快猫短视频

NEXT week, the launch window opens for our next shot at Mars 鈥� and we will be ready for it. NASA鈥檚 Mars Explorer Rovers are sitting on the launch pad at Cape Canaveral in Florida, and Mars Express, Europe鈥檚 first attempt to land on the Red Planet, is ready for blast-off from the Baikonur Cosmodrome in Kazakhstan.

For planetary scientists, these missions represent the opportunity of a lifetime. The craft will be landing at sites that look like they were fashioned by flowing water in days long ago when Mars was warm and wet. On Earth, scientists find life wherever they find liquid water. If life ever evolved in some prebiotic Martian soup, then these are the places where a lucky robotic explorer could stumble upon its traces.

But what if the landers fail to find evidence of liquid water on the surface of Mars? Maybe they will land in the wrong place. Maybe they do not have the right tools to find water. Or maybe the evidence just isn鈥檛 there.

That鈥檚 unlikely, say mission scientists, with just a bead of sweat glistening on their foreheads, but not unthinkable. Despite the flood of enthusiastic publicity from NASA and the European Space Agency (ESA), the scientists admit privately that even the most obvious, seemingly water-carved features on their hand-picked landing sites may have an alternative explanation. And therein lies the challenge.

The main reason for thinking water once flowed on Mars comes from orbiting craft. Images from NASA鈥檚 Mars Odyssey, launched in 2001, show features that look like former lake beds, river beds and flood plains. In 2002, sensors on the probe picked up neutrons emitted as a result of cosmic rays striking the hydrogen atoms in huge bands of icy permafrost beneath the dusty surface (快猫短视频, 8 June 2002, p 11). If Mars was warmer in the past, lots of this ice may have been liquid.

The key places to look for more direct evidence is in the gullies, basins and canyons. If these were indeed carved by water, landers should be able to see obvious hallmarks of water erosion on the rocks. The catch is that craft cannot land just anywhere. They need a flat surface that must be near the Martian equator to maximise the light their solar panels receive and therefore the power that they generate. They also need to be at low altitude to give the thin Martian atmosphere as much time as possible to slow the craft before touchdown.

A second problem is that the craft cannot be aimed very accurately. ESA鈥檚 best shot would be to hit a target around 200 kilometres across, and NASA鈥檚 aim is only slightly better. As a result, both agencies have had to set their sights on areas where water seems to have flowed in bulk, producing large-scale features such as giant lake beds and alluvial plains.

Fortunately there are plenty of these, and NASA and ESA say that they have picked ideal landing sites. But independent scientists are not so sure, and point to a number of explanations for the observed features 鈥� only some of which involve water.

Take the Gusev Crater. NASA scientists first earmarked Gusev as a potential landing site in the 1970s. It鈥檚 not hard to see why. Gusev is an ancient impact crater in the Martian tropics that is up to 1600 metres deep and 150 kilometres across. That makes it just about big enough for NASA to target. But what immediately catches the eye is that Gusev lies at the end of a channel known as the Ma鈥檃dim Vallis that stretches 900 kilometres to the south.

Many scientists believe that Ma鈥檃dim Vallis is an ancient river bed carved into the Martian surface by fast-flowing water billions of years ago. This water would have flowed directly into the Gusev Crater, turning it into a lake a kilometre deep in places. The landscape to the north of the crater looks choppy. Perhaps, speculate these scientists, the lake sometimes overflowed at its northern rim, flooding and dramatically eroding the plain beyond.

The floor of Gusev is surprisingly shallow and smooth, not peppered with mini craters from meteor hits. This would make sense in an ancient lake bed. Any sediment carried into the lake from the Ma鈥檃dim Vallis would settle to the bottom, creating a smooth surface, just like dry lake beds on Earth. Taken together, the evidence for water starts to seem persuasive.

But there is another explanation. Channels like Ma鈥檃dim Vallis also exist on the Moon, where they are called sinuous rilles. One of the most famous is the Hadley Rille, which was visited by astronauts during the Apollo 15 mission in 1971. There is some dispute over exactly how sinuous rilles form but geologists are pretty sure that liquid water plays no role in the process. Instead, they interpret them as the result of lava flows carving their way across the lunar surface billions of years ago.

The smooth, shallow nature of the crater floor may also have another explanation. It could be made of ash deposits from the Apollinaris Patera volcano a couple of hundred kilometres to the north. Or it might be ordinary Martian dust that settled into the crater as wind passed over its rims. It might even have been filled in by landslides from collapsed crater walls.

These scenarios are not easy to tell apart with images from orbit, particularly when the features have been weathered over billions of years. But until they are ruled out, they must be considered as real alternatives to the water hypothesis. 鈥淭hat鈥檚 why we鈥檙e going there,鈥� says Ron Greeley, a planetary geologist at Arizona State University in Tempe.

NASA鈥檚 Rover A, due to lift off early in June, will land in the crater and examine the floor using cameras, a microscope and a variety of spectrometers to build up a signature of the rocks. If the crater floor is largely ash, it should be made up of volcanic particles that are sharp and angular. These particles should be small and all roughly the same size because bigger rocks would not be able to travel all the way from the volcano to the crater in the Martian atmosphere. Ordinary wind-blown dust from other parts of the planet should also be small and evenly sized, but will have been frosted and dulled by weathering, making it easy to spot. Landslide debris, on the other hand, should consist of rocks of a variety of sizes and shapes. Telling these scenarios apart should be straightforward as long as the surface within the crater is not covered too thickly with recent layers of dust.

