文章来源:未知 文章作者:enread 发布时间:2011-06-24 05:01 字体: [ ]  进入论坛

Huygens mission has directly sampled the water plumes2 jetting into space from Saturn3's moon Enceladus. The findings from these fly-throughs are the strongest evidence yet for the existence of large-scale saltwater reservoirs beneath the moon's icy crust. Enceladus' water plumes shoot water vapour(水蒸汽) and tiny grains of ice into space. They originate from the 'tiger stripe' surface fractures at the moon's south pole, and create the faint E-ring, which traces the orbit of Enceladus around Saturn.

The Cassini spacecraft discovered the plumes in 2005 and more recently has been able to fly directly through them.

During three of Cassini's passes in 2008 and 2009, the Cosmic Dust Analyser measured the composition of freshly ejected plume1 grains. The icy particles hit the detector4 target at speeds of 6.5 .5 km/s, and vapourised instantly. Electrical fields inside the instrument then separated the various constituents5 of the resulting impact cloud for analysis.

Far away from Enceladus, the data show that the ejected grains are relatively6 small and mostly salt-poor, closely matching the composition of the E-ring. Closer to the moon, however, Cassini has found that relatively large, salt-rich ice grains dominate.

It appears as though more than 99% of the total mass of ejected solids is in salt-rich grains, but most of these are heavy and fall back to the moon, so never make it into the E-ring.

The salt-rich particles have an 'ocean-like' composition which indicates that most, if not all, of the expelled ice comes from liquid saltwater, rather than from the icy surface of the moon.

When salty water freezes slowly, the salt is squeezed out, leaving pure water ice behind. So, if the plumes were coming from the surface ice, there should be very little salt in them.

"There currently is no plausible7(貌似可信的) way to produce a steady outflow of salt-rich grains from solid ice across the tiger stripes other than saltwater under Enceladus' icy surface," says Frank Postberg, Universität Heidelberg, Germany, who is the lead author on the paper announcing these results.

The picture the team envisages8(正视,面对) instead is that deep underneath9 Enceladus' surface, perhaps 80 km down, there is a layer of water between the rocky core and the icy mantle10, kept liquid by tidal forces owing to Saturn and some neighbour moons, as well as by the heat generated by radioactive decay.

Salt in the rock dissolves into the water, which accumulates in liquid reservoirs beneath the icy crust. When the outermost11 layer cracks open, the reservoir is exposed to space. The drop in pressure causes the liquid to evaporate, with some of it flash-freezing into salty ice grains: together these create the plumes.

Roughly 200 kg of water vapour is lost every second in the plumes, with smaller amounts of ice grains. According to the team's calculations, the water reservoirs must have large evaporating surfaces, otherwise they would easily freeze over, stopping the plumes.

"Enceladus is a tiny icy moon located in a region of the outer Solar System where no liquid water was expected to exist, because of its large distance from the Sun," says Nicolas Altobelli, ESA's Project Scientist for the Cassini-Huygens mission.

"This finding is therefore a crucial(重要的) new piece of evidence showing that environmental conditions favourable12 to the emergence13 of life may be sustainable on icy bodies orbiting gas giant planets."


1 plume H2SzM     
  • Her hat was adorned with a plume.她帽子上饰着羽毛。
  • He does not plume himself on these achievements.他并不因这些成就而自夸。
2 plumes 15625acbfa4517aa1374a6f1f44be446     
羽毛( plume的名词复数 ); 羽毛饰; 羽毛状物; 升上空中的羽状物
  • The dancer wore a headdress of pink ostrich plumes. 那位舞蹈演员戴着粉色鸵鸟毛制作的头饰。
  • The plumes on her bonnet barely moved as she nodded. 她点点头,那帽子的羽毛在一个劲儿颤动。
3 Saturn tsZy1     
  • Astronomers used to ask why only Saturn has rings.天文学家们过去一直感到奇怪,为什么只有土星有光环。
  • These comparisons suggested that Saturn is made of lighter materials.这些比较告诉我们,土星由较轻的物质构成。
4 detector svnxk     
  • The detector is housed in a streamlined cylindrical container.探测器安装在流线型圆柱形容器内。
  • Please walk through the metal detector.请走过金属检测器。
5 constituents 63f0b2072b2db2b8525e6eff0c90b33b     
n.选民( constituent的名词复数 );成分;构成部分;要素
  • She has the full support of her constituents. 她得到本区选民的全力支持。
  • Hydrogen and oxygen are the constituents of water. 氢和氧是水的主要成分。 来自《简明英汉词典》
6 relatively bkqzS3     
  • The rabbit is a relatively recent introduction in Australia.兔子是相对较新引入澳大利亚的物种。
  • The operation was relatively painless.手术相对来说不痛。
7 plausible hBCyy     
  • His story sounded plausible.他说的那番话似乎是真实的。
  • Her story sounded perfectly plausible.她的说辞听起来言之有理。
8 envisages ea73402f4b85e7991e1a7bf6243f6fca     
想像,设想( envisage的第三人称单数 )
  • The board envisages that there will be a high profit. 董事会预期将会获得高额利润。
  • The paper reviews and envisages the development trend of carbon electrode. 本文对炭电极的发展方向做了回顾与展望。
9 underneath VKRz2     
  • Working underneath the car is always a messy job.在汽车底下工作是件脏活。
  • She wore a coat with a dress underneath.她穿着一件大衣,里面套着一条连衣裙。
10 mantle Y7tzs     
  • The earth had donned her mantle of brightest green.大地披上了苍翠欲滴的绿色斗篷。
  • The mountain was covered with a mantle of snow.山上覆盖着一层雪。
11 outermost w4fzc     
  • He fired and hit the outermost ring of the target.他开枪射中了靶子的最外一环。
  • The outermost electron is shielded from the nucleus.原子核对最外层电子的作用受到屏蔽。
12 favourable favourable     
  • The company will lend you money on very favourable terms.这家公司将以非常优惠的条件借钱给你。
  • We found that most people are favourable to the idea.我们发现大多数人同意这个意见。
13 emergence 5p3xr     
  • The last decade saw the emergence of a dynamic economy.最近10年见证了经济增长的姿态。
  • Language emerges and develops with the emergence and development of society.语言是随着社会的产生而产生,随着社会的发展而发展的。
TAG标签: water icy plumes