文章来源:未知 文章作者:enread 发布时间:2012-10-18 03:21 字体: [ ]  进入论坛
The giant impact believed to have formed the Earth-Moon system has long been accepted as canon(标准,教条). However, a major challenge to the theory has been that Earth and Moon have identical oxygen isotope1 compositions, even though earlier impact models indicated they should differ substantially. In a paper published October 17 in the journal Science online, a new model by Southwest Research Institute (SwRI), motivated by accompanying work by others on the early dynamical history of the Moon, accounts for this similarity in composition while also yielding an appropriate mass for Earth and the Moon. In the giant impact scenario2, the Moon forms from debris3 ejected into an Earth-orbiting disk by the collision of a smaller proto-planet with the early Earth. Earlier models found that most or much of the disk material would have originated from the Mars-sized impacting body, whose composition likely would have differed substantially from that of Earth.
The new models developed by Dr. Robin4 M. Canup, an associate vice5 president in the SwRI Space Science and Engineering Division, and funded by the NASA Lunar Science Institute, involve much larger impactors than were previously6 considered. In the new simulations, both the impactor and the target are of comparable mass, with each containing about 4 to 5 times the mass of Mars. The near symmetry(对称,整齐) of the collision causes the disk's composition to be extremely similar to that of the final planet's mantle7 over a relatively8 broad range of impact angles and speeds, consistent with the Earth-Moon compositional similarities.
The new impacts produce an Earth that is rotating 2 to 2.5 times faster than implied by the current angular momentum9 of the Earth-Moon system, which is contained in both Earth's rotation10 and the Moon's orbit. However, in an accompanying paper in Science, Dr. Matija Ćuk, SETI Institute, and Dr. Sarah T. Stewart, Harvard University, show that a resonant11 interaction between the early Moon and the Sun -- known as the evection resonance12 -- could have decreased the angular momentum of the Earth-Moon system by this amount soon after the Moon-forming impact.
"By allowing for a much higher initial angular momentum for the Earth-Moon system, the Ćuk and Stewart work allows for impacts that for the first time can directly produce an appropriately massive disk with a composition equal to that of the planet's mantle," says Canup.
In addition to the impacts identified in Canup's paper, Ćuk and Stewart show that impacts involving a much smaller, high-velocity impactor colliding into a target that is rotating very rapidly due to a prior impact can also produce a disk-planet system with similar compositions.
"The ultimate likelihood of each impact scenario will need to be assessed by improved models of terrestrial planet formation, as well as by a better understanding of the conditions required for the evection resonance(共振) mechanism," adds Canup.
Canup used smoothed-particle hydrodynamics (SPH) to simulate the colliding planetary objects using 300,000 discrete13 particles whose individual thermodynamic(热力学的) and gravitational interactions were tracked with time.


1 isotope isotope     
  • The isotope ratio is directly used for comparing oils or gases.同位素比率直接用于比较各种石油或天然气。
  • How to apply a radio isotope?如何运用放射性同位素?
2 scenario lZoxm     
  • But the birth scenario is not completely accurate.然而分娩脚本并非完全准确的。
  • This is a totally different scenario.这是完全不同的剧本。
3 debris debris     
  • After the bombing there was a lot of debris everywhere.轰炸之后到处瓦砾成堆。
  • Bacteria sticks to food debris in the teeth,causing decay.细菌附着在牙缝中的食物残渣上,导致蛀牙。
4 robin Oj7zme     
  • The robin is the messenger of spring.知更鸟是报春的使者。
  • We knew spring was coming as we had seen a robin.我们看见了一只知更鸟,知道春天要到了。
5 vice NU0zQ     
  • He guarded himself against vice.他避免染上坏习惯。
  • They are sunk in the depth of vice.他们堕入了罪恶的深渊。
6 previously bkzzzC     
  • The bicycle tyre blew out at a previously damaged point.自行车胎在以前损坏过的地方又爆开了。
  • Let me digress for a moment and explain what had happened previously.让我岔开一会儿,解释原先发生了什么。
7 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。
8 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。
9 momentum DjZy8     
  • We exploit the energy and momentum conservation laws in this way.我们就是这样利用能量和动量守恒定律的。
  • The law of momentum conservation could supplant Newton's third law.动量守恒定律可以取代牛顿第三定律。
10 rotation LXmxE     
  • Crop rotation helps prevent soil erosion.农作物轮作有助于防止水土流失。
  • The workers in this workshop do day and night shifts in weekly rotation.这个车间的工人上白班和上夜班每周轮换一次。
11 resonant TBCzC     
  • She has a resonant voice.她的嗓子真亮。
  • He responded with a resonant laugh.他报以洪亮的笑声。
12 resonance hBazC     
  • Playing the piano sets up resonance in those glass ornaments.一弹钢琴那些玻璃饰物就会产生共振。
  • The areas under the two resonance envelopes are unequal.两个共振峰下面的面积是不相等的。
13 discrete 1Z5zn     
  • The picture consists of a lot of discrete spots of colour.这幅画由许多不相连的色点组成。
  • Most staple fibers are discrete,individual entities.大多数短纤维是不联系的单独实体。
TAG标签: moon earth impact