月球發電機的壽命至少要再增加10億年

原文網址：http://news.mit.edu/2017/lunar-dynamo-lifetime-extended-least-1-billion-years-0809

月球發電機的壽命至少要再增加10億年

研究顯示可能有兩種機制驅動了月球許久以前翻騰且熔化的核心

Jennifer Chu

從古老月岩中得出的新證據顯示，從前攪動月球熔化金屬核心而運轉的發電機(dynamo)，產生的磁場持續時間至少比以往認為的還要長了10億年。發電機是指產生類地星球周圍磁場的自然作用，其動力來源為許多行星或恆星內部導電性流體的翻騰滾動。

在今日刊登於《科學進展》(Science Advances)的論文中，麻省理工學院(MIT)和羅格斯大學的研究人員發表由美國航太總署(NASA)阿波羅15號任務採集到的月岩，呈現出來的跡象顯示它們在10至25億年前形成時，環境當中具有相對微弱的磁場，強度約為5微特斯拉。雖然跟地球目前的磁場相比弱了大概10倍，但仍然比現在星際空間的磁場強了1000倍以上。

數年前，同一群研究人員辨識出年代為40億年的月岩是在強勁許多，大約100微特斯拉的磁場下形成，並測出該磁場的強度大約是在30億年前開始急遽下降。當時研究人員不確定月球發電機以及連帶產生的磁場是否在此之後就迅速停止活動，還是完全消散之前曾苟延殘喘一段時間。

今日發表的結果支持後者描繪出來的景象：月球磁場開始變得衰弱之後卻還是持續存在了至少另一個10億年，其存在的總時間至少有20億年。

研究共同作者，MIT地球、大氣與行星科學系(EAPA)的行星科學教授Benjamin Weiss表示歷時更久的總時間有助於確定提供月球發電機動力的現象。精確來說，此研究提高了有兩種不同機制提供動力的可能性――其中一種較為早期，驅動的發電機功率強上許多；第二種則讓月球的核心在邁向臨終時，仍然可以維持十分和緩的滾沸狀態。

Weiss表示：「行星磁場是由液態金屬流動而形成的概念不過只有數十年的歷史。究竟是什麼力量驅動了地球和其他星體，尤其是月球內部金屬的流動，科學家的理解仍然不多。透過瞭解月球發電機的歷史，我們或許能找出答案。」

Weiss的共同作者包括之前為MIT的研究生，現於羅格斯大學擔任助理教授的主要作者Sonia Tikoo、加州大學柏克萊分校的David Shuster、EAPS的Clément Suavet和Huapei Wang；以及EAPS的地質學教授暨副主任Timothy Grove。

阿波羅任務帶回的玻璃紀錄

自從NASA阿波羅任務的太空人從月球表面帶回樣品開始，科學家就發現其中有些岩石可以當作月球古代磁場的精準「紀錄」。這類岩石之中含有的無數微小顆粒就像羅盤的指針一樣，會沿著岩石在數十億年前結晶當時的磁場方向排列。因此科學家可以藉由這些顆粒來量測月球古代磁場的強度。

Weiss和其他研究人員直到最近都還無法找到年代小於32億年，有精確記載磁場的岩石樣品。因此，他們只能度量月球在32億至42億年前的磁場強度。

Weiss解釋：「問題在於年代小於30億年的月岩非常罕見，因為那個時間點附近月球已經冷卻下來，大部分的火山也停止活動，連帶造成月球表面新生成的火成岩大幅減少。所以我們沒有辦法測量年代較近的樣品來觀察30億年之後月球是否還有磁場。」

然而，阿波羅任務取回的岩石中有一小群並非由月球古代的火山噴發形成，而是在月球稍後的歷史中經由小行星撞擊產生。這些岩石受到撞擊釋放的高熱融化，之後再結晶時月球磁場會決定其晶體排列方向。

Weiss和他的同事分析了一塊這樣的岩石，稱作「阿波羅15第15498號樣品」，其為1971年8月1日從月球隕石坑「沙丘」(Dune Crater)南緣採集而來。這具樣品是由玻璃質基質將礦物與岩石碎屑的混和物熔接而成，基質的顆粒保存了岩石組成時月球的磁場紀錄。

Weiss表示：「我們發現將碎屑熔接在一起的玻璃質材料，具有可以良好保存磁場紀錄的特性。」

烘烤岩石

團隊測出該岩石樣品的年代大約為10億至25億年，跟他們先前分析的樣品相比年輕許多。接著他們發展出一種方法可以解析岩石的玻璃質基質保存的古代磁場紀錄。首先，他們先用非常靈敏的磁強計來測量岩石本身的磁場性質。

