* Astronomy

A new model of the ancient Martian climate has revealed that the glacial deposits of the planet’s tropics were laid down by snow carried to equatorial regions by monsoon winds. The findings, published today in Science, resolve the mystery about the source of the rocks and debris at the foot of Mars’s tropical mountains and volcanoes spotted by the Viking mission in 1976.

In the last two years, Brown University planetary geologist James Head and other Mars experts have offered up mounting evidence that these ice-rich landforms – which appear to ooze out of valleys in the Eastern Hellas region or puddle on the western flanks of the three giant volcanoes known as the Tharsis Montes – are the remnants of geologically recent glaciers.

But how could ice form so far from the planet’s poles? Long-ago landslides? Upwelling from an underground reservoir?

"What we found was that the glaciers were formed from snow brought from the polar regions" - James Head.



A few million years ago the axis of Mars was tilted in such a way that the poles were pointing dramatically closer to the sun. Sun rays hit the polar ice caps nearly head on, releasing massive amounts of water vapour into the atmosphere. Monsoon-like winds carried the water vapour south, up and over the soaring slopes of the Tharsis Montes volcanoes and Olympus Mons, the solar system’s largest volcano. The vapour cooled, condensed and fell in the form of snow. Over time, the snow turned to ice, the ice formed glaciers, and the glaciers created the deposits seen today.
The Martian precipitation cycle described in Science is similar to the one on Earth that routinely blankets mountainous regions such as the Rockies in snow. Another Earthly analogy: the tropical mountain glaciers described in the article can be found in places such as Mount Kilimanjaro in Africa or the Andean peaks in South America.



The team arrived at their finding using a climate model that simulated the present-day Mars water cycle but assumed a 45-degree axial tilt found on the planet millions of years ago. The model created a near-perfect match of predicted ice accumulation and direct observational evidence from images taken by the Mars Express, Mars Global Surveyor and Mars Odyssey orbiters.

"The findings are important because they tell us that Mars has experienced big climate changes in the past, the kinds of climate change that led to the Great Ice Age here on Earth. The findings are also interesting because this precipitation pattern may have left pockets of ice scattered across Mars. This is good information for NASA as officials plan future space missions, particularly with astronauts" - James Head , Louis and Elizabeth Scherck Distinguished Professor at Brown.

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GEOLOGIC INVESTIGATIONS OF THE EASTERN HELLAS REGION OF MARS: PAST, PRESENT, AND FUTURE. (PDF)

-- Edited by Blobrana at 03:06, 2006-01-20

University of Arkansas researchers have become the first scientists to show that liquid water could exist for considerable times on the surface of Mars.

Julie Chittenden, a graduate student with the Arkansas Centre for Space and Planetary Sciences, and Derek Sears, director of the Space Centre and the W.M. Keck Professor of Planetary Sciences, will report their findings in an upcoming issue of the Geophysical Research Letters.

"These experiments will help us understand how water behaves on Mars" - Julie Chittenden.

Researchers have debated whether or not liquid water could exist on the surface of Mars because of the low temperatures and pressures found on the planet. Based on previous experiments and hypotheses, scientists have speculated that pure water on the planet's surface would evaporate from solid to gas, bypassing the liquid phase, at the low pressures found on Mars - 7 millibars as opposed to about 1,013 millibars on Earth. However, the planet's surface sports features like gullies and channels that look as though they might have been created by the movement of liquid. Terrestrial experiments designed to simulate Mars-like conditions have been performed to help answer this question of whether or not liquid water exists on Mars, but until this point they have only been done with pure water at high pressures.


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Chittenden and Sears used a planetary environmental chamber in the W.M. Keck Laboratory for Space Simulation to simulate the conditions found on Mars - an atmosphere of carbon dioxide, 7 millibars of pressure and temperatures from zero degrees Celsius to 25 degrees below - and examined the evaporation rates of brine solutions expected to be found on Mars. Most water on Earth contains salts that leech into the water when it comes in contact with soil, and similar processes might be expected to occur in any surface water found on the Red Planet. Salts in the water lower the freezing point of the solution.

The University of Arkansas team placed the salt solutions in the planetary environmental chamber simulating Mars-like conditions, and then measured the evaporation rates at varying temperatures.

