Lunar dust collected by Apollo 17 astronauts in the 1970s has revealed that the Moon is 40 million years older than previously believed.

After landing on the moon Dec. 11, 1972, NASA astronauts Eugene Cernan and Harrison Schmitt collected rocks and dust from the lunar surface. A new analysis of that sample detected zircon crystals and dated them to 4.46 billion years old. Previous estimates put the Moon, formed by a massive celestial collision, at 4.425 billion years old.

The findings were published Monday in the journal Geochemical Perspectives Letters.

“These crystals are the oldest known solids that formed after the giant impact. And because we know how old these crystals are, they serve as an anchor for the lunar chronology,” said senior study author Philipp Heck, Robert A. Pritzker Curator for Meteoritics and Polar Studies at the Field Museum of Natural History in Chicago, in a statement.

The early days of our solar system — when Earth was still forming and growing in size — were chaotic, with rocky bodies often colliding in space. During that time more than 4 billion years ago, a Mars-size object crashed into Earth, flinging off a large rocky piece that became the Moon, according to the researchers. But scientists have struggled to precisely date this pivotal event.

The energy from the impact of the Mars-size object hitting Earth melted the rock that would eventually form the Moon’s surface.

“When the surface was molten like that, zircon crystals couldn’t form and survive. So any crystals on the Moon’s surface must have formed after this lunar magma ocean cooled,” said Heck, who is also the senior director of the museum’s Negaunee Integrative Research Center and a professor in the department of geophysical sciences at the University of Chicago.

This research marks the first use of the analytical method of dating the crystals with atom probe tomography and was carried out using instruments at Northwestern University in Evanston, Illinois, according to the study authors.

The analysis showed how many uranium atoms within the zircon crystals experienced radioactive decay. Elements can transform if their atoms contain an unstable configuration of protons and neutrons, causing some of them to decay — such as how uranium decays to become lead. By tracking how long this process takes, scientists can determine the age of something by comparing the ratio of uranium to lead atoms.

The research team used lead isotopes within the lunar dust sample to determine that the crystals were 4.46 billion years old, indicating that the Moon must also be at least that old.(SD-Agencies)

Words to Learn 相关词汇

【岩浆】yánjiāng magma molten rock material

【关键】guānjiàn pivotal central and important

阿波罗17号宇航员20世纪70年代收集的月壤显示，月球的年龄比之前认为的要早4000万年。

1972年12月11日登陆月球后，NASA宇航员尤金•塞尔南和哈里森•施密特收集了月球表面的岩石尘埃。一项新研究对这些样本进行分析后检测到了锆石晶体，它们已经存在了44.6 亿年。之前认为月球在一次大规模天体碰撞中形成，距今有44.25亿年的历史。

研究发表在周一出版的《地球化学展望通讯》杂志上。

研究的资深作者、芝加哥菲尔德自然历史博物馆罗伯特•普利兹克流星学和极地研究馆馆长菲利普•赫克在一份声明中说：“这些晶体在撞击后形成，是已知最古老的固体。知道这些晶体的年龄，也就锚定了月球的年龄。”

太阳系早期是混乱的，那时候地球逐渐形成、变大，岩石天体经常在太空中碰撞。研究人员说，40多亿年前的那段时间里，一个火星大小的天体撞上了地球，甩出一大块岩石，成了月球。科学家们一直在努力确定这一关键事件的准确日期。

火星大小的天体撞击地球产生的能量熔化了月球表面的岩石。

赫克说：“月球表面岩石熔化时，锆石晶体不可能形成并保存下来。因此，月球表面的任何晶体一定是在月球的岩浆海冷却后形成的。” 赫克也是菲尔德博物馆尼高尼综合研究中心高级主任以及芝加哥大学地球物理系教授。

报告作者称，这项研究标志着首次使用原子探针层析成像法对晶体进行年代测定，在伊利诺伊州埃文斯顿的西北大学完成。

分析结果揭晓了锆石晶体中有多少铀原子经历了放射性衰变。元素的原子中如果含有不稳定的质子和中子构型，就会发生变化，其中一些原子会衰变，比如铀会衰变成铅。通过比对这一过程所需的时间，科学家可以通过测定铀原子和铅原子的比例来确定某物的年龄。

研究小组利用月壤样本中的铅同位素确定了晶体的年龄为44.6亿年，这表明月球的年龄至少也是 44.6 亿年。

(Translated by Debra)