標 題
Not that sort of monolith
不是那種巨石
An enormous—and unexpected—lump of granite has been found on the Moon
月球上發現了一塊巨大的花崗岩–出乎意料
The discovery sheds light on lunar history, and suggests how other moons might be explored
這一發現揭示了月球的歷史，並提出了探索其他衛星的方法
正 文
IN “2001: A SPACE ODYSSEY”, Stanley Kubrick and Arthur C. Clarke imagined a monolith buried beneath the surface of the Moon which turned out to be an alien artefact that set humankind on a path to the stars. The “batholith” that has been discovered below Compton-Belkovich, a volcanic-looking set of features on the far side of the Moon, hardly promises that. But it sheds some interesting light on the Moon’s past, and shows the power of a new way of peering into the crusts of other planets.
斯坦利-庫布里克（Stanley Kubrick）和阿瑟-克拉克（Arthur C. Clarke）在《2001：太空漫遊》（2001: A SPACE ODYSSEY）中想象了一塊埋藏在月球表面下的巨石。在康普頓-貝爾科維奇（Compton-Belkovich）下面發現的 "浴石"–月球遠側的一組火山岩地貌–很難讓人相信這一點。但它揭示了月球過去的一些有趣的資訊，並顯示了一種窺探其他行星地殼的新方法的威力
A batholith is a geological formation created when a vast quantity of molten rock rises through a planet’s crust, spreading out sideways as it does so. On Earth, these batholiths are composed mostly of granite; the rocks of Yosemite, for example, are parts of the Sierra Nevada batholith uncovered and spectacularly sculpted by subsequent erosion.
熔岩是一種地質構造，當大量熔岩在行星地殼中上升，並向兩側擴散時就形成了熔岩。在地球上，這些熔岩主要由花崗岩組成；例如，優勝美地的岩石就是內華達山脈熔岩的一部分，這些熔岩在隨後的侵蝕作用下被揭露出來，形成了壯觀的雕刻。
The Compton-Belkovich batholith is also made of granite, which is what gave it away. Granite is richer in thorium than other igneous—that is, once molten—rocks, such as basalt, and thorium is radioactive. Its decay heats the granite that hosts it. Measurements of microwave emissions made by the first two Chinese satellites to orbit the Moon, Chang’e 1 and Chang’e 2, allowed a team of scientists led by Matthew Siegler and Jianqing Feng of the Planetary Science Institute in Tucson, Arizona and Southern Methodist University in Dallas, Texas to calculate how much heat was being given off by rocks under the surface at Compton-Belkovich.
康普頓-貝爾科維奇巖床也是由花崗岩構成的，這就是它的秘密所在。花崗岩比其他火成岩（即曾經熔融的岩石，如玄武岩）富含釷，而且釷具有放射性。釷的衰變會加熱容納它的花崗岩。由亞利桑那州圖森市行星科學研究所的馬修-西格勒（Matthew Siegler）和馮建青（Jianqing Feng）以及得克薩斯州達拉斯市南衛理公會大學（Southern Methodist University）的馮建青領導的科學家小組透過對中國首批兩顆繞月衛星–嫦娥一號和嫦娥二號–發射的微波進行測量，計算出了康普頓-貝爾科維奇表面下的岩石釋放出多少熱量。
Assuming that the thorium was present at the sort of concentrations typically seen in granite allowed the scientists to calculate a likely shape for the source. Their best guess, published in Nature last week, was that there is one smallish hot blob, maybe 10-20km across and a few kilometres deep, balanced on top of a much bigger, deeper one that might be 50km across and go down for tens of kilometres.
假設釷在花崗岩中通常存在的濃度，科學家們就能計算出熱源的可能形狀。他們的最佳猜測發表在上週的《自然》（Nature）雜誌上，即有一個直徑約 10-20 千米、深幾千米的小熱球，平衡在一個直徑約 50 千米、深幾十千米的更大更深的熱球之上。
This suggests interesting goings-on in the depths of the Moon of a sort not previously recognised. Igneous rocks are made through a sort of geological distillation. When rocks far below the surface are heated, some components melt more easily than others; when the molten rock solidifies its mineral make-up is not the same as that of the original parent.
