Passage 1 Spectacular Mars Images Reveal Evidence of Ancient Lakes 031
火星上的湖泊 《卫报》2010-1-7

[00:01]Spectacular Mars images reveal evidence of ancient lakes
[00:07]Recent spectacular satellite images suggest that Mars was warm enough to
[00:12]sustain lakes three billion years ago,
[00:16]a period that was previously thought to be too cold and arid to sustain water
[00:21]on the surface, according to research published in the journal Geology.
[00:27]The research, by a team from Imperial College London
[00:31]and University College London (UCL),
[00:33]suggests that during the Western Epoch,
[00:38]approximately 3 billion years ago, Mars had lakes made of melted ice,
[00:44]each around 20 kilomters wide, along parts of the equator.
[00:49]Earlier research had suggested that Mars had a warm and wet early history
[00:55]but that between 4 billion and 3.8 billion years ago, before the Western Epoch,
[01:02]the planet lost most of its atmosphere and became cold and dry.
[01:08]In the new study, the researchers analysed detailed images
[01:12]from NASA's Mars Reconnaissance Orbiter,
[01:16]which is currently circling the red planet,
[01:18]and concluded that there were later episodes where Mars experienced warm
[01:24]and wet periods.
[01:25]The researchers say that there may have been increased volcanic activity,
[01:30]meteorite impacts or shifts in Mars' orbit during this period to warm Mars'
[01:36]atmosphere enough to melt the ice.
[01:39]This would have created gases that thickened the atmosphere for a temporary period,
[01:45]trapping more sunlight and making it warm enough for liquid water to be sustained.
[01:51]Lead author of the study, Dr Nicholas Warner,
[01:55]from the Department of Earth Science and Engineering at Imperial College London,
[02:00]says: "Most of the research on Mars has focused on its early history
[02:06]and the recent past. Scientists had largely overlooked the Western Epoch
[02:11]as it was thought that Mars was then a frozen wasteland.
[02:15]Excitingly, our study now shows that this middle period in Mars' history
[02:21]was much more dynamic than we previously thought."
[02:24]The researchers used the images from the Mars Reconnaissance Orbiter
[02:29]to analyse several flat-floored depressions located above Ares Vallis,
[02:35]which is a giant gorge that runs 2,000 kilometers across the equator of Mars.
[02:42]Scientists have previously been unable to explain how these depressions formed,
[02:49]but believed that the depressions may have been created by a process
[02:53]known as sublimation, where ice changes directly from its solid state
[02:58]into a gas without becoming liquid water. The loss of ice
[03:03]would have created holes between the soil particles,
[03:06]which would have caused the ground to collapse into a depression.
[03:10]In the new study, the researchers analysed the depressions
[03:14]and discovered a series of small winding channels that connected them together.
[03:20]The researchers say these channels could only be formed by running water,
[03:25]and not by ice turning directly into gas.
[03:29]The scientists were able to lend further weight to their conclusions
[03:33]by comparing the Mars images to images of thermokarst landscapes that
[03:38]are found on Earth today, in places such as Siberia and Alaska.
[03:44]Thermokarst landscapes are areas where permanently frozen subsoil is melting,
[03:50]creating lakes that are interconnected by the same type of drainage channels
[03:55]found on Mars.
[03:58]The team believe the melting ice would have created lakes
[04:02]and that a rise in water levels may have caused some of the lakes
[04:06]to burst their banks, which enabled water to carve a pathway
[04:11]through the frozen ground from the higher lakes
[04:14]and drain into the lower lying lakes, creating permanent channels between them.
[04:20]Prof. Jan-Peter Muller, Mullard Space Science Laboratory,
[04:25]Department of Space Climate Physics at University College London,
[04:30]was responsible for mapping the three dimensional (3D)
[04:34]shape of the surface of Mars. He adds:
[04:38]"We can now model the three dimensional shape of Mars' surface
[04:43]down to sub-meter fineness of detail that can be distinguished in an image,
[04:48]at least as good as any commercial satellite orbiting the Earth.
[04:54]This allows us to test our hypotheses in a much more rigorous manner
[04:59]than ever before."
[05:01]The researchers determined the age of the lakes by counting crater impacts,
[05:06]a method originally developed by NASA scientists
[05:10]to determine the age of geological features on the moon.
[05:14]More craters around a geological feature indicate that an area is older
[05:20]than a region with fewer meteorite impacts.
[05:23]In the study, the scientists counted more than 35,000 crater impacts
[05:29]in the region around the lakes, and determined
[05:32]that the lakes formed approximately three billion years ago.
[05:37]The scientists are unsure how long the warm and wet periods lasted
[05:42]during the Western Epoch or how long the lakes sustained liquid water in them.
[05:49]The researchers say that their study may have implications for astrobiologists
[05:56]who are looking for evidence of life on Mars.
[05:59]The team says that these lake beds indicate regions on the planet
[06:05]where it could have been warm and wet, potentially creating habitats
[06:09]that may have once been suitable for microbiological life. The team says
[06:15]that these areas may be good targets for future robotic missions.
[06:20]The next step will see the team extend their survey to other areas
[06:25]along the equator of Mars so that they can ascertain
[06:29]how widespread these lakes were during the Western Epoch.
[06:34]The team will focus their surveys on a region at the mouth of Ares Vallis
[06:40]called Chryse Planitia, where preliminary surveys of satellite images
[06:45]have suggested that this area may have also supported lakes.
[06:51]The study was a collaboration between the Department of Earth Science
[06:55]and Engineering at Imperial College London and Space Physics at UCL.
[07:02]The project was funded by the Science and Technology Facilities Council,
[07:07]the Royal Society and the Leverhulme Trust.
[07:11]Space exploration might have begun as a competition among nations,
[07:16]but in the 21st century it is an international enterprise,
[07:21]with scientists from many countries participating in each mission.