月球縮水可能產生「月震」

原文網址：https://www.nasa.gov/press-release/goddard/2019/moonquakes

月球縮水可能產生「月震」

By Bill Steigerwald / Nancy Jones

隨著月球內部逐漸冷卻，月球正在慢慢縮水，數億年來已經縮小了約50公尺。就像葡萄做成的葡萄乾有大量皺紋，月球縮水的時候也會長出皺紋。但跟柔軟的葡萄皮不一樣，月球表面的地殼相當堅硬，因此會在月球收縮時破裂，造成一部分的地殼被推到鄰近地殼的上方，形成逆衝斷層。

這是一道十分明顯的圓弧形逆衝斷層崖。在月球勘測軌道飛行器照相機(LROC)的影像中發現了數千道斷層崖，它們在月球表面就像是一個個台階(向左指的箭頭)。月球收縮的時候，地表附近的地殼會互相推擠、破裂，最後沿著斷層往上推而形成這些斷層崖。在崖面以及斷層崖後方的地形(較高的那側，向右指的箭頭)可以看到礫原、明亮的表土或風化物。圖片編號：LROC NAC frame M190844037LR。來源：NASA/GSFC/Arizona State University/Smithsonian

「我們的分析首度證實了由於月球還在冷卻收縮，這些斷層仍然處於活躍狀態，而且可能會產生月震。」Thomas Watters表示。他是美國國家航空太空博物館的地球及行星研究中心的資深科學家。「某些月震相當強烈，可以達到芮氏規模五左右。」

這些斷層崖在月球表面就像是階梯狀的小斷崖，大概有數公尺高並延伸數公里。1972年阿波羅17號降落在陶拉斯―利特羅谷的任務期間，太空人Eugene Cernan和Harrison Schmitt駕駛月球車的時候，就必須之字形前進才能攀上並跨過Lee-Lincoln斷層崖。

阿波羅任務在月球上架設了幾座地震儀，這些儀器可以量到地震產生的振動，並記錄不同震波的到達時間以及強度，加以估計地震發生地點，也就是震央。以Watters為主要作者的這篇研究，分析了四臺月球地震儀記錄到的數據。他們還利用了一種演算法(數學程序)來更加精確地估計觀測網相當稀疏時，偵測到的地震震央。研究結果發表在5月13日的《自然―地球科學》(Nature Geoscience)。

在阿波羅11、12、14、15、16號任務期間，太空人在月球表面架設了地震儀。雖然阿波羅11號架設的地震儀只運作了三週，但其他四座從1969年到1977年總共記錄到了28次淺層地震――此類地震的成因可能就是逆衝斷層。這些地震的強度從芮氏規模2到5不等。

運用新演算法重新估算震央的位置，團隊發現28次地震有8次的震央，距離在月球影像中可以見到的斷層不到30公里。此距離使團隊暫時推論這些地震是由斷層產生，因為團隊的模擬結果顯示從斷層崖的大小來看，強烈地震應該發生在這個範圍之內。此外，新的分析也發現8次地震有6次發生時，月球正在遠地點附近，也就是月球軌道中距地球最遠的地方。此時地球重力造成的潮汐力增加而讓斷層受到的總應力達到高峰，使斷層滑動事件更加容易發生。

Watters表示：「我們認為這八次地震很有可能是因為月球整個都在收縮加上潮汐力的影響，使得月球地殼受到壓縮，斷層累積應力最終滑動。這代表阿波羅任務架設的地震儀確實記錄到了月球的收縮過程，也證實月球的板塊構造活動依然活躍。」團隊進行了10000次模擬以計算應力達到最大的同時，斷層附近也發生許多地震是巧合的機率有多高，結果不到百分之四。此外，雖然其他事件，像是流星體撞擊也會造成震動，但它們產生的地震訊號和斷層滑動事件的並不一樣。

月球勘測軌道飛行器(LRO)拍下的高解析度月球影像也證實了這些斷層仍然相當活躍。月球勘測軌道飛行器照相機(LROC)拍下了超過3500個斷層崖的影像。其中一些影像顯示斷層崖或附近的地形有出現較為明亮的部分，而且下方有崩塌或者礫石堆積。由於太陽輻射和宇宙輻射會讓月球表面的物質逐漸變黑，所以明亮的區塊便是近期才露出來的地區，有可能是因為近期發生月震，土石從斷層崖上滑落的結果。在Vitello cluster一座斷層崖的邊坡上可以發現新鮮的礫原；而和斷層可能有關的明亮地形例子，則出現在傑馬•弗里西斯撞擊坑和穆謝撞擊坑附近。LROC的其他斷層影像可以看到礫石崩落的痕跡，如果斷層滑動並產生地震，造成礫石從坡面滑落就會形成此現象。由於微小的流星體持續成群撞擊月球，這類痕跡很快就會消失(以地質時間尺度來說)，所以它們的出現證實了月球近期仍有月震發生。在薛丁格盆地，斷層附近出現的礫石帶就是因為地震晃動，近期才形成的落石堆積。

