2018-08-31 09:50 供稿單位: 新航道 原創作者:王少娟 新航道上海學校
In the 19th century Percival Lowell, anAmerican astronomer, popularised the idea that there were canals criss-crossingthe Martian surface, carrying water from the poles to feed a thirsty desertcivilisation. Better telescopes, and the arrival of space probes in the 1960s,revealed the canals as a mirage. Mars’s frigid temperatures, and the feeblepressure exerted by its wispy atmosphere, mean that no liquid water couldsurvive on the surface for long. Nevertheless, in 2006 seasonal changes in apair of Martian craters led astronomers to speculate that small amounts ofliquid water might be bubbling briefly to the surface in the Martian summer.Over a decade later, though, the case remains unproven.
Now the question seems to have been settled in spectacular style. In a paper published in Science on July 25th, Roberto Orosei of the National Institute for Astrophysics in Italy, and his colleagues, report the discovery of a lake of liquid water 20km across, buried 1.5km beneath Mars’s surface, close to its southern polar ice cap. The lake seems to be a Martian cousin of familiar Earthly features such as Lake Vostok, a subterranean lake in Antarctica.
The team used radar waves to peer beneath the planet’s surface. Different materials reflect the radar waves with different intensities, allowing scientists to detect what is there. The team used a radar sensor on Mars Express, an orbiting probe, to survey a 200km-wide area of Planum Australe, the planet’s southern polar plain. The sensor lacked the sensitivity of those used on Earth, but after more than three years of collecting data Dr. Orosei felt confident enough to claim that water was the only explanation for the team’s readings.
That the lake is underground is key to its survival, says Susanne Schwenzer, a planetary scientist at the Open University, in Britain, who was not involved with the work. As in Antarctica, the thickness of the ice sheet insulates the water from the sub-freezing temperatures on the planet’s surface. At the same time, the pressure exerted by the ice lowers the water’s melting point below 0°C. “And if the water is spiced with salts of sodium, magnesium and calcium—all of which have been found on Mars—its melting point could drop still further.
The discovery is exciting from a purely geological point of view. But the biggest question is whether anything might be “alive down there. There is plenty of life in Lake Vostok, even though it has been cut off for tens of millions of years. Mars was much warmer and wetter in the past. Fossilised river deltas and lakebeds are visible on the planet’s surface. If microbial life did arise on Mars in the distant past, it might be clinging on in just such an isolated pocket of water below the planet’s surface.
Perhaps. Mars has been dry for around 3.8bn years. That is a long time for a life-preserving lake to have endured. Dr. Schwenzer points out that Mars’s axis has wobbled sufficiently over the planet’s history that the polar caps have wandered widely over its surface. On the other hand, the existence of one such lake suggests there may be more. Alien hunters have, in recent years, been shifting their attentions to the icy moons of Jupiter and Saturn, which sport oceans beneath their surfaces. The discovery of liquid water on Mars will shift some of that attention back.