火星北极存在“近乎纯净的冷水冰”
2005年12月02日10:10 【字号 大 中 小】【留言】【论坛】【打印】【关闭】新华社今日上午7时24分专电 欧洲航天局11月30日宣布，“火星快车”探测器发现火星曾经有水存在的重要证据，并探测到火星地表下储量丰富的冰层。
“火星快车”装备有高强度的地面扫描雷达“MARSIS”，能探测出地下1公里内的物体。“火星快车”于今年6月26日，7月6日和9日3次围绕火星运行，通过雷达采集数据。
科学家分析数据后发现，火星北极区域存在“层状沉淀物”，并认为沉淀物是冰层，“近乎纯净的冷水冰”，只有2%受到灰尘污染。沉淀物下面是大量的沙子，而且可能与冰层“凝结”在一起。同时，科学家还通过近红外谱仪“OMEGA”在火星上发现水存在的有力证据――页硅酸盐。当火山喷发出的玄武岩在水中长时间浸泡后，就会形成页硅酸盐。
科学家据此推断，35亿至38亿年前，火星表面曾被海洋覆盖。如果这一推论成立，将意味着46亿年前形成的火星初期曾是颗“水星”。
“火星快车”探测器2003年6月发射升空，它携带的“猎兔犬2”号登陆器当年12月在登陆火星时丢失。
欧洲航天局去年1月宣布，“火星快车”探测器发现火星南极存在冰冻水，这些冰冻水部分裸露在火星表面，没有被由二氧化碳凝固形成的干冰全部覆盖。这是人类首次直接在火星表面发现水。
英文版
Radar exposes buried ice on MarsOrbiting craft spots underground crater, but no melt water.
Mark Peplow
The first experiment to look for water deep below the surface of Mars has revealed hints of a subterranean ice lake, trapped in a buried crater.
The MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument, carried on board the European Space Agency's orbiting Mars Express probe, uses radar to see beneath the surface of the red planet.
Radio waves from its 20-metre antennae penetrate the planet's surface before reflecting off layers of different materials. The strength and time delay of these echoes can help scientists to locate deposits of ice, or even trapped aquifers of liquid water.
The results of its first scan of the planet, made in July this year, show what scientists believe to be a impact crater, up to 250 kilometres across and buried 1.5-2.5 kilometres below the surface.
The basin is at least partly filled with what could be an ice-rich material. This is surprising because it lies in a relatively temperate mid-latitude region called Chryse Planitia, where no ice is seen at the surface, says Jeff Plaut, a member of the MARSIS team from NASA's Jet Propulsion Laboratory in Pasadena, California.
Finding more of these buried craters could help to reveal what Mars looked like billions of years ago, before the holes were covered by dust or lava flows.
Ice magic
"It's really interesting because it allows us to reconstruct the geological history of Mars in much more detail," says John Murray, a planetary scientist at the Open University in Milton Keynes, UK.
"Chryse is at the downslope end of Valles Marineris, the Grand Canyon of Mars," Murray says, "and there have obviously been flooding events there in the past." He thinks that the ice in the crater could once have flowed as water through the enormous channels of Valles Marineris, before pooling in the crater and being covered by sediment.
MARSIS has also looked at the ice sheet at the martian north pole, and found that it is about 1.8 kilometres thick. Plaut says that there was a chance of finding melted water at the base of this layer, but MARSIS has found no evidence of liquid there. The results are presented in this week's edition of Science1.
Rocky start
The MARSIS experiment was delayed for more than a year, after mission engineers feared that deploying the antennae might destabilize the spacecraft. The arms were finally unfurled in June, and more results are expected to follow soon. "What we have so far is just the first few weeks of data," says Plaut.
ADVERTISEMENT
He explains that MARSIS can only see through Mars's surface when its closest approach to the planet is during the martian night, because sunlight increases the amount of charged particles in the ionosphere, which can disrupt the radio waves bouncing between MARSIS and the planet. And the orbit of Mars Express has kept its closest approach in the sunlight for the past few months.
"But this month, we return to the night side," enthuses Plaut.
页:
[1]