嫦娥四号在月球上的发现| Nature Podcast

来源：Nature自然科研

又到了每周一次的 Nature Podcast 时间了！欢迎收听本周由Benjamin Thompson和 Shamini Bundell 带来的一周科学故事，本期播客片段讨论嫦娥四号在月球背面发现的物质。欢迎前往iTunes或你喜欢的其他播客平台下载完整版，随时随地收听一周科研新鲜事。

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Host: Shamini Bundell

Next up, reporter Lizzie Gibney has been looking at the first results from China’s mission to the far side of the Moon. 

Interviewer: Lizzie Gibney

More than 20 probes have landed on the Moon, but only one has ever landed on its mysterious far side – the hemisphere, that due to a tidal lock with Earth, always faces away from us. That probe was China’s Chang’E-4, which made a historic landing there in January. Probes don’t tend to go to the far side because communicating directly with a lander there is impossible. Chang’E-4 gets around that by talking to Earth via a relay spacecraft, based well beyond the Moon. Chang’E-4 landed in a 2,500-kilometre-wide dip called the South Pole-Aitken basin. It’s a site that’s particularly interesting for scientists. Here’s Bill Bottke, a planetary scientist from the Southwest Research Institute. 

Interviewee: Bill Bottke

It’s probably the oldest terrain we have on the Moon. We see the greatest density of craters and basins on the far side, and that tells us the oldest terrains on the Moon are located there. The biggest impact crater on the Moon, that everyone agrees is am impact crater, is this one called South Pole-Aitken basin. It’s over 2,000 kilometres across and we think it’s the oldest impact structure on the Moon. 

Interviewer: Lizzie Gibney

Chang’E-4 landed earlier this year in the Von Kármán crater – a smaller dip within the South Pole-Aitken basin – and its rover, Yutu-2, set out to study the terrain around it. Using a spectrometer, it looked at the light reflected off the Moon’s surface. Different materials absorb light in characteristically different ways, revealing their composition. The near side of the moon is mostly covered with mare basalt – a type of rock made of solidified lava. But Yutu-2 may have seen something different on the far side. Here’s Dawei Liu, of the National Astronomical Observatories of the Chinese Academy of Sciences. 

Interviewee: Dawei Liu

Because Yutu-2 landed on the floor of the Von Kármán crater, much of the floor should be covered by lava flows, whose dominate mineral should be high-calcium pyroxene and maybe similar to the lunar near-side mare basalts. However, what we found are quite different from near-side mare basalts. We found that they are mainly composed of olivine and a low-calcium pyroxene. So, this is kind of surprising to us. 

Interviewer: Lizzie Gibney

The findings are unlike anything ever detected before in the near-side samples, but they do match with evidence seen by orbiters from above. And the team have a theory about what they might be seeing. They think the material might be from the interior of the Moon –matter from within the lunar mantle that could have been churned up when a huge asteroid hit, creating the South Pol-Aitken basin, and then later scattered across the surface by another impact, which created the nearby Finsen impact crater. Here’s Bill again. 

Interviewee: Bill Bottke

It could very well be that the Chinese mission has managed to sample or at least understand some of the material that may have come from the very deep interior of the Moon. Now, I will say, with that said, there’s some other possibilities. This may not necessarily be lunar mantle and they discuss that in the paper, but I think it’s an exciting possibility and I think everyone’s going to be very interested to see where this goes when this paper hits the street in a few days. 

Interviewer: Lizzie Gibney

If true that this dense material comes from inside the Moon, this backs up something called the lunar magma ocean theory, which says that soon after its formation, the Moon’s surface was hot and molten and then later separated into layers as it solidified. That would have left lighter materials in the surface crust, burying deeper ones within its mantle. Here’s Dawei again. 

Interviewee: Dawei Liu

According to this theory, the lighter materials should float to form the lunar crust while the denser minerals, such as olivine, should sink to form the lunar mantle. However, this theory was debated because no direct evidence has been found to indicate that the mantle was dominated by olivine. Thus, whether this theory is correct requires further validating and also it also requires further evidence. Our result attempts to prove that olivine-rich lunar mantle could be right, and thus we’re supporting the lunar magma ocean hypothesis. 

Interviewer: Lizzie Gibney

Teasing apart the fingerprints of different materials is complicated, and Dawei is keen to stress that the rover needs to collect a lot more data before his team can confirm that this really is material from inside the mantle. Over the next few months the little rover will analyse the spectra of many more samples, as well as map the geology of the landing site.

Bill says that there’s a lot more we can learn from the Moon. 

Interviewee: Bill Bottke

The Moon, in a sense, is like a little accessible planet which is smaller than the Earth, but it did go through this extensive melting, so by studying the Moon and the Moon’s mantle, we can hopefully get at some of the same processes that will tell us how the planets in our solar system came to be the way they are. 

Interviewer: Lizzie Gibney

Chang’E-4 still has a lot to do. But it’s not China’s only Moon mission. It will soon be followed by Chang’E-5 in December this year, which will aim to bring back samples from the Moon to Earth. 

Interviewee: Dawei Liu

I think in future China has planned many missions – maybe Chang’E-6 or subsequent Chang’E missions. These missions will help us to build a lunar base in future. And also, we try to go to Mars and now the Mars exploration programme is in progress, and our asteroid exploration is also in planning. 

Interviewer: Lizzie Gibney

That’s a long shopping list of exciting future missions. The Moon is just the first stepping stone to all of these, says Dawei. 

Interviewee: Dawei Liu

The Moon will be one of the most important targets for China’s future deep space exploration. 

Host: Shamini Bundell

That was Dawei Liu, and you also heard from Bill Bottke. You can read Dawei’s paper and a News and Views article over at nature.com. 

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