自古以来，我们对移民外星球的想法从未停歇。火星的自转周期与地球几乎相等，昼夜交替、季节更迭与地球的情况十分类同，重力和温度条件虽然稍恶劣些，但并不是极端难耐的程度......种种这些相似点让浪漫的人类不禁畅想：火星上，可不可以住人呢？

　　19世纪，美国天文学家Percival Lowell让我们知道在火星表面有纵横交错的水道，然而火星上酷寒的温度加上稀薄大气层带来的低气压，让其表面任何液态水都无法存在太久。

　　2006年，火山坑里的气候变化让天文学家猜测少量的液态水也许会在火星的夏季短暂地冒出来，但一直未被证实。

　　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.

　　然而最近，一项研究却让不少外星迷们振奋不已—意大利天体物理研究学家Roberto Orosei称，在火星表面靠近南极冰盖1.5km以下，发现一个宽20km的液态湖泊。

　　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.

　　该小组利用雷达波窥探火星表面之下。不同的物质会以不同的强度反射雷达波，使科学家可以探测出下面是什么。他们通过火星快车号，一种探测器上面的雷达传感器来勘测火星南极高原200km宽面积的地区。这种传感器并不像地球上的那些传感器那么灵敏，但是经过三年多的数据收集，Orosei博士有足够信心认为数据解读的解释是水。

　　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.

　　Susanne Schwenzer，英国开放大学行星科学家，并未参与到研究中，他说湖泊在地下是它能存在的关键。就像是在南极洲（的湖泊），冰盖的厚度将湖水与星球表面的零下温度的水隔离开来。同时，冰所施加的压力将湖水的融点降低至零度以下。如果水中掺杂钠盐，镁还有钙的话—这些已经在火星上发现，那么融点将下降的更多。

具体研究方式如下：

　　发现了湖泊，能否说明有生命呢？火星过去更暖和湿润，已经成为化石的河流三角洲还有湖床在火星表面依稀可见。如果微生物确实在遥远的过去出现过，那么这些微生物有可能附着在这片星球地表之下与世隔绝的水洼中。

　　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.

　　火星已经干旱30亿年了，这对于能保存生命的湖泊来说很漫长。火星中是否存在、能否存在生命，科学家们一直在探索中，或许将来有，我们真的能像去美国一样，去火星。

　　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.

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