"There's a huge decrease in the evaporation rate the colder it gets, more than anyone realized" - Julie Chittenden.
With the dissolved sodium and calcium in the water, the freezing point for the brine mixtures drops to 21 degrees below zero Celsius for salt water and 50 degrees below zero for water containing calcium chloride.

Temperatures on Mars vary between 125 degrees below zero Celsius and 28 degrees above at different latitudes and different times of the day. Thus, there is a possibility that liquid water could exist on the planet's surface at different locations and times of day.

"Brine formation could considerably increase the stability of water on Mars by both extending the temperature range over which liquid water is stable to negative-40 degrees Celsius and by decreasing the evaporation rates by two orders of magnitude" - the researchers.

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A large team of NASA scientists, led by earth and planetary scientists at Washington University in St. Louis details the first solid set of evidence for water having existed on Mars at the Gusev crater, exploration site of the rover Spirit.

Using an array of sophisticated equipment on Spirit, Alian Wang, Ph.D., Washington University senior research scientist in earth and planetary sciences in Arts & Sciences, and the late Larry A. Haskin, Ph.D., Ralph E. Morrow Distinguished University Professor of earth and planetary sciences, found that the volcanic rocks at Gusev crater near Spirit's landing site were much like the olivine-rich basaltic rocks on Earth, and some of them possessed a coating rich in sulphur, bromine, chlorine and hematite, or oxidized iron. The team examined three rocks and found their most compelling evidence in a rock named Mazatzal.

The rock evidence indicates a scenario where water froze and melted at some point in Martian history, dissolving the sulphur, chlorine and bromine elements in the soil. The small amount of acidic fluids then react with the rocks buried in the soil and formed these highly oxidized coatings.



During its traverse from landing site to Columbia Hills, the rover Spirit dug three trenches, allowing researchers to detect relatively high levels of magnesium sulphate comprising more than 20 percent of the regolith - soil containing pieces of small rocks - within one of the trenches, the Boroughs trench. The tight correlation between magnesium and sulphur indicates an open hydrologic system - these ions had been carried by water to this site and deposited.

Spirit's fellow rover Opportunity earlier had detected a history of water at another site on Mars, Meridiani planum. This study (by Haskin et al.) covered the investigation of Spirit rover sols 1 through 156, with the major discoveries occurring after sol 80. After the findings were confirmed, Spirit traversed to the Columbian hills, where it found more evidence indicating water. The science team is currently planning for sol 551 operation of Spirit rover, which is only 55 meters away from the summit of Columbia Hills.

Spirit on sol 597 on Sept 6 and on the summit of Husband Hill.

"We will stay on the summit for a few weeks to finish our desired investigations, then go downhill to explore the south inner basin, especially the so-called 'home-plate,' which could be a feature of older rock or a filled-in crater. We will name a major geo-feature in the basin after Larry" - Alian Wang.

Wang, Haskin, their WUSTL colleague Raymond E. Arvidson, chair of earth and planetary sciences, and James S. McDonnell Distinguished University Professor, and Bradley Jolliff, Ph.D., research associate professor in earth and planetary sciences, and more than two dozen collaborators from numerous institutions, reported their findings in the July 7, 2005 issue of Nature magazine (Larry A. Haskin et al. Nature 436, 66-69 (7 July 2005) doi:10.1038/nature03640). The paper was the last one that lead author Haskin, a highly regarded NASA veteran and former chair of earth and planetary sciences at WUSTL, submitted before his death on March 24, 2005.



"We looked closely at the multiple layers on top of the rock Mazatzal because it had a very different geochemistry and mineralogy.
This told us that the rock had been buried in the soil and exposed and then buried again several times over the history. There are chemical changes during the burial times and those changes show that the soil had been involved with water.
The telltale thing was a higher proportion of hematite in the coatings. We hadn't seen that in any previous Gusev rocks. Also, we saw very high chlorine in the coating and very high bromine levels inside the rock. The separation of the sulphur and chlorine tells us that the deposition of chlorine is affected by water" - Alian Wang

While the multilayer coatings on rock Mazatzal indicates a temporal occurrence of low quantity water associated with freezing and melting of water, the sulphate deposition at trench sites indicates the involvement of a large body of water.

