每日跟讀#606: Quakes show moon, gradually shrinking, is tectonically active

The moon may be dynamic and tectonically active like Earth — not the inert world some scientists had believed it to be — based on a new analysis disclosed on Monday of quakes measured by seismometers in operation on the moon from 1969 and 1977.

週一披露的新分析顯示，月球可能具有動能，地殼活動如地球般活躍，而非科學家先前認為，月球是一個毫無生氣的世界。這項分析來自月球上的地震儀，自一九六九年以及一九七七年開始運作後，測量到的「月震」。

Researchers examining the seismic data gathered during NASA’s Apollo missions traced the location of some of the quakes to step-shaped cliffs called scarps on the lunar surface that formed relatively recently, in geological terms, due to the ongoing subtle shrinking of the moon as its hot interior cools. “It means that the moon has somehow managed to remain tectonically active after 4.5 billion years,” said Smithsonian Institution planetary scientist Thomas Watters, who led the research published in the journal Nature Geoscience.

研究人員仔細檢驗美國太空總署在阿波羅任務期間蒐集到的地震數據，追蹤其中幾次地震發生的地點，找到月球表面上被稱為「陡坡」的階梯形峭壁。從地質學的角度而言，這些「陡坡」的形成時間相對晚近，是肇因於月球──隨著灼熱的內部構造逐漸冷卻──持續而且細微地縮水。史密森尼學會的行星科學家湯瑪士‧華特斯是這份研究的主持人，他指出：「這個現象意味著月球經過四十五億年後，因為某種原因，地殼構造仍然持續活躍著。」此研究日前刊登於期刊《自然地球科學》。

Earth’s tectonic activity is driven by its hot interior. The moon, which orbits our planet at a distance of about 385,000km, has a diameter of about 3,475km, a bit more than a quarter of Earth’s diameter. Images from NASA’s Lunar Reconnaissance Orbiter showed that the moon has delicately shriveled as its interior has cooled over the eons, akin to a plump grape transforming into a smaller raisin. As a result, it has acquired thousands of small surface wrinkles in the form of surface features called thrust fault scarps.

地球的地殼活動是由炙熱的內部構造所驅使。月球在大約三十八萬五千公里的距離外繞行著我們這顆行星，其直徑約為三千四百七十五公里，略大於地球直徑的四分之一。根據美國太空總署「月球勘測軌道飛行器」傳回的影像顯示，隨著內部結構在極漫長的時間流逝中慢慢冷卻，月球開始出現細緻的萎縮，就像是一顆豐滿的葡萄逐漸變成小粒的葡萄乾。也因此，月球表面產生數以千計的小小皺紋，呈現出「逆斷層陡坡」的地貌特徵。

These faults push one part of the lunar crust up and over the adjoining part, said University of Maryland geologist and study co-author Nicholas Schmerr. They can reach up to about 100m tall and extend for many kilometers. “This is exciting as it wasn’t clear if the moon had already gone through this period billions of years ago and was tectonically dead, or if it was still active in the present,” Schmerr said.

馬里蘭大學的地質學家尼可拉斯‧史梅爾是研究的共同作者，他表示這些逆斷層會把一部份的月球地殼向上推，蓋住毗鄰的部份地殼。斷層推擠的結果可能達到一百公尺高，延伸數公里長。史梅爾說：「這項發現真的很讓人興奮，因為我們仍不清楚月球是否在數十億年前就經歷過這段時期，然後地殼活動已死，還是目前仍在活動中。」

US astronauts placed seismometers on the lunar surface during the Apollo 11, 12, 14, 15 and 16 missions, recording 28 shallow quakes up to almost 5 magnitude, which is moderate strength. Eight quakes occurred close to faults. Other events such as meteorite impacts can produce quakes, but those would produce different seismic signatures. Boulder movements and disturbed soil near the scarps also indicated tectonic activity. Watters said experts must be mindful that quakes may strike near these scarps when planning sites for future lunar exploration and a long-term human presence on the moon.

在阿波羅十一號、十二號、十四號、十五號和十六號的任務期間，美國太空人都曾在月球表面安裝地震儀，並記錄到二十八次的淺層地震，規模最大來到五級，力度還算適中。其中，有八次地震發生在接近斷層的地方。其他事件當然也會造成地震，例如隕石撞擊月球表面，但這些情況會產生不同的地震訊號。巨石位移以及接近陡坡處發生的土壤擾動，也是地殼活動的徵兆。華特斯指出，專家學者在籌畫未來月球探勘以及人類長期居住的地點時，務必要特別留意這些陡坡附近可能會發生地震。

Source article: http://www.taipeitimes.com/News/lang/archives/2019/05/19/2003715373