這表明月球深處發生了有趣的變化，而這種變化是以前從未認識到的。火成岩是透過一種地質蒸餾法形成的。當遠在地表以下的岩石被加熱時，某些成分比其他成分更容易熔化；當熔岩凝固時，其礦物組成與原始母體的礦物組成並不相同。
To turn rock from the mantle of Earth, or the Moon, into basalt requires just one cycle of distillation; basalt thus created makes up the solid crust under the oceans of Earth and the smooth lava plains of the Moon. To make granite, though, requires a number of separate distillations (which is what gives it high thorium concentrations). On Earth this is easily arranged; the collisions of tectonic plates provides lots of opportunities. On the placid, plateless Moon there is no such mechanism, and it has been easy to assume that basalt is for the most part all the planet has to offer. The granite batholith suggests something new at play. Perhaps the rocks in the area were unusually rich in water. Perhaps pulses of heat from below made them melt and freeze repeatedly.
將地球或月球地幔中的岩石轉化為玄武岩只需要一個蒸餾週期；這樣形成的玄武岩構成了地球海洋下的固體地殼和月球上光滑的熔岩平原。不過，要製造花崗岩，則需要多次單獨的蒸餾（這也是花崗岩釷濃度高的原因）。在地球上，這很容易安排；構造板塊的碰撞提供了很多機會。在沒有板塊的平靜月球上，沒有這樣的機制，人們很容易認為玄武岩是地球上的全部。花崗岩熔岩表明有新的東西在起作用。也許該地區的岩石含有異常豐富的水。也許來自地下的熱脈衝使它們反覆融化和凍結。
將地球或月球地幔中的岩石轉化為玄武岩只需要一個蒸餾週期；這樣形成的玄武岩構成了地球海洋下的固體地殼和月球上光滑的熔岩平原。不過，要製造花崗岩，則需要多次單獨的蒸餾（這也是花崗岩釷濃度高的原因）。在地球上，這很容易安排；構造板塊的碰撞提供了很多機會。在沒有板塊的平靜月球上，沒有這樣的機制，人們很容易認為玄武岩是地球上的全部。花崗岩熔岩表明有新的東西在起作用。也許該地區的岩石含有異常豐富的水。也許來自地下的熱脈衝使它們反覆融化和凍結。
The lunar discovery suggests that similar microwave measurements might provide novel insights into the innards of other planets. That this has not been tried before is due to a preconception. Satellites orbiting Earth use microwave receivers to measure water in the atmosphere. Because the solar system’s other rocky planets do not have watery atmospheres, such instruments have not been sent to them.
月球上的發現表明，類似的微波測量可能會讓人們對其他行星的內部結構有新的認識。以前之所以沒有嘗試過，是因為先入為主。圍繞地球執行的衛星使用微波接收器測量大氣中的水。由於太陽系的其他巖質行星沒有含水的大氣層，因此還沒有向它們傳送過此類儀器。
But when China launched Chang’e 1 and 2 its aim was to show that it could do science round the Moon, not necessarily to do the best such science. Dr Siegler thinks the microwave antennae were put on board simply because they had been shown to work and there were no more suitable instruments to hand. Now they have proved their worth, he hopes to see the designers of future spacecraft take note. One target where they might come in handy would be the moons of Jupiter, where their sensitivity to heat and moisture might be a useful addition to other sub-surface probes. Fans of “2001” will remember that its monolith spurred a new mission to Jupiter. Maybe the batholith will, too. ■
但是，中國發射嫦娥一號和二號的目的是為了證明它可以在月球上進行科學研究，而不一定是要進行最好的科學研究。Siegler博士認為，在月球上安裝微波天線只是因為它們已經被證明是有效的，而且手頭也沒有更合適的儀器。現在它們已經證明了自己的價值，他希望看到未來的航天器設計者注意到這一點。木星衛星是它們可能派上用場的一個目標，它們對熱量和溼度的敏感性可能會成為其他地表下探測器的有益補充。2001》的影迷們一定還記得，該片中的一塊巨石推動了一項前往木星的新任務。也許浴石也會如此。■
