As Nasa's rover Curiosity ambled across the arid Martian surface last month it made a momentous discovery - a deposit of smooth, oval stones lying in a long depression in Gale Crater near the base of Mount Sharp on the Red Planet.
The smoothness and flatness of the pebbles indicate that had the rover arrived just three billion years earlier, it would have landed knee-deep in a running river - the one that shaped the stones.
The find is persuasive evidence that water once flowed on the planet, fuelling speculation possibility that the Martian environment could once have supported life - and may still.
While water on the Red Planet's surface is now largely imprisoned in ice caps, scientists are on a hunt to discover if liquid water - and life - exists in warm spots beneath the planet's surface.
''Life - its origin and possible existence in the universe - is a jigsaw puzzle. It requires energy, liquid water and organic carbon,'' said Professor Craig Simmons, the director of the National Centre for Groundwater Research and Training and Flinders University.
''In recent decades, groundwater has emerged as a vital piece of the puzzle. On Earth it is by far the biggest source of fresh water, accounting for about 97 per cent of what's available on the planet.
''Surface water and groundwater are basically a single source of water: they interconnect constantly. So if we want to understand life better, we need to look at every piece of the puzzle, including especially the pieces that are underneath our feet.''
To support life as we know it, water has to be liquid and flowing. The only way for it to exist in this state is when a planet has, or previously had, a hydrological cycle, said Professor Victor Baker from the University of Arizona, who has spent most of his career chasing floods - large and small - across the universe.
In a hydrological cycle, water circulates the planet in three states - ice, liquid and vapour.
It evaporates from ocean surfaces, condenses as clouds and falls back to the surface as rain.
Some of that water then heads straight back into the atmosphere, some escapes across the surface in rivers, some falls as snow and is held frozen in ice caps, but most of it seeps into the subsurface and becomes groundwater.
The groundwater, in turn, helps to fill rivers and lakes and to support life at the surface.
''Recent space missions reveal that Earth is not alone in having a hydrological cycle,'' Baker said.
''The cycles, however, run in many different versions. The moon of Saturn, Titan, has such as cycle, but it consists of methane and other hydrocarbons instead of water.
''Most of the recently discovered extra-solar systems have what are called 'hot Jupiters', which are giant gas planets very close to their suns.
‘‘However, some hard surface planets may have liquid water on their surfaces.
''Cycles that are similar to Earth's used to run, on Mars and Venus,'' Baker said, however, as Venus is 460 degrees at the surface, it is considered too hot to sustain life. Researchers are therefore investigating the next best candidate: Mars.
Mars - Earth's other sibling
While present-day Mars is like a frozen desert, satellite observations and space missions, including the first evidence of water-transported gravel from Curiosity, show that water once flowed on its surface.
This raises the possibility that life either existed or still exists on this planet, and for Professor Malcolm Walter, an astrobiologist at the University of New South Wales, the search begins with its underground source.
''Currently, water on the Martian surface is mostly held in two polar ice caps, and due to the cold environment and the low atmospheric pressure, any liquid water rapidly evaporates or freezes ,'' Walter said.
''Below the icy crust, however, it is a different story - we know that the Red Planet, like Earth, has a warm interior due to radioactive decay of some of its elements.
‘‘Since the temperature there is above the melting point of water ice, it's suggested that Martian groundwater exists in a liquid state,'' he said.
''Consistent with this theory, the Mars Orbiter Camera have captured high-resolution images of little gullies… on the walls of impact craters.
‘‘These gullies, found mostly in the highlands of the southern hemisphere, indicate that from time to time, liquid water from shallow depths trickles to the surface before it freezes.
''The 'seepage' is actually similar to how water emerges from aquifers on Earth.
‘‘All this evidence points to the presence of liquid groundwater on the Red Planet, and in turn, enhances the prospects of finding life,'' Walter said.
''Groundwater is sufficient alone for living beings to survive, because it already contains dissolved nutrients such as phosphorus, nitrogen and sulphur,'' he said.
Apart from dissolved nutrients, there are other reasons for researchers to consider groundwater as an ideal cradle for life.
Being underground means the surface acts as a protective layer and shields the water from ultraviolet (UV) solar radiation, which destroys organic compounds. Its warmth creates a ''womb-like'' environment.
''When you compare Mars to Earth, you'll find that the Red Planet resembles certain locations on our planet, such as Canada, where liquid water is flowing hundreds of metres below the permafrost on the surface. And since we know that organisms can survive many kilometres beneath the Earth's crust, it's quite likely that there is a living ecosystem in Mars's groundwater as well,'' Walter said.