然後他們在實驗室把岩石置於已知強度的磁場，再把岩石加熱到將近它原來形成時的極度高溫，並測量隨著岩石周遭的溫度上升，其磁化性質會如何改變。

Weiss解釋：「你可以觀察岩石在已知磁場中加熱後的磁化性質，然後將此磁場跟你事先量測到岩石本身的磁場相比，就可以得出過往的磁場強度。」

研究人員還做了一項重大調整來讓實驗更能重現月球本來的環境，特別是它的大氣。雖然地球的大氣成份約有20%是氧氣，但月球僅有微乎其微的氧氣存在。Suavet跟Grove合作特別建造了一座可以在缺氧條件下加熱岩石的烤箱，它可以在避免岩石生鏽的同時模擬岩石原先磁化時所處的無氧環境。

Weiss表示：「如此一來，我們終於可以得到月球磁場的精確測量結果。」

從冰淇淋機變成熔岩燈

研究人員從他們的實驗測出大約在10億至25億年前，月球擁有的磁場比較微弱，強度大約為5微特斯拉；跟30億至40億年前的月球磁場相比弱了2個數量級。如此劇烈的下降讓Weiss和他的同事提出月球發電機可能是由兩種不同的機制驅動。

科學家過往就已經提出月球發電機可能是由地球的引力驅動。在月球歷史上它以相當近的距離環繞地球時，在如此接近的情況下，地球的重力或許強到足以牽動並旋轉月球的岩石內部。月球的液態核心可能會跟著被外殼拖曳，在過程中產生相當強勁的磁場。

一般認為大約30億年前月球可能已經移動到離地球一定遠的地方，使得透過此機制來驅動發電機的能量變得不夠充足，剛好發生在月球磁場強度急遽下降的時候。這時另一種機制可能開始作用來維持變弱的磁場。當月球越來越遠離地球，其核心或許利用一種緩慢冷卻的機制來維持緩緩滾沸的狀態至少有10億年的時間。

Weiss表示：「當月球逐漸冷卻下來，其核心就像熔岩燈一樣――由於低密度的物質溫度較高或者成分跟周遭液體不同，它們會開始往上升。這是我們認知中的地球發電機運轉方式，而我們主張月球發電機晚期也是以相同的方式運作。」

研究人員正籌畫分析更年輕的月岩以找出月球發電機是什麼時候完全停止運作。

Weiss表示：「今日月球的磁場幾乎為零。而我們現在知道它是在該岩石的形成和今天之間的某個時刻吹起熄燈號。」

此研究有一部分係由NASA資助。

Lunar dynamo’s lifetime extended by at least 1 billion years

Findings suggest two mechanisms may have powered the moon’s ancient churning, molten core.

New evidence from ancient lunar rocks suggests that an active dynamo once churned within the molten metallic core of the moon, generating a magnetic field that lasted at least 1 billion years longer than previously thought. Dynamos are natural generators of magnetic fields around terrestrial bodies, and are powered by the churning of conducting fluids within many stars and planets.

In a paper published today in Science Advances, researchers from MIT and Rutgers University report that a lunar rock collected by NASA’s Apollo 15 mission exhibits signs that it formed 1 to 2.5 billion years ago in the presence of a relatively weak magnetic field of about 5 microtesla. That’s around 10 times weaker than Earth’s current magnetic field but still 1,000 times larger than fields in interplanetary space today.

Several years ago, the same researchers identified 4-billion-year-old lunar rocks that formed under a much stronger field of about 100 microtesla, and they determined that the strength of this field dropped off precipitously around 3 billion years ago. At the time, the researchers were unsure whether the moon’s dynamo — the related magnetic field — died out shortly thereafter or lingered in a weakened state before dissipating completely.

The results reported today support the latter scenario: After the moon’s magnetic field dwindled, it nonetheless persisted for at least another billion years, existing for a total of at least 2 billion years.

Study co-author Benjamin Weiss, professor of planetary sciences in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS), says this new extended lifetime helps to pinpoint the phenomena that powered the moon’s dynamo. Specifically, the results raise the possibility of two different mechanisms — one that may have driven an earlier, much stronger dynamo, and a second that kept the moon’s core simmering at a much slower boil toward the end of its lifetime.

“The concept of a planetary magnetic field produced by moving liquid metal is an idea that is really only a few decades old,” Weiss says. “What powers this motion on Earth and other bodies, particularly on the moon, is not well-understood. We can figure this out by knowing the lifetime of the lunar dynamo.”

Weiss’ co-authors are lead author Sonia Tikoo, a former MIT graduate student who is now an assistant professor at Rutgers; David Shuster of the University of California at Berkeley; Clément Suavet and Huapei Wang of EAPS; and Timothy Grove, the R.R. Schrock Professor of Geology and associate head of EAPS.

