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.
Mars is a planet where scale is redefined. It is half the size of Earth, but home to what is probably the largest mountain in the solar system, the 16-mile-high Olympus Mons. Just beneath its equator is a truly grand canyon that also sets a solar system record. The Valles Marineris is as long as the US is wide, and in places reaches 10km deep.
The dream that refuses to die is that one day humans will climb Olympus Mons, and descend into Marineris. But in the meantime, a different sort of exploration is going on - less glamorous, but arguably far more revelatory. Earlier this month, Nasa celebrated the anniversary of the landing of two rovers that were sent to opposite ends of the planet exactly five years before, to study its ancient equatorial geology. 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. "If you were here billions of years ago, it would be exploding, fire-fountaining, with ash coming up and shallow pools of hot water evaporating into the Martian atmosphere," says Ray Arvidson, deputy principal investigator on the rover mission at the University of Washington in St Louis. 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.
The rovers are guided across the Martian landscape by a team of 14 drivers based at the Jet Propulsion Laboratory in Pasadena, California. They arrange their working day to coincide with the Martian night, so they can work out the details for the next day's excursions while the rovers are asleep. The Martian day is 40 minutes longer than on Earth, so the team is constantly shifting the hours it works.
Before shutting down each night, the rovers send pictures and other data of their positions to an orbiter called Mars Odyssey, which relays them back to mission control. There, the drivers confirm the rovers' locations and decide where to send them next. When the rovers wake up, they receive a day's worth of commands telling them what direction to drive in and for how long. All of the moves are run through a simulator on Earth to ensure the rovers won't crash into any boulders or drive off the edge of a cliff.
The process is arduous but it is the only way to drive a golf cart-sized buggy around a planet 100m miles away. Because the driving instructions are relayed via Mars Odyssey, they can take between 1½ hours and a day to reach the rovers.
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.
Despite intense radiation (unlike Earth, Mars has no magnetic field to deflect particles spraying out of the sun) and temperatures that dip below -95C, the rovers keep going. So far, they have sent back a quarter of a million pictures from Mars and 36 gigabytes of scientific data.
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.
Nasa lost contact with the Phoenix lander on 2 November last year, after it had sent back 25,000 images from the planet. The probe died, or perhaps went into hibernation, when a dust storm darkened the sky and blocked light from reaching the lander's solar panels. The probe's batteries are now drained, and there is only a minute chance that it will power up when the sun is high in the sky again later this year.
In Arizona, Smith's team is turning its attention to an unsolved puzzle in the Phoenix data. Ironically, the presence of perchlorate in Martian soil ruined the probe's chances of detecting organic molecules, which would surely be there if life is present. By testing Martian soils concocted in the lab, and comparing the results with those from Phoenix, Smith believes he may answer the question yet.
Arvidson, who was on the Phoenix team and continues to work on the Nasa rovers, says the evidence from the latest missions points to a Mars that at least in the past could well have supported life. "We've been to three locations and each has its own story of water interacting with the surface and beneath," he says. "The question now is, was it habitable for long enough for life to synthesise and establish an ecosystem? We're paving the way to answer that question, for the future missions that will really dig in and look for the smoking gun."
Mars: the facts
Mars is the fourth rock from the sun and our outer neighbour in the solar system.
Mars has two moons, named Phobos and Deimos, Greek for "fear" and "panic". They were named after the horses that pulled the chariot of the Greek war god Ares.
Mars is nearly half the size of Earth but around one tenth the mass.
Mars and Earth have the same land surface area.
The gravity on Mars is a third of that on Earth.
The Martian year lasts 687 Earth days, and each Martian day is 40 minutes longer.
The Martian atmosphere is 95% carbon dioxide, 2.7% nitrogen and 0.13% oxygen.
Mars has no magnetic field, so the surface is constantly bombarded by radiation from the sun.
2009 - Phobos-Grunt
A Russian mission to the Martian moon, Phobos, to bring back soil samples.
2011 - Mars Science Laboratory
A Nasa rover twice the size of the Spirit and Opportunity probes. It will collect and analyse rocks and soil. Chief among its objectives is to find organic compounds and to check for environmental conditions that could have supported microbial life now or in the past.
2013 - Maven
A Nasa orbiter designed to look at gases being given off from the Martian atmosphere.
2016 - ExoMars
A European Space Agency mission to send an orbiter and rover to look for Martian life and possible surface hazards for future manned missions.