此外，其中一件重新定位的月震震央，離阿波羅17號太空人穿越的Lee-Lincoln斷層崖只有13公里。這些太空人當時也檢視了登陸地點附近，北丘邊坡上的礫石和礫石帶。而南丘上把Lee-Lincoln斷層崖南段掩埋的大型山崩，也是另一筆這條斷層可能曾經滑動並產生月震的證據。

參與LRO計畫的科學家John Keller任職於NASA戈達德太空飛行中心，他說：「看到將近50年前的數據可以和LRO任務的數據結合起來，進而使我們更加了解月球，實在令人印象深刻。而且未來我們想執行研究月球內部作用的任務時，這項結果也可以告訴我們應該前往哪裡。」

LRO從2009年開始拍攝月球表面，團隊想要比較特定斷層帶在不同時間的相片，藉此看出近期月震活動的證據。共同作者，NASA馬歇爾太空飛行中心的行星地震學家Renee Weber表示：「如果人類想要探索月球，應該先在月球表面建立新的地震觀測網，這不但可以讓我們更加了解月球內部，也能測出會造成災害的月震多常發生。」

月球並不是太陽系中唯一隨著年紀增長而稍微縮水的世界。水星擁有巨大的逆衝斷層，長度可以到達1000公里且高度超過3公里。從兩者的大小比例來看，水星上的逆衝斷層比月球大出許多，代表水星的縮水幅度遠遠超出月球。由於岩石星體會熱漲冷縮，水星上的大型斷層顯示水星形成之後，溫度可能曾經高到使整個水星融化。科學家重建月球的起源時猜想月球是否曾經有同樣經歷；或是月球只有一部分融化，可能表面是岩漿海，深部的加溫速度則較為緩慢。月球較小的斷層崖顯示它的收縮幅度較小，符合部分熔融假說。

NASA計畫在2024年以前讓史上第一位女性，接著是一名男性登陸月球。他們會先從月球軌道上的Gateway太空站取得人類用的登陸系統，然後降落在月球南極。NASA計畫在2028年前完成可以讓人類持許往返月球的任務，接著將從月球學到的經驗套用在前往火星的任務。

研究經費來自於NASA的LRO計畫，以及美國國家科學基金會和加拿大自然科學暨工程研究委員會。LRO是NASA華盛頓總部科學任務局之下的計畫，由戈達德太空飛行中心管理。LROC則是由亞利桑那州立大學管理。

Shrinking Moon may be generating moonquakes

The Moon is shrinking as its interior cools, getting more than about 150 feet (50 meters) skinnier over the last several hundred million years. Just as a grape wrinkles as it shrinks down to a raisin, the Moon gets wrinkles as it shrinks. Unlike the flexible skin on a grape, the Moon’s surface crust is brittle, so it breaks as the Moon shrinks, forming “thrust faults” where one section of crust is pushed up over a neighboring part.

“Our analysis gives the first evidence that these faults are still active and likely producing moonquakes today as the Moon continues to gradually cool and shrink,” said Thomas Watters, senior scientist in the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum in Washington. “Some of these quakes can be fairly strong, around five on the Richter scale.”

These fault scarps resemble small stair-step shaped cliffs when seen from the lunar surface, typically tens of yards (meters) high and extending for a few miles (several kilometers). Astronauts Eugene Cernan and Harrison Schmitt had to zig-zag their lunar rover up and over the cliff face of the Lee-Lincoln fault scarp during the Apollo 17 mission that landed in the Taurus-Littrow valley in 1972.

Watters is lead author of a study that analyzed data from four seismometers placed on the Moon by the Apollo astronauts using an algorithm, or mathematical program, developed to pinpoint quake locations detected by a sparse seismic network. The algorithm gave a better estimate of moonquake locations. Seismometers are instruments that measure the shaking produced by quakes, recording the arrival time and strength of various quake waves to get a location estimate, called an epicenter. The study was published May 13 in Nature Geoscience.

Astronauts placed the instruments on the lunar surface during the Apollo 11, 12, 14, 15, and 16 missions. The Apollo 11 seismometer operated only for three weeks, but the four remaining recorded 28 shallow moonquakes – the type expected to be produced by these faults – from 1969 to 1977. The quakes ranged from about 2 to around 5 on the Richter scale.

