ON 27 AUGUST, at 9.51 am GMT, Mars will be a mere 56 million kilometres from Earth, the closest it has been since 57,617 BC. The planet鈥檚 stellar magnitude will be 鈭�2.9 on the astronomers鈥� scale of brightness, on which negative numbers are reserved for only the brightest celestial objects. On the night of the 27th, Mars鈥檚 orange glow will be unmissable, the brightest thing in the sky. The moon, which is magnitude 鈭�12 when full, will be new, and hence invisible. Venus, with maximum brightness around 鈭�4, will be too close to the sun to be out that night. It goes without saying that astronomers, both amateur and professional, are very excited about this rare chance to see the Red Planet so clearly.
Roughly every two years, Mars and Earth make a close pass called opposition, when Earth passes between the sun and Mars, falling in a straight line with the sun at one end and Mars at the other. Mars and Earth are at their closest near times of opposition, but since both orbits are slightly elliptical, some oppositions are closer than others. The orbits change shape gradually over thousands of years. At the moment, Mars鈥檚 orbit is becoming much more elongated, so that the approaches have been getting closer, leading up to this special opposition when Earth鈥檚 nearest planetary neighbour will be just about 56 million kilometres away. At a time like this, journey times between Earth and Mars fall from as long as 2 years to just a few months. It is an unrivalled opportunity for scientific study, which explains why a host of spacecraft is under way to the planet right now.
It鈥檚 good timing. A lot of scientific hope is riding on the missions, as Mars is proving more enigmatic than ever at the moment. The latest images of the Martian surface taken by NASA鈥檚 orbiting Mars Global Surveyor (MGS) have revealed profoundly mysterious landforms that have left geologists scratching their heads. The features include a combination of surprisingly stable dunes, canyons without craters and rapidly eroding ice caps. All point to amazingly fast processes taking place on the surface.
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Mars has changed considerably in the past few thousand years 鈥� in some places, even the past two years. Yet nobody knows why. Unravelling the mystery will require a radical leap in theoretical thinking, says Michael Malin, the geologist in charge of the MGS camera. He points to the earth science revolution that began in 1837 when Swiss-born geologist Louis Agassiz proposed that lots of surprising observations could be explained if Earth had been subjected to a series of ice ages. For example, boulders found far from their origins and bedrock that had been scoured are due to the action of advancing and retreating glaciers. At the time, such a fast-changing geological environment was completely unthought of.
On Mars today, it looks as if glaciers are receding after an ice age. At the planet鈥檚 south pole, alternate layers of ice and dust are vanishing before our eyes. These long, sweeping, arm-like peninsulas were deposited as a result of past climate oscillations. According to MGS images from 1999 and 2001, they are eroding at a rate of 3 metres per year or more. The images show peninsulas of ice narrowing, and occasionally being pinched off into islands, with some islands disappearing altogether. By measuring the amount of erosion seen over two years, Malin calculates one entire layer will disappear within 20 years.
鈥淲e were absolutely shocked by that,鈥� said Malin when he presented his results at a meeting of the American Association for the Advancement of Science in Denver, Colorado, in February. The magnitude of the changes implies an enormous amount of energy is being pumped into the ice to melt and vaporise it. And the speed of the vaporisation has helped to resolve a long-standing controversy over whether the ice is frozen water or carbon dioxide. 鈥淐alculations showed the only material that could have changed that rapidly is carbon dioxide,鈥� says Malin. It is hard to tell from above how thick each layer of ice is, but best estimates are that with every layer eroded, the thickness of the Martian atmosphere increases by 1 per cent.
More questions remain. How many layers were there in the first place, before the erosion started? How many remain below? Nobody knows. But the implications for one of Mars鈥檚 best-known surface features are astounding. 鈥淎ll the visible ice, all the carbon dioxide that we see in this 鈥榩ermanent鈥� ice cap could be eroded in less than a century,鈥� Malin says.
Other features indicate a changing world, too. For example, huge fields of granular dunes preserve detailed features that show that they once marched across the landscape like sand dunes on Earth, blown by the wind. Yet these dunes are frozen in place, without a trace of motion over a two-year interval.
The only plausible explanation is, again, climate change. If the atmosphere was much thicker in the recent past, its winds may have been able to push along dunes that today鈥檚 winds can no longer even ruffle. Mars may have lost much of that thicker atmosphere in the past and perhaps it is now regaining it from the evaporation of its polar caps.
Perhaps the most mysterious new-found feature on Mars lies inside its version of the Grand Canyon, the huge Valles Marineris, a 2000-kilometre-long canyon near the equator. In a side canyon called Candor Chasma, the floor lies 3.5 kilometres below the surrounding plateau and the walls are spectacularly layered. But there are few impact craters on Candor Chasma鈥檚 floor, implying that it is less than a million years old, as it has not had time to be bombarded by many meteorites. But if it is that young, Malin asks, 鈥渉ow did it get exposed from under three and a half kilometres of material?鈥� So far, there is no answer.
鈥淎ltogether,鈥� says Malin, 鈥渨e have maybe eight to ten landforms that indicate that what you see on Mars today, in terms of the environment, is not what formed the features we see.鈥� That points to climate change, agrees planetary scientist Chris McKay of NASA鈥檚 Ames Research Center in California, who viewed Malin鈥檚 images at a Mars conference in Pasadena, California, last month. But until scientists develop a detailed hypothesis that describes the type of climate change and links it to the features observed, the images don鈥檛 make sense, says McKay. 鈥淲e鈥檝e reached a point of diminishing returns from orbital imaging,鈥� he says.
Malin and McKay aren鈥檛 the only ones feeling puzzled. 鈥淭he problems are becoming more difficult, instead of becoming easier,鈥� said Bruce Jakosky, a planetary scientist at the University of Colorado at Boulder, who was at the meeting in Pasadena. 鈥淧eople are seeing things they just don鈥檛 understand, and coming up with wild ideas to try to explain them,鈥� he says. Many suggestions invoke glaciation, but none can explain all the enigmatic features.
One thing is clear: the rapid erosion on the surface and the possibility of a thickening atmosphere make this a unique time to watch as Mars shakes off its own ice age, or perhaps undergoes a radical and unknown surface transformation. The next time the planet passes this close, on 28 August 2287, it could look completely different. Better mark it on your calendar.