With thedozens of orbiters, landers and rovers that scientists have sent to Mars, youwould think we'd have a good handle on just what makes our planetary neighbortick. But even with all the pictures snapped of its rocky terrain, all themeasurements taken from orbit and the soil samples scooped up, we've stillbarely begun to solve the puzzle that is the red planet.
The sheeramount of surface area of the planet left to investigate speaks to howdifficult the project is. Mars'diameter is only half that of the Earth and its mass is only 1/10th of theEarth's . But it has no oceans. Its surface area is equivalent to all of thecontinents on Earth, "so it's going to take awhile to understand,"said Phoenix landing site working group chairman Ray Arvidson of Washington University in St. Louis.
In the longlist of questions that make up this puzzle, two related queries have long stoodout in the minds of scientists and the public alike: Is there liquid water onMars? And does this seemingly barren planet harbor some kind of life?
NASA's MarsPhoenix Lander, slated to touch down in the northern polar region of Marson Sunday, will aim to help answer these two key questions as it surveys a tinypiece of the planet.
Scientistshave been keen to find evidence of water on Mars because it is essential tolife as we know it, and having an "onsite" source of H2O would becrucial to any future manned missions to the planet.
"Liquidwater is the holy grail on Mars. Where is it? Does it exist at all?" said Phoenix principal investigator Peter Smith of the University of Arizona.
For thefirst half of the 20th century, it was thought that liquid water sloshed aroundall over the surface of Mars, in dark patches assumed to be seas coveringportions of the planet's surface (not to mention astronomer PercivalLowell's infamous canals, later shown to be optical illusions). Mariner 4's1965 flyby, which returned the first images of the planet's surface, dashedhopes of finding any Martian seas: The surface looked as inactive andpockmarked with craters as the moon.
Mariner 9,however, found signs that liquid water had once flowed across the Martianlandscape through ancient river beds, as well as evidence of water erosion.Other missions, including two current rovers, Spirit and Opportunity,have found ample evidence that water once flowed through rivers, pooled inlakes and spewed from hydrothermal vents.
But thisliquid water flowed mostly in very ancient times, when conditions on Mars were muchdifferent than they are today. Now, the planet's atmospheric pressure is toolow (about 1/100th of Earth's) for liquid water to last on the surface. Theonly place on the surface where water exists is at the poles, and there it isfound only in its frozen form.
In February2002, NASA's Mars Odyssey orbiter extended the known regions of water on Marswhen it detected the signatureof water ice just under the surface of the Martian arctic regions, and lotsof it.
"It'snot just a little bit that you might expect to get frozen into the ground fromthe atmosphere, but it's like 70 to 80 percent of the upper meter of thesurface is ice," Smith said. "The amount of ice was a hugesurprise."
Because thearctic regions of the planet haven't been explored from the surface and theunderground ice has so far only been detected indirectly, this subsurface arenais "all of a sudden this mystery zone in my opinion," Smith said.
Exactly howthese substantial subsurface layers of water ice formed is unknown. Somescientists say it could be a remnant of an ancient northern sea that has beentheorized to have existed when Mars was much warmer. It may also have formed aswater vapor froze out of the atmosphere, which is partly how the polar caps onMars form today. But this deposition typically only creates a small amount ofsoil-trapped ice.
"Soyou wonder how you can get 70 percent water in just the pore spaces [betweensoil grains], it doesn't make any sense!" Smith said. "So there mustbe some other way that you're getting this large amount of water in thatarea."
Phoenix will aim to shed some light on theorigin of this ice, which is expected to be so cold that it will be as hard asconcrete, and to characterize it by chipping away pieces for analysis. Smithand the other Phoenix investigators also hope the lander will help determinewhether or not the ice periodically melted and wet the Martian soil to create ahabitable zone that could have possibly supported some form of Martian life.
"Youknow there's ice there now, it's probably too cold to melt ? the question is,in the last million or couple million years has there been a 'wet zone' upthere, if you like, where you really did get liquid water, you wet the soil,and would that be a habitable zone on Mars?" Smith told SPACE.com.
Hunt fora habitable zone
The idea of"little green men" living on Mars has captured the human imaginationin one form or another for decades. Countless science fiction books and movies,such as H.G. Wells' "The War of the Worlds," have featured Earthinvasions by menacing Martian neighbors. Even Percival Lowell thought hiscanals were dug by an intelligent Martian civilization.
But when Viking2 landed on the surface of the red planet in 1976 and sampledthe ruddy soils, it found no signs of life: no microbes, no organicmaterials and certainly no "little green men" with laser guns.
The barrensurface, frigid temperatures, thin atmosphere and lack of liquid water decreasethe likelihood that Martian life might exist or have survived if it onceexisted, at least in any biological form we're familiar with. But recent discoveriesof microbes living in extreme environments on Earth renewed speculation thatlife could be hiding away somewhere on Mars.
The soil atthe surface of Mars ? the type sampled by the Vikingmission ? is actually dust that gets blown all over the planet and isconstantly bombarded by ultraviolet radiation from the sun, which is known tobe harmful to life on Earth.
But thesubsurface ice layer discovered by Odyssey is under about 2 to 4 inches (5 to10 centimeters) of dust, so it would be protected from UV rays (though not fromDNA-damaging cosmic rays), so the soil-ice layers would be more likely to haveorganics, Arvidson said ? if they even exist.
Smith notedthat there are microbes on Earth that freeze during the polar winter, butsurvive and thaw outagain come spring, "like nothing happened." Similarly, Marsexperiences much warmer periods when the wobble of its axis sets it at adifferent angle with respect to the sun ? but this happens on much longer timescales than Earth seasons.
"Canthat [microbe thawing] happen over 500,000-year periods? I don't know. Whoknows? Life can evolve to all kinds of extreme environments and this one wouldbe especially extreme," Smith added.
If life didthrive on Mars during more hospitable times, it could perhaps have left sometelltale sign of organic compounds in the Martian soil. Phoenix is designed to analyzethe soil near the subsurface ice layers for traces of organics; finding thesechemicals wouldn't prove the existence of life on Mars, but it would bolster plansto probe the planet further and to find an answer to this long-standingquestion.
- Video: NASA's Phoenix: Rising to the Red Planet
- Video: Looking for Life in All the Right Places
- The Top 10 Martian Landings of All Time