter (MRO), HiRISE has brought us unparallelled views of Martian landscape, geology, active erosion processes and even our own surface missions.
After nearly 8 years of orbiting Mars, HiRISE has amassed a huge archive of observations and, in many cases, observations can be combined to provide a unique insight to the planet's topography -- an observation that can be difficult to make with a single top-down snapshot.
Therefore, the HiRISE team use "stereo pairs" of observations from different orbital passes (and therefore different viewing angles) of the same locations on the Martian surface. This can produce topographical maps of surface features accurate to within 10s of centimeters in height. These high resolution digital terrain models, or DTMs, provide an incredible scientific insight as well as constructing an aesthetically pleasing perspective of an otherwise "flat" vista. In all images a color spectrum of purple-white is used, where the purple/blue hues are the lowest lying land and the red/white hues are the highest. Here are some of our favorite DTM images.
Shown here are the stunning "moving dunes" of Nili Patera (catalog number: ESP_017762_1890)
Elevation range: 55 meters (purple/blue - lowest) to 275 meters (red/white - highest) above mean Mars surface elevation.
DTMs can be very useful when trying to understand the morphology of craters on the Martian surface. This is Raga Crater, featuring very steep crater slopes in its interior (ESP_014011_1315).
Elevation range: 1,311 meters (purple/blue - lowest) to 1,966 meters (red/white - highest) above mean Mars surface elevation.
This is one of the stereo pairs of images used to compose the DTM of Raga Crater (see previous slide). Although this HiRISE image provides incredible high-resolution imagery of the feature, there is little elevation data, something the DTM provides through its topographical color spectrum (ESP_014011_1315).
The rim of Endeavour Crater in Meridiani Planum. Since 2011, Mars rover Opportunity has been extensively studying the crater's rim, turning up exciting evidence of past water on the Martian surface. The HiRISE DTMs have played a key role in mapping the rover's drive in the region (ESP_018701_1775)
Elevation range: -1,695 meters (purple/blue - lowest) to -1,380 meters (red/white - highest) below mean Mars surface elevation.
The barchan dunes on Mars can be monstrous structures. This example is nearly 300 meters high and features a steep slip face where there appear to be obvious signs of avalanches having taken place (PSP_006899_1330).
Elevation range: 1,031 meters (purple/blue - lowest) to 1,321 meters (red/white - highest) above mean Mars surface elevation.
This may look like a shooting star cartoon, but it's actually an old impact crater plus ridge of dunes in Athabasca Valles. The "tail" of material is likely caused by prevailing winds shaping the landscape (PSP_002661_1895).
Elevation range: -2,611 meters (purple/blue - lowest) to -2,441 meters (red/white - highest) below mean Mars surface elevation.
Victoria Crater in Meridiani Planum, a crater explored by Mars Exploration Rover Opportunity from September 2006 to August 2008 (PSP_001414_1780).
Elevation range: -1,453 meters (purple/blue - lowest) to -1,373 meters (red/white - highest) below mean Mars surface elevation.
Small cones in an ancient volcanic region of mars, formed by molten lava flowing over ice or water (ESP_018747_2065).
Elevation range: -3,262 meters (purple/blue - lowest) to -3,196 meters (red/white - highest) below mean Mars surface elevation.
A deep channel formed by the ancient flow of water in the Tartarus Colles Region. A small island is evident in the meandering channel (ESP_012444_2065).
Elevation range: -3,301 meters (purple/blue - lowest) to -3,189 meters (red/white - highest) below mean Mars surface elevation.
Zooming in on Gasa Crater reveals gullies formed through erosion (ESP_021584_1440)
Elevation range: -704 meters (purple/blue - lowest) to 581 meters (red/white - highest) above mean Mars surface elevation.
The "inverted valleys" near Juventae Chasma were once the floor of valleys. But over time, the topographic low regions, which are composed of material resistant to erosion (likely cemented there by water sedimentation), become ridges as the softer material around them eroded below the ancient valley floors (PSP_007627_1765).
Elevation range: 2,128 meters (purple/blue - lowest) to 2,234 meters (red/white - highest) above mean Mars surface elevation.
A well-preserved 3 kilometer-wide impact crater (ESP_012991_1335).
Elevation range: 1,114 meters (purple/blue - lowest) to 1,742 meters (red/white - highest) above mean Mars surface elevation.
A mound in Ganges Chasma. Using the topographical color reference, this feature is approximately 800 meters high from base to peak. The arcing structure around the mound may be a wind-blown ridge of material surrounding the obstacle (ESP_017173_1715).
Elevation range: -3,716 meters (purple/blue) to -2,711 meters (red/white) below mean Mars surface elevation.
Inside a crater in Western Arabia Terra with stair-stepped hills and dunes.
Elevation range: -2,575 meters (purple/blue) to -2,259 meters (red/white) below mean Mars surface elevation.
A fresh impact crater. Newly formed craters on Mars have smooth ridges and are often circular. Older craters undergo atmospheric erosion processes, often causing the ridges to appear broken, frayed and slumped (PSP_005837_1965).
Elevation range: -4,304 meters (purple/blue) to -3,658 meters (red/white) below mean Mars surface elevation.
Layered surface deposits of material in the north polar region of Mars leave a step-like pattern (ESP_018870_2625).
Elevation range: -3,555 meters (purple/blue) to -3,027 meters (red/white) below mean Mars surface elevation.
A deep fissure scars the Martian surface, a possible source of ancient floodwater (PSP_010361_1955).
Elevation range: -2,747 meters (purple/blue) to -1,577 meters (red/white) below mean Mars surface elevation.
At the base of shield volcano Ascraeus Mons' slopes in the Tharsis Region, ancient river and tributary channels carve up the landscape (PSP_002486_1860).
Elevation range: 6,432 meters (purple/blue) to 6,675 meters (red/white) below mean Mars surface elevation.
A distributary channel -- a river that branches off and flows away from a main channel -- can be seen flowing down the base of Ascraeus Mons (ESP_011373_1865).
Elevation range: 6,568 meters (purple/blue) to 6,766 meters (red/white) below mean Mars surface elevation.