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曙光號造訪竈神星及穀神星  

2013-05-28 00:16:52|  分类: 小行星及彗星 |  标签: |举报 |字号 订阅

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Dawn's Targets -- Vesta and Ceres
 
Ceres and Vesta are the two most massive residents of the asteroid belt. Vesta is a rocky body, while Ceres is believed to contain large quantities of ice. The profound differences in geology between these two protoplanets that formed and evolved so close to each other form a bridge from the rocky bodies of the inner solar system to the icy bodies, all of which lay beyond in the outer solar system. At present, most of what we now know about Vesta and Ceres comes from ground-based and Earth-orbiting telescopes like NASA's Hubble Space Telescope. The telescopes pick up sunlight reflected from the surface in the ultraviolet, visible and near-infrared, and by emitted radiation in the far-infrared and microwave regions.

穀神星及竈神星是小行星帶中兩顆最大的天體。竈神星由岩石構造,而穀神星据信含有大量的水冰。兩顆原行星形成演化如此類似,其地質學上的重大差異構成了鏈接内太陽系岩石天體與所有位於外太陽系的冰天體之間的橋梁。如今,我們對竈神星與穀神星所知的絕大部分源于地基及繞地運行的望遠鏡,如NASA的哈勃空間望遠鏡。望遠鏡以紫外、可見光、近紅外(的方式),並通過遠紅外及微波區域的輻射散發搜集從其表面反射的太陽光。
Vesta 竈神星
piece of asteroid Vesta

Image above: This meteorite is a sample of the crust of the asteroid Vesta, which is only the third solar system object beyond Earth where scientists have a laboratory sample (the other extraterrestrial samples are from Mars and the Moon). The meteorite is unique because it is made almost entirely of the mineral pyroxene, common in lava flows. Image credit: R. Kempton (New England Meteoritical Services)

上圖:此隕星為小行星竈神星的地殼樣本,竈神星是科學家僅有的三個地外太陽系物體實驗樣本中的一個(其它天外來客樣本來自火星及月球)。此隕星獨一無二特性在於其幾乎全部由常見于岩漿流的礦物煇石構成。圖像來源:羅伯特-開普敦(新英格蘭隕石服務項目)


The asteroid's official name is "4 Vesta" because it was the fourth asteroid discovered. About the length of Arizona, it appears to have a surface of basaltic rock -- frozen lava -- which oozed out of the asteroid's presumably hot interior shortly after its formation 4.5 billion years ago, and has remained largely intact ever since. Telescopic observations reveal mineralogical variations across its surface.

由于竈神星是第四顆發現的小行星,其官方名稱為“4 號竈神星”。它的長度和亞利桑那州差不多,似乎擁有玄武質岩的外層,此冰凍岩漿是在竈神星45億年前剛剛形成后從其灼熱的内部流出的,且直到現在大部分一直完好如初。望遠鏡觀測結果揭示了其表層的礦物學變化(現象)。

Vesta has a unique surface feature which scientists look forward to peering into. At the asteroid's south pole is a giant crater - 460 kilometers (285 miles) across and 13 kilometers (8 miles) deep. The massive collision that created this crater gouged out one percent of the asteroid's volume, blasting over one-half million cubic miles of rock into space.

科學家一直期待對竈神星獨一無二的表層特徵進行觀測(研究)。小行星南極是一個巨大的火山口-460公里(285英里)闊,13公里(8英里)深。形成此火山口的大規模撞擊將相當於小行星體積1%的物質,即超過50万立方公里的岩石,鑿出地表並抛進宇宙。


What happened to the one percent that was propelled from its Vesta home? The debris, ranging in size from sand and gravel to boulder and mountain, was ejected into space where it began its own journey through the solar system. Scientist believe that about 5 percent of all meteorites we find on Earth are a result of this single ancient crash in deep space.

 從竈神星本身抛出的1%物質結果呢?這些從沙礫碎石到巨石山體大小的殘骸被抛入宇宙,並開始其在太陽系的旅程。科學家相信,我們在地球上發現的約5%的隕石是這一古老深空撞擊的結果。


To get an idea of the size of the crater on Vesta's south pole, the longest dimension of the main-belt asteroid Eros (which the Near Earth Asteroid Rendezvous Shoemaker spacecraft studied in 2000) is 30 kilometers long. That entire asteroid would quite easily be lost in the awesome maw of the crater near Vesta's south pole. Or, as another analogy, if Earth had a crater that was proportionately as large as the one on Vesta, it would fill the Pacific Ocean.

想要瞭解竈神星南極火山口的大小,(對比下),主帶小行星厄洛斯(舒梅克號探測器在2000年對厄洛斯近地小行星交會作過研究)最長処為30公里,那麽大的小行星可以輕易掉入竈神星南極附近的火山口。另外打個比喻,如果地球有和竈神星一樣比例大小的火山口,可以容納整個太平洋。


Ceres 穀神星
Ceres

Image above: View of Ceres from Keck Observatory. Image credit: NASA/JPL
上圖:愷克天文臺觀測的穀神星。圖像來源:NASA/JPL噴氣推進實驗室
The object is known by astronomers as "1 Ceres" because it was the very first minor planet discovered. As big across as Texas, Ceres' nearly spherical body has a differentiated interior - meaning that, like Earth, it has denser material at the core and lighter minerals near the surface. Astronomers believe that water ice may be buried under Ceres' crust because its density is less than that of the Earth's crust, and because the dust-covered surface bears spectral evidence of water-bearing minerals. Ceres could even boast frost-covered polar caps.
Astronomers estimate that if Ceres were composed of 25 percent water, it may have more water than all the fresh water on Earth. Ceres' water, unlike Earth’s, is expected to be in the form of water ice located in its mantle.

由於是最先被發現的小行星,天文學家稱穀神星為“1號穀神星”。穀神星和德克薩斯州大小類似,其近乎圓形的主體有著差異化的内核,即像地球一樣,它内核物質更重,接近表層的礦物更輕。由於其表層密度低於地球表層,且其灰塵覆蓋的表層呈現含水礦物的光譜證據,天文學家相信穀神星表層下可能埋有水冰。穀神星甚至可能有霜雪覆蓋的極冠。天文學家估計,如果穀神星由四分之一的水構成,它擁有的水量可能超出地球上的所有淡水(總量)。與地球不同,穀神星上的水很有可能以水冰的形式存在於其地幔。

 

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