Apollo’s glassy recorders

Since NASA’s Apollo astronauts brought back samples from the lunar surface, scientists have found some of these rocks to be accurate “recorders” of the moon’s ancient magnetic field. Such rocks contain thousands of tiny grains that, like compass needles, aligned in the direction of ancient fields when the rocks crystallized eons ago. Such grains can give scientists a measure of the moon’s ancient field strength.

Until recently, Weiss and others had been unable to find samples much younger than 3.2 billion years old that could accurately record magnetic fields. As a result, they had only been able to gauge the strength of the moon’s magnetic field between 3.2 and 4.2 billion years ago.

“The problem is, there are very few lunar rocks that are younger than about 3 billion years old, because right around then, the moon cooled off, volcanism largely ceased and, along with it, formation of new igneous rocks on the lunar surface,” Weiss explains. “So there were no young samples we could measure to see if there was a field after 3 billion years.”

There is, however, a small class of rocks brought back from the Apollo missions that formed not from ancient lunar eruptions but from asteroid impacts later in the moon’s history. These rocks melted from the heat of such impacts and recrystallized in orientations determined by the moon’s magnetic field.

Weiss and his colleagues analyzed one such rock, known as Apollo 15 sample 15498, which was originally collected on Aug. 1, 1971, from the southern rim of the moon’s Dune Crater. The sample is a mix of minerals and rock fragments, welded together by a glassy matrix, the grains of which preserve records of the moon’s magnetic field at the time the rock was assembled.

“We found that this glassy material that welds things together has excellent magnetic recording properties,” Weiss says.

Baking rocks

The team determined that the rock sample was about 1 to 2.5 billion years old — much younger than the samples they previously analyzed. They developed a technique to decipher the ancient magnetic field recorded in the rock’s glassy matrix by first measuring the rock’s natural magnetic properties using a very sensitive magnetometer.

They then exposed the rock to a known magnetic field in the lab, and heated the rock to close to the extreme temperatures in which it originally formed. They measured how the rock’s magnetization changed as they increased the surrounding temperature.

“You see how magnetized it gets from getting heated in that known magnetic field, then you compare that field to the natural magnetic field you measured beforehand, and from that you can figure out what the ancient field strength was,” Weiss explains.

The researchers did have to make one significant adjustment to the experiment to better simulate the original lunar environment, and in particular, its atmosphere. While the Earth’s atmosphere contains around 20 percent oxygen, the moon has only imperceptible traces of the gas. In collaboration with Grove, Suavet built a customized, oxygen-deprived oven in which to heat the rocks, preventing them from rusting while at the same time simulating the oxygen-free environment in which the rocks were originally magnetized.

“In this way, we finally have gotten an accurate measurement of the lunar field,” Weiss says.

From ice cream makers to lava lamps

From their experiments, the researchers determined that, around 1 to 2.5 billion years ago, the moon harbored a relatively weak magnetic field, with a strength of about 5 microtesla — two orders of magnitude weaker than the moon’s field around 3 to 4 billion years ago. Such a dramatic dip suggests to Weiss and his colleagues that the moon’s dynamo may have been driven by two distinct mechanisms.

Scientists have proposed that the moon’s dynamo may have been powered by the  Earth’s gravitational pull. Early in its history, the moon orbited much closer to the Earth, and the Earth’s gravity, in such close proximity, may have been strong enough to pull on and rotate the rocky exterior of the moon. The moon’s liquid center may have been dragged along with the moon’s outer shell, generating a very strong magnetic field in the process.

It’s thought that the moon may have moved sufficiently far away from the Earth by about 3 billion years ago, such that the power available for the dynamo by this mechanism became insufficient. This happens to be right around the time the moon’s magnetic field strength dropped. A different mechanism may have then kicked in to sustain this weakened field. As the moon moved away from the Earth, its core likely sustained a low boil via a slow process of cooling over at least 1 billion years.

“As the moon cools, its core acts like a lava lamp — low-density stuff rises because it’s hot or because its composition is different from that of the surrounding fluid,” Weiss says. “That’s how we think the Earth’s dynamo works, and that’s what we suggest the late lunar dynamo was doing as well.”

The researchers are planning to analyze even younger lunar rocks to determine when the dynamo died off completely.

“Today the moon’s field is essentially zero,” Weiss says. “And we now know it turned off somewhere between the formation of this rock and today.”

This research was supported, in part, by NASA.

原始論文：Sonia M. Tikoo, Benjamin P. Weiss, David L. Shuster, Clément Suavet, Huapei Wang and Timothy L. Grove. A two-billion-year history for the lunar dynamo. Science Advances, 2017 DOI: 10.1126/sciadv.1700207

引用自：Massachusetts Institute of Technology. "Lunar dynamo's lifetime extended by at least 1 billion years: Findings suggest two mechanisms may have powered the moon's ancient churning, molten core."