"We examined the regolith at different depths within the Big Hole and the Boroughs trenches and saw an extremely tight correlation between magnesium and sulphur, which was not observed previously. This tells us that magnesium sulphate formed in these trench regoliths. The increasing bromine concentration and the separation of chlorine from sulphur also suggests the action of water. We don't know exactly how much water is combined with that. The fact that the magnesium sulphate is more than 20 percent of the examined regolith sample says that the magnesium and sulphur were carried by water to this area from another place, and then deposited as magnesium sulphate. A certain amount of water would be needed to accomplish that action" - Alian Wang.

The surface coatings, at least, are not the result of Mazatzal being exposed during part of its lifetime to underground liquid water, as first thought. It had been thought all of the lighter-coloured rocks visible at Gusev were just ordinary basalt boulders with a somewhat thicker coating of windblown Martian dust - but at the MER press conference on April 8, science team member Ray Arvidson of Washington University announced that quite a few genuinely light-coloured rocks similar to Mazatzal have now been recognized in Spirit's pictures and long-range infrared spectra, among the rocks thrown out onto the plain by the meteoroid impact that excavated Bonneville Crater.
And, judging from their shapes, these rocks have been substantially eroded by blowing sand BEFORE the light-coloured weathered crust formed on them. They must therefore have been coated with at least a very thin film of liquid water for fairly long periods of time while still sitting on the surface of Mars.
Arvidson said that this meshes with the new portrait of Mars' climatic "obliquity cycles", such as I described in my recent piece about the Mars papers delivered at last year's Division of Planetary Sciences Conference.

Mars' obliquity (the tilt of its spin axis) slowly rocks back and forth, over cycles of roughly 100,000 years, between maximal and minimal tilts. For the past 3 million years, it has been rocking between tilts of 13 - 35 degrees. Earlier, there was a period of several million years in which computer analyses indicate that its tilt was varying instead between 26 - 46 degrees.
The effect on Mars' weather patterns during its periods of more extreme tilt can be extraordinary. Since the two poles are alternatively titled much more directly toward the Sun than is now the case, during summer all the water ice located on or near the surface at either of Mars' polar regions actually tends to sublimate into vapour and migrate down toward lower latitudes to refreeze.

During tilts of 40 degrees, it may migrate all the way down to the equator (and Gusev), leaving Mars with an equatorial "ice belt" rather than two polar ice caps. When it reaches those lower latitudes, it either refreezes as frost on the planet's surface, or actually falls as snow. And - even given modern Mars' extremely thin air - during daily and seasonal temperature shifts, there will be periods when the ice melts to form thin and short-lived films of liquid water on the planet's rocks and soil which can nevertheless weather them significantly.

It's also possible that, during the periods of highest obliquity, carbon dioxide that has been frozen near the poles, or adsorbed in large amounts by the cold polar soil, is released into the atmosphere to raise Mars' surface air pressure several fold. This would still make it, at absolute most, about one-twentieth as dense as Earth's surface air - but that would be enough to greatly increase the ability of Mars' winds to blow dust and sand around.
Indeed, a German team has recently concluded, by counting the small craters on top of some of Mars' sand dunes, that most of them were formed several hundred thousand years ago and have been pretty much unmoving since then. It's possible that the rocks at Gusev have been carved by blowing sand only during high-obliquity periods - and that, as the tilt of Mars' spin axis then diminishes, there's a brief period during which the planet's air pressure and thus the erosive force of its winds greatly drops but there is still a layer of frost or snow covering the rocks. This would explain why the light-coloured rocks at Gusev have an aqueous weathered crust that formed on them after their last erosion by wind.

