A bright window into our past, present and future

A small track for a roverLook up at a clear night sky and you won't miss it, shining bright like nothing else---the Moon.

Its romantic, unique looks have fascinated humans for centuries: we've studied it for as long as we've been able to, we paced our months to its cycle, we've sent probes to orbit it and land on its surface, and we've even visited in person. Yet, there is still so much we don't know about our attractive next-door space neighbor.

Ground truths

After the lunar exploration golden age of the 60s, the interest waned. Manned missions ended, spacecrafts stopped landing, and all research went through orbiter probes. This changed in 2013, when a small Chinese rover by the name of Yutu (which cutely translates to "Jade Rabbit") wheeled itself out of its unmanned lander (called Chang'e 3), and started exploring the surface. The rover's journey lasted a few months before the brutal cold cut it short, but was enough for Yutu and Chang'e to do plenty of research and snap a number of pictures.

When the photos were released to the public in December 2014, the Planetary Society's blog editor, Emily Lakdawalla, went to retrieve the archives---somewhat tortuously: "It was challenging for me to get the data because I don't read Chinese," she says, plus there were some hoops to jump through, though "Really no more than there were for the ESA data when they started putting it online."

Now, thanks to her work, we can easily enjoy a collection of the photos on The Planetary Society website, without going through the same ordeal.

Because Chang'e landed in a similar spot as NASA's Apollo missions did in the 60s---one of the large, visibly dark areas that spot the lunar surface---the landscape in the pictures looks somewhat familiar.

Yet, Lakdawalla says, they did not look quite the same: "To me, the landing site looked different geologically than the Apollo sites, but unfortunately I'm not a lunar expert so I can't really say for sure." But a detailed study, published in Nature Communications, proved her intuition right.

Billions of years ago, the Moon had volcanos and lava was flooding vast craters, forming these dark plains. In the Chang'e crater, particularly, lava bubbled up towards the end of lunar volcanism, quite a bit later than in the Apollo sites. According to Yutu's soil analysis, volcanic activity changed a lot over that time, creating very different rocks in different areas.

The researchers, in fact, determined that the Chang'e crater contains previously unknown basalt (volcanic rock), with a mineral makeup unlike that of any sample brought back by astronauts of Apollo missions or by the Soviet probes. Not only that, since different minerals tend to crystallize at different times, the scientist managed to reconstruct a timeline of the crater eruptions.

Each rock sample we fetch from the Moon, then, is literally solid information, a treasure for years of science. For example, we can use the Yutu results to get a firmer grasp on remote observations: from their altitudes, orbiters mostly measure the top layer of dusty, ground-up rock (called regolith), which is a mixture of everything below, and is very hard to interpret. Direct ground analysis clarifies the ambiguities, and provides a firm baseline for future observation, also in different areas.

Time travel with rocks

Likewise, even after decades, the Apollo samples keep giving precious information, and let us travel even further back in time, to when the Moon came to be.

We can relive the story of when a young Earth smacked into another small planet, called Theia, and the two scraped each other. Tethered to Earth's gravity, Theia couldn't escape and so it became the Moon. Or so the theory went.

An international collaboration, led by UCLA's Prof. Edward Young, published in Science a different story, a gorier one, of head-on crashes and planets disappearing.

The researchers analyzed some Apollo samples (the same used as a yardstick for Yutu's measurements) and compared them with volcanic rocks that surfaced recently (in geologic times) in Hawaii and Arizona. Instead of looking at the rock composition, they went down to the atomic level, to see what elements made up the minerals, and especially what "types" of oxygen were present.

Each "type", or "isotope", has a different number of neutrons in its nucleus: oxygen normally has 8, but sometimes (less than 0.1% of the times) it comes with 9 or 10.

As Prof. Young explains, "Each rocky body that we have been able to sample, Mars, Earth and various asteroids, has distinctive oxygen isotope ratios. The differences are many, many times the precision of our measurements." So each planet, moon and asteroid should have its unique signature composition. But, as they discovered, not the Earth and the Moon---they have the exact same signature.

If Theia hit the Earth sideways, as the theory said, the Moon would share most of Theia's signature, different than Earth's. Since it doesn't, Young says, "it means that Moon and Earth came from the same well-mixed 'reservoir' of oxygen.

This in turn indicates that the collision that made the Moon was violent enough to thoroughly mix up Theia and the proto-Earth." In other words, it was no grazing collision: Theia splashed into the Earth head-on, disappearing completely. The two planets became one, and stirred up a colossal cloud that coalesced to become the Moon.

High tide and sunshine

The Moon, then, formed relatively close to Earth. So close, in fact, that its gravity pulls stronger on the nearer side of Earth than the other; not enough to deform the crust much, but enough to make the light, flowing waters bulge. Or, as we call it: tides.

The atmosphere, too, can flow around and have tides of its own, which affect the weather, as researchers from the University of Washington discovered. According to their results published in Geophysical Research Letters, when the Moon is right overhead (high Moon) or opposite to us, the atmospheric tidal bulge increases the pressure, so air holds on to more of its water vapor, which cannot condense into clouds and rain.

The scientists analyzed the average tropical rainfall data over several years and found that showers increase and decrease twice a day, following along with the position of the Moon.

The change is imperceptible (about 1% more or less rainfall), and doesn't really decide between rain and shine: "I don't think the effect can be any cause for tipping the balance," says Tsubasa Kohyama, lead author of the paper.

So don't take an umbrella just because the Moon is setting: "The effect is negligible compared to the weather variations," he adds. The scientists, however, think the Moon may play a role at larger scales, and are trying to work its effects into more articulate climate and weather models.

The road to the stars

So, from tides to weather, the better we know about the countless lunar influences on our everyday life, the more we will understand our own planet.

And the better we know the Moon itself, the more we can understand the history of our Solar System, which is key to interpret what we see in other star systems.

The Moon is a fundamental stepping-stone for space exploration, too. "It has many of the challenges of deep-space operations, but not all of them," says Lakdawalla. "So it's wise for nations that are newly venturing beyond Earth to send missions to the Moon first."

Whether we want to just look at the stars or actually visit them, the road there begins from the Moon.

Image credit: CC-BY-NC Chinese Academy of Sciences/The Planetary Society

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