If the rover spots layered, sedimentary rocks, they would be a certain sign of water. But here鈥檚 the snag. Such rocks, if they exist, are likely to be buried under layers of dust and since the rover cannot dig there is no way to uncover them. Everything depends on luck. Perhaps wind erosion will have scoured the surface to reveal the layered rocks beneath. Or maybe recent meteor impacts will have unearthed sedimentary boulders and sent them flying across the crater floor for the rover to stumble across. Who knows?

In case Gusev turns out to be a disappointment, NASA鈥檚 second rover, to be launched later in June, will go to a site that looks even more hopeful. Meridiani Planum is also in the Martian tropics, roughly halfway around the planet from Gusev. It is a good place to look for water for two reasons. First, from orbit the landscape appears layered, as if it had been laid down in regular intervals over the millennia. This could have happened if the area had been flooded in the past, with sediment being deposited as the flood waters receded.

The second reason for interest in Meridiani is an unexpected discovery at the site in 2000. Infrared images from a camera on board NASA鈥檚 Mars Global Surveyor revealed deposits of a grey crystalline mineral rich in iron, which was identified as crystalline haematite, a form of iron oxide. Most of Mars is a rusty red, because of the presence of the same chemical in a disorganised, non-crystalline form. Instead of this, Rover B should see a grey surface, possibly with outcrops of so-called platy haematite that is smooth and shiny 鈥� almost metallic-looking. This will be an alien landscape unlike any other ever seen.

To geologists, the significance of the observations made in 2000 was immediately clear. On Earth, crystalline haematite usually forms as a precipitate in iron-rich water, either at high temperatures near hydrothermal vents or at low temperatures in lakes and seas. If that was the case here, Meridiani must have been flooded. These observations pushed it to the top of NASA鈥檚 list of most desirable landing sites.

Flood waters or volcanic ash?

But here again, there are alternative explanations. Flood waters can indeed lay down rocks in layers, but so can repeated volcanic eruptions. Geologists say that there do not appear to be volcanic vents near Meridiani. But they could be buried beneath the surface, and in any case, volcanic ash could travel for thousands of kilometres through the Martian atmosphere and still form a noticeable layer when it lands. And while haematite can form in iron-rich water, it can also form by oxidation when hot, iron-rich lava comes to the surface, without ever coming into contact with water. The rover will look for other materials, such as carbonates, that scientists would expect to see in a haematite layer formed by water, as well as for chemical evidence of the harsh oxidising conditions that would support the alternative explanation.

The final exploration site is Isidis Planitia, a large flat basin near the Martian equator, where the Beagle 2 lander, part of ESA鈥檚 Mars Express mission, will touch down on 26 December. NASA scientists dismiss Isidis as a large, flat basin with no sign of water erosion, but European scientists are keen. 鈥淚sidis is one of the more exciting sites on Mars,鈥� says John Bridges of the Natural History Museum in London and a member of the Beagle 2 landing-site selection team.

One feature on Isidis that catches the attention is the large number of tiny volcanic cones, some as small as 20 metres across. On Earth, these cones are formed by the explosions that occur when magma comes into contact with water beneath the surface. Repeated explosions create a cone-shaped structure of ash on the surface. In 1997, Kenneth Tanaka and colleagues at the US Geological Survey in Flagstaff, Arizona, proposed that these volcanoes may have formed from wet mud.

Geologists believe that the surface of Isidis is only 10 million years old, because there is less weathering from asteroid impacts. So, if the cones were formed when this surface was being laid down as the result of magma mixing with water or ice, then some ice may still be around and the best place to look for it will be near the cones. Finding the ice would point to a former hydrothermal system, with volcanic heat and ice present in the same place. 鈥淚deally we鈥檇 like to land with a cone in the mid distance about 1 or 2 kilometres away,鈥� says Bridges.

Given the landing accuracy of the vehicle, that will take more luck than judgement. But even if Beagle 2 does make it, the European team could be in for a surprise because once again there is another explanation for the formation of these cones.

The thin Martian atmosphere is made almost entirely of carbon dioxide, as are the planet鈥檚 icy poles. Perhaps CO₂ could be responsible for creating many of the features that look as if they were formed by water, says Nick Hoffman, a geologist at the University of Melbourne in Australia. That鈥檚 not as crazy as it sounds. If CO₂ ice in the Martian soil came into contact with lava, it would vaporise explosively, creating exactly the kind of cones that we see. And Hoffman points out that although CO₂ usually sublimes directly from a solid to a gas, the conditions on Mars may allow liquid CO₂ to flow on or near the Martian surface. Formation of liquid CO₂ requires a slightly higher pressure than occurs on the surface of Mars but this could build up if the CO₂ were trapped beneath the planet鈥檚 surface, he says.

If the missions are successful, they should give us their answers soon after touching down at the end of this year. Even trace amounts of water will put a big smile on the faces of mission scientists, and set the stage for a future mission to pick up rocks that show evidence of water action and possibly life, and bring them back to Earth. 鈥淚n many ways, the landers are preparing the way for a sample return mission,鈥� says Jim Garvin, NASA鈥檚 Mars programme scientist.

Even if there is no evidence of water at these sites, it does not necessarily mean Mars is barren. More hopeful landing sites at higher latitudes and altitudes, which are unsuitable for today鈥檚 landers, would be prime targets for future missions. Canyons and gullies in the ancient highlands and in the permafrost layer far from the equator look tempting and would be within the reach of craft that did not rely on solar power and had better landing accuracy.

One thing is for certain: this year鈥檚 crop of landers will not give planetary scientists all the answers they need. They will want to go back again and again, until we understand exactly how the river beds and lake beds of Mars were fashioned, and whether water, or wishful thinking, played the greater role.