There is another complicating factor: it's considered increasingly likely that sulphur - and sulphuric acid in particular - play a major role in the surface chemistry all over Mars. Theories of planetary formation suggest that Mars may originally have formed out of a part of the original near-solar nebula that was more sulphur-rich than the part of the nebula that formed Earth. In any case, what volcanic vents Mars has left (and it certainly has some active vents even now) emit various sulphurous gases that are virtually certain to react with the 0.15% of free oxygen that exists in Mars' atmosphere right now. (The oxygen is briefly liberated by the Sun's ultraviolet light from the carbon dioxide and water vapour in Mars' air, although it quickly combines chemically again with the atmosphere's other components.)
That oxygen - when it reacts with Mars' tiny traces of volcanic sulphurous gases - quickly turns them into sulphur trioxide, which in turn reacts immediately and eagerly with any water it contacts to form sulphuric acid. (It's this same kind of process - working on the tiny traces of water vapour and sulphur dioxide that still exist in Venus' air - that forms that planet's sulphuric-acid cloud deck.)
Sulphuric acid, in turn, is extremely "hydrophilic" - it's powerfully chemically drawn to blend with the rest of Mars' surface water, whether liquid or frozen. And mixtures of sulphuric acid and water can have amazingly low melting points.
In fact, as such an acid-water mixture drops to temperature levels enough to refreeze part of the water in it, the remaining liquid portion of the mixture becomes a more and more concentrated liquid solution of sulphuric acid until it finally becomes about 39% H2SO4 - at which point it will remain liquid down to temperatures as low as -74 deg C, 10 deg C lower than the lowest average yearly surface temperature on Mars at the current time.
Moreover, such an acid solution is so eagerly chemically attracted to more molecules of water vapour in the air that - even in modern Mars' near-vacuum trace of air - it will not evaporate; it pulls more water vapour molecules out of the thin air as fast as it releases them into it. In short, it appears increasingly likely that small moist traces of sulphuric acid solution may frequently exist on or near the surface of many parts of Mars right now, even far from any volcanic vents, remaining stubbornly liquid even in the planet's current hostile environment.
And during those high-obliquity periods when Mars had significant amounts of water ice alternately refreezing and evaporating on its surface on a seasonal basis, it would be even easier for significant amounts of sulphuric acid solution to exist there, weathering the planet's rocks and soil particles more efficiently than water alone could do.
Such a phenomenon would also very nicely explain the thin surface crust of salts that seems to cement soil particles together all over Mars. This crust seems to be composed largely of magnesium sulphate - natural Epsom salt - and this is precisely the substance produced in large amounts when the mineral olivine, found in Martian basalt, is weathered by sulphuric acid. The dark weathered crust of Mazatzal is indeed richer in sulphur and chlorine salts than the unaltered original rock.
And at the Lunar and Planetary Science Conference in Houston earlier in March, it was announced that the "OMEGA" near-infrared spectrometer on Europe's orbiting Mars Express, just while mapping the first one percent of Mars' surface, has found particularly dense concentrations of Mg sulphate in many of the light-coloured patches seen on the floor of Mars' great Marineris Valley - suggesting that these regions, like Meridiani, may have been exposed to particularly large amounts of groundwater mixed with sulphuric acid.

(OMEGA, by the way, is far more sensitive to sulphates than that longer wavelength IR spectrometer on our own MGS orbiter that discovered the Meridiani hematite concentration in the first place. This explains why MGS did not detect the very large amounts of sulphates in the light-coloured Meridiani Etched rock from orbit.)

The presence of large amounts of sulphuric acid on the surface and near-surface of Mars could also explain one of the planet's biggest puzzles: its so-called "carbonate paradox". It's known with reasonably certainty that Noachian Mars had a fairly dense carbon dioxide atmosphere - maybe considerably denser than Earth's total present-day air. But if the planet was also warm enough during these days for large amounts of liquid water to exist on or near its surface (thanks to the greenhouse effect from all this CO2), then that same liquid water should have caused most of the CO2 to react with Mars' silicate rocks and form large permanent beds of carbonate minerals. In effect, the early habitable Mars would have self-destructed - something that doesn't happen on Earth only because our crustal tectonic cycle (which Mars is too small and thus too internally cool to have) constantly drags Earth's accumulated beds of carbonates back down into its hot interior and break them back down into CO2, which its volcanoes then belch back up into the atmosphere.

However, repeated searches from Earth-based telescopes and the orbiting MGS and Mars Odyssey spacecraft have failed to reveal any large carbonate deposits - which has led many scientists to conclude that even Noachian Mars' dense CO2 blanket wasn't powerful enough to warm the planet's surface above freezing, so that there were never more than small and temporary patches of liquid water on or near its surface - maybe always concealed, when they did exist, beneath a layer of ice. This, of course, would have made ancient Mars vastly less promising as a possible spot for the evolution of life. (It would also mean that Mars instead lost most of its early CO2 when it was splashed into space by the giant meteoroid impacts of the period, or swept into space by the solar wind that has blown directly over Mars' upper atmosphere ever since its very early initial magnetic field shut down.