Using the revised location estimates from the new algorithm, the team found that eight of the 28 shallow quakes were within 30 kilometers (18.6 miles) of faults visible in lunar images. This is close enough to tentatively attribute the quakes to the faults, since modeling by the team shows that this is the distance over which strong shaking is expected to occur, given the size of these fault scarps. Additionally, the new analysis found that six of the eight quakes happened when the Moon was at or near its apogee, the farthest point from Earth in its orbit. This is where additional tidal stress from Earth’s gravity causes a peak in the total stress, making slip-events along these faults more likely.

“We think it’s very likely that these eight quakes were produced by faults slipping as stress built up when the lunar crust was compressed by global contraction and tidal forces, indicating that the Apollo seismometers recorded the shrinking Moon and the Moon is still tectonically active,” said Watters. The researchers ran 10,000 simulations to calculate the chance of a coincidence producing that many quakes near the faults at the time of greatest stress. They found it is less than 4 percent. Additionally, while other events, such as meteoroid impacts, can produce quakes, they produce a different seismic signature than quakes made by fault slip events.

Other evidence that these faults are active comes from highly detailed images of the Moon by NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft. The Lunar Reconnaissance Orbiter Camera (LROC) has imaged over 3,500 of the fault scarps. Some of these images show landslides or boulders at the bottom of relatively bright patches on the slopes of fault scarps or nearby terrain. Weathering from solar and space radiation gradually darkens material on the lunar surface, so brighter areas indicate regions that are freshly exposed to space, as expected if a recent moonquake sent material sliding down a cliff. Examples of fresh boulder fields are found on the slopes of a fault scarp in the Vitello cluster and examples of possible bright features are associated with faults that occur near craters Gemma Frisius C and Mouchez L. Other LROC fault images show tracks from boulder falls, which would be expected if the fault slipped and the resulting quake sent boulders rolling down the cliff slope. These tracks are evidence of a recent quake because they should be erased relatively quickly, in geologic time scales, by the constant rain of micrometeoroid impacts on the Moon. Boulder tracks near faults in Schrödinger basin have been attributed to recent boulder falls induced by seismic shaking.

Additionally, one of the revised moonquake epicenters is just 13 kilometers (8 miles) from the Lee-Lincoln scarp traversed by the Apollo 17 astronauts. The astronauts also examined boulders and boulder tracks on the slope of North Massif near the landing site. A large landslide on South Massif that covered the southern segment of the Lee-Lincoln scarp is further evidence of possible moonquakes generated by fault slip events.

“It’s really remarkable to see how data from nearly 50 years ago and from the LRO mission has been combined to advance our understanding of the Moon while suggesting where future missions intent on studying the Moon’s interior processes should go,” said LRO Project Scientist John Keller of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Since LRO has been photographing the lunar surface since 2009, the team would like to compare pictures of specific fault regions from different times to see if there is any evidence of recent moonquake activity. Additionally, “Establishing a new network of seismometers on the lunar surface should be a priority for human exploration of the Moon, both to learn more about the Moon’s interior and to determine how much of a hazard moonquakes present,” said co-author Renee Weber, a planetary seismologist at NASA's Marshall Space Flight Center in Huntsville, Alabama.

The Moon isn’t the only world in our solar system experiencing some shrinkage with age. Mercury has enormous thrust faults -- up to about 600 miles (1,000 kilometers) long and over a mile (3 kilometers) high -- that are significantly larger relative to its size than those on the Moon, indicating it shrank much more than the Moon. Since rocky worlds expand when they heat up and contract as they cool, Mercury’s large faults reveal that is was likely hot enough to be completely molten after its formation. Scientists trying to reconstruct the Moon’s origin wonder whether the same happened to the Moon, or if instead it was only partially molten, perhaps with a magma ocean over a more slowly heating deep interior. The relatively small size of the Moon’s fault scarps is in line with the more subtle contraction expected from a partially molten scenario.

NASA will send the first woman, and next man, to the Moon by 2024. These American astronauts will take a human landing system from the Gateway in lunar orbit, and land on the lunar South Pole. The agency will establish sustainable missions by 2028, then we’ll take what we learn on the Moon, and go to Mars.

This research was funded by NASA’s LRO project, with additional support from the Natural Sciences and Engineering Research Council of Canada. LRO is managed by NASA Goddard for the Science Mission Directorate at NASA Headquarters in Washington. The LROC is managed at Arizona State University in Tempe.

原始論文：Thomas Watters, Renee Weber, Geoffrey Collins, Ian Howley, Nicholas Schmerr and Catherine Johnson. Shallow seismic activity and young thrust faults on the Moon. Nature Geoscience, 2019 DOI: 10.1038/s41561-019-0362-2

引用自：NASA/Goddard Space Flight Center. "Shrinking moon may be generating moonquakes."