A possible water world
Apart from Mars, liquid groundwater possibly also exists on the Jovian moons Europa, Ganymede and Callisto, Professor Victor Baker said.
And if given the opportunity, Professor Craig Cary from The University of Waikato said he would willingly dive into the underground lakes and ocean on Europa in his pursuit of extraterrestrial life.
''The four Galilean moons of Jupiter are Io, Europa, Ganymede and Callisto,'' Cary explained.
''They are actually quite large bodies, and are often described as a miniature version of the solar system.
''Io is the most volcanic body in the solar system because its orbit around the massive Jupiter causes it to stretch and crack. What's fascinating is that you then have Europa right next to this volcanic moon, and it's a frozen ball of ice.''
Decades of observations from the Galileo Orbiter and the Voyager spacecraft indicate that there are lakes and a liquid ocean of 100 kilometres beneath the icy surface of Europa, Cary said.
''We have beautiful close-up shots of Europa's ice sheets, showing the surface as a big mosaic of floating ice pieces.
Actually, these aren't floating crusts, but fractures caused by the ice as it's moved by flowing liquid underneath.
''The Galileo Orbiter also detected that Europa's magnetic field changes as it rotates, indicating that there's some sort of electrical conducting material, which we think could be a salty liquid ocean.
''The satellites have also found a chaotic terrain consisting of small rotated and tilted blocks of deformed ice crust that covers nearly half of Europa,'' Baker said.
''This terrain could have been caused by water circulating from the warm ocean bottom areas of the planet to just beneath its crust.''
''Given how cold Europa is, the only way for it to keep that much water to exist in a liquid state is for there to be a lot of internal heat,'' Cary said. ''This could have been generated by the moon's orbit around Jupiter, or by radioactive decay from its metallic core. So it's possible that there's volcanic activity in its subsurface, with mud pools, hot springs, or hydrothermal vents. Scientists consider all these locations as possible birthing suites for life on Earth - and maybe elsewhere too.''
Is this our time?
As every telescope, satellite, robotic spacecraft and human spaceflight takes us deeper into space, researchers are increasingly hopeful this is our time to determine if life exists outside of Earth.
''Within the next 20 years, we could know if there's life on Mars,'' Walter said.
''We're sending robots to take soil samples from the gullies and analyse them for organic compounds, which is what Curiosity will do on Mount Sharp.
‘‘The plutonium-powered robot will spend two years on the Red Planet to explore whether Mars could have been a habitable environment, however, we still have to examine the planet's groundwater, which is the aim of future robotic missions.''
As for Europa, Cary eagerly anticipates the launch of the Jupiter Icy Moon Explorer spacecraft in 2022.
''The challenge is getting through 7km of surface ice, and engineers are developing tools to do that,'' he said.
''There's a 'melt probe', known as the cryobot, to bore through the icy crust.
‘‘Once it reaches liquid water, you can send a remotely operated underwater vehicle [ROV], which will… send its analyses back up through a cable.''
To reach further into the heavens, researchers are also building giant telescopes that will have the power to image objects in other solar systems.
These include the Thirty Meter Telescope, the European Extremely Large Telescope and the Giant Magellan Telescope.
With such powerful equipment, we can study the spectra from distant stars and galaxies, identify if they have accompanying planets and analyse their chemical composition, which will reveal if water is present - and in turn, possibly life, too.
''Scientists speculate that life in a microbial form may occur on other planets, but we don't know this for sure,'' Walter said.
''Having evolved under different conditions from Earth, life could be significantly different - but that's part of the anticipation.
''Having more samples can help us understand more about life - including how it started on Earth.
''The other big question is: has life happened only once in the universe, as a result of a stupendous accident?
‘‘If we find out that it's happened twice, we'll know that it wasn't an accident at all, and that there's probably life all over the universe.''
As we continue our quest, we should take deeper thought for our groundwater on Earth, Simmons said. After all, it supports much of the life on this planet.
''We're currently extracting groundwater from sources where we don't know when - or if - it will be replenished,'' he said.
''While water is a renewable resource and can be recharged by the hydrological cycle, this process can take from weeks to millions of years.
''Currently, more about 97 per cent of Earth's fresh water is beneath the surface, making it the largest resource on our planet. It is our 'last reserve' should our surface supplies fail.''
''We're excited to find out more about extraterrestrial hydrological cycles,'' Simmons and Baker say, ''as each discovery gives us new insight into water-related processes on Earth’’.
‘‘What we learn from other planets may help us to understand, manage and look after our home planet better.''
- Sydney Morning Herald
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