However - as Jeffrey Moore points out in the April 15 "Nature" - if Noachian Mars' surface water bodies actually consisted of even a very weak 0.1% solution of sulphuric acid, that acid would prevent any CO2 dissolved in the water from reacting with Mars' silicate rocks to form carbonates. It's thus possible that Noachian Mars really might have had large bodies of surface liquid water without leaving any carbonate evidence of this behind today. Indeed, since such an adulteration by sulphuric acid would considerably lower the freezing point of water, it would also make the existence of such bodies more likely.

And since such a body of weak sulphuric acid solution, as it did finally start to freeze from the top down, would leave a more and more concentrated layer of liquid acid solution underneath the thickening top layer of pure water ice, Steve Squyres has pointed out that this gives us another possible mechanism by which the sediment layer at Meridiani might have been exposed to a layer of H2SO4 concentrated enough to break down basalt sand or volcanic ash into the mess of amorphous silicates, sulphate salts, and iron oxides that the Opportunity rover has found in the Etched rock layer there today. There would still have had to be some currents running through the liquid layer beneath the ice to produce the "cross-bedded" ripples found by the rover in the sediment layers, but he thinks this possible.

Another talk at the Conference concerned yet another complication in this story. Phil Christensen of Arizona State University - the chief investigator for the "TES" thermal IR spectrometers on both the orbiting MGS spacecraft and the two MER rovers - has recently announced his belief that his instruments have finally detected evidence for small amounts of carbonates (probably magnesium carbonate) lacing the soil of Mars planet wide. These apparently make up only 2 to 3 percent of Mars' soil globally, although the "Mini-TES" on the Spirit rover has found somewhat larger amounts of the substance in the soil at Gusev. But even this small percentage - if it's also spread through the material of Mars' rocks down to a depth of one to three kilometres - would be enough after all to have absorbed one to three bars (that is, Earth atmospheric pressures' worth) of atmospheric CO2.

If this is so, then - to quote Jeffrey Moore in "Nature" - "It could be that Mars sustained a thick CO2 atmosphere and supported liquid water, so long as the CO2 was precluded from forming carbonates by (small amounts of ) sulphur dioxide in the air and water
Once the abundance of SO2 (from Mars' early volcanism) dropped below the critical level to suppress carbonate formation, the atmosphere would have rapidly collapsed to near its present size, leaving carbonates very little time to form as layered marine deposits" of the sort that have been unsuccessfully looked for on Mars.

The discovery of presumably geologically recent gully features on Mars has spawned a wide variety of proposed theories of their origin including hypotheses of the type of erosive material.

A new study suggests small gullies on Mars were carved by water recently and would be prime locations to look for life,
The new study suggests water may still bubble to the surface of Mars now and then, flow for a short stretch, then boil away in the thin, cold air.
The conclusion is based on computer modelling of the atmosphere and how water would behave.


Portion of MOC image M17-00423 located at 200.86°W, 39.16°S showing the alcove, channel, and debris apron structures of recent gullies on Mars.


"The gullies may be sites of near-surface water on present-day Mars and should be considered as prime astrobiological target sites for future exploration. The gully sites may also be of prime importance for human exploration of Mars because they may represent locations of relatively near surface liquid water, which can be accessed by crews drilling on the red planet" - Jennifer Heldmann, lead researcher from NASA's Ames Research Centre.

"If liquid water pops out onto Mars' surface, it can create short gullies about 500-meters long. We find that the short length of the gully features implies they did form under conditions similar to those on present-day Mars, with simultaneous freezing and rapid evaporation of nearly pure liquid water" - Jennifer Heldmann.

Some of the gullies taper off into very small debris fields or leave no debris at all. That implies the water rapidly froze or evaporated.
Given the low air pressure on Mars, water would boil in a flash, so it is doubtful that ice accumulates in the gullies.
The findings will be presented next month at a meeting of the American Astronomical Society's Division for Planetary Sciences in Cambridge, England.

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