Understanding Mars

I liked Ian Sample's well-written article describing the importance of the various recent Mars probes:

High in the sky above Mars, it is snowing right now. Very gently snowing. The snow does not settle on the rubble-strewn land below - not these days, anyway - but instead vaporises into the thin atmosphere long before it reaches the ground.

The first flakes of snow, on a planet that until fairly recently was believed to be waterless, were spotted just a few months ago. A Nasa lander near the planet's north pole was scanning the sky with a laser when it noticed the telltale signs of snowfall. The probe, called Phoenix, announced the news in a radio signal that was picked up by an overhead orbiter and beamed back to Earth. Nothing like it had ever been seen before.

The news of snow falling is just one piece of an extraordinary wealth of information that has recently been sent back from Mars by orbiters, landers and rovers. Together, they have mapped the surface in unprecedented detail, cracked open rocks, sniffed the atmosphere and dug down into the soil. What they have found points to an unimagined Martian history, one where life may have once gained a foothold and may even cling on still in the frigid soils of the permafrost.

...The rovers, Spirit and Opportunity, were designed to survive the harsh environment for just 90 days, but somehow they have survived, and they continue to astound Nasa scientists with the new data they send home.

Spirit has explored a world as fantastic as any imagined by JG Ballard. It touched down a short distance from the Columbia Hills in a region named Home Plate, a plateau 80 metres wide. Spirit found the plateau to be surrounded by deposits of opal. As the rover trundled around, its wheels kicked up soil rich in sulphate. Together, these two pieces of information identify Home Plate as an old volcano. ... Warmth and water rank highly on Nasa's checklist of criteria for the emergence of life, and scientists have been making a credible case for water on Mars since at least the mid-1970s, when pictures sent back from the Viking orbiters showed deep channels, canyons and even features that resembled ancient lake shorelines. In 2006, Nasa had announced the then strongest evidence yet for liquid water on the planet, when its Mars Global Surveyor orbiter spotted what appeared to be stains on gully walls caused by gushing water.

On the other side of the planet, Spirit's twin, Opportunity, has been having its own solar-powered adventures, driving around and analysing rocky features on an expanse called the Meridiani Planum. From data sent back to Earth, scientists know that the rover landed on several hundred metres of sedimentary deposits that must have formed in ancient lakes. The soil is inhospitable, similar to that in parts of Rio Tinto in southern Spain, where water bubbles up through iron sulphide deposits, forming sulphuric acid that dries into an acidic mud over the long, dry summer.

...Remarkably, though long out of warranty, Spirit and Opportunity are carrying only a few mechanical injuries. Spirit, which parked up for the Martian winter with its solar panels angled towards the sun, has recently been ordered to drive south to investigate what look like once-exploding volcanoes that have since been eroded. Opportunity has scrambled out of the 800m-wide Victoria crater and is now setting off on a journey of more than a kilometre to a giant crater called Endeavour. The rocks scattered around the basin of the Endeavour crater have been scanned by cameras aboard orbiters hurtling overhead. They are unlike anything scientists have seen before.

...While Spirit and Opportunity poked and prodded rocks dating back billions of years, the Mars Phoenix lander was sent to explore more recent conditions on the planet. The probe touched down after a flawless descent in the Martian arctic last year, in an area known as Vastitas Borealis, or "northern waste".

On arrival, Phoenix's lead scientist, Peter Smith, of the University of Arizona, expected the lander to find precisely what Spirit and Opportunity had already seen: a landscape smothered in acidic, salty soils that could hardly be considered hospitable. There was good reason to think as much: the planet is frequently hit by giant dust storms that can measure thousands of kilometres wide. When they strike, they whip up the soil and scatter it around on a global scale. The soil in one place, scientists thought, would be similar to that in another.

But built into Phoenix was a robotic arm that allowed it to reach down and gather clods of Martian soil to test with its onboard chemistry lab. And as Phoenix's arm scraped away at the frozen surface, it revealed clear patches of ice that quickly evaporated, making it the first probe to touch water on another planet.

On closer inspection, it became clear that the soil at Phoenix's feet, in the planet's arctic circle, was very different to that found by Spirit and Opportunity at the equator. It was slightly alkaline, like sea water, and contained calcium carbonate, which usually forms in the presence of water. "It told us that Mars is not the same everywhere, as people were suggesting," says Smith. "If the soil was acidic and salty everywhere, you would have trouble imagining life even getting started in a place like that, but we found conditions much like those you see in the Earth's oceans, and for those of us looking for habitable zones on Mars, that's good news."

Scientists poring over data coming back from Phoenix are using it to work out what may have happened in the planet's past. Their best guess links the soil conditions to wild swings in the planet's orientation.

As the Earth orbits the sun, it leans over on its axis at an almost constant 23.5 degrees, and in doing so underpins the regularity of our seasons. Today, Mars is leaning over at around 25 degrees, but five million years ago, that could have been 40 or even 50 degrees. By showing more of its polar caps to the sun, the Martian ice will have warmed up and vaporised. The atmosphere would have become thick with ice clouds that later released snow, which fell to the ground and made the ground damp. That, at least, is the leading theory.

Further tests by the Phoenix lander found traces of a substance called perchlorate in the Martian soil. On Earth, some microbes use perchlorate as a source of energy, Smith says. The picture that emerges from Phoenix is that millions of years ago, when Mars was tilted more toward the sun, the planet may have been hospitable to life. Whether it remained so for long enough for primitive life to get started is another matter.

"We have nutrients in the soil, energy sources, and if there was liquid water five millions years ago, we're getting close to an environment where life could live," says Smith. "If you tossed some Earthly life up there that hadn't evolved for the climate, then it probably couldn't survive. But you have to wonder if, over a long period of time - say a billion years - if Mars slowly transformed itself from a more benign place to what we see today, whether little creatures could have evolved and maybe learned to survive. Life does amazing things on Earth. You can unfreeze the permafrost in Siberia and bring things back to life that have been encased in ice for a million years, so who knows?"

Last week, the evidence for life on Mars received another boost, when scientists at Nasa reported enormous plumes of methane emanating from the planet's north during the summer months. Methane is not proof of life - it can just as well be released by geological processes - but the prospect that life might be responsible is tantalising none the less.