Not long after the Earth cooled down, a few hundred million years after its crust solidified, life showed up. Fossils called stromatolites, which are sediments stuck together by ancient bacterial colonies, tell the story of our oldest known ancestors on this planet. But the record of life on Earth is not confined to the Earth. Life has plenty of calling cards, and some of them—like radio waves, for instance—escape the planet and head to the stars.
Even the most ancient signatures of life have arrived somewhere else, according to a new study in Nature. The moon is dusted in a fine layer of oxygen, suggesting Earth’s companion is continually polluted by the byproducts of life.
Far from a barren rock, the moon is actually rich in many of the materials we take for granted on Earth, like water ice and noble gases, which are hidden within its shadowed craters. But the provenance of its oxygen has been unclear. Some of the gas is probably native to the moon, based on its elemental form. Some of it probably comes from the flow of charged particles the sun blows into the solar system, called the solar wind. And some must come from Earth, carried along the solar wind like dust in a breeze. Nobody has been able to demonstrate this, in part because it’s hard to tease apart whether the oxygen is from Earth or the sun itself. To do that, scientists in Japan used data from a 10-year-old lunar orbiter, along with some convenient cosmic geometry.
Kaguya, or SELENE, which is the spacecraft’s more common name in English-speaking countries, launched in 2007, orbited the moon for two years, and then crashed into it in 2009. But scientists are still sifting through the wealth of data Kaguya collected there. Last October, for instance, the Japanese Aerospace Exploration Agency released some stunning HD movies and images of our planet viewed from the moon.
Recently, Kentaro Terada and colleagues at Osaka University went back through Kaguya’s ion measurements, looking for charged particles that were moving toward the moon and those that were coming from the moon at different times.
Their key finding relied on a special arrangement among the Earth, the moon, and the sun. Periodically, they block one another, which is why we have eclipses. The solar wind streams onto the moon and Earth all the time, but about five days every month, the Earth is in the sun’s way. It’s during this time that more atoms from Earth freely fly to the moon.
When Kaguya looked at oxygen ions in April 2008, it was clear the oxygen was coming from the Earth, according to Terada. There was a lot more oxygen than the solar wind typically contains, and Earth was blocking most of the solar particles, so it was easier to see the difference. “The upper atmosphere consists of oxygen ions that are easily picked up by the solar wind and transported to the moon,” Terada says. “We think it is going into the lunar soil, based on the observations of the energy of the oxygen ions. Maybe some portion is implanted on the moon, and some portion is lost into interplanetary space.”
This has been happening for billions of years, meaning the first exhalations of life might still exist on the moon. Maybe, the moon is a place we can visit to study what the early Earth was like, billions of years ago.
The first bacteria that built those ancient stromatolites wanted nothing to do with oxygen, which was toxic to them. It wasn’t until algae became complex and photosynthetic that life on Earth started breathing free oxygen, pumping great quantities of it into the atmosphere. This killed many other bacteria in the process, in our planet’s first mass extinction, the Great Oxygenation Event of 2.3 billion years ago. Oxygen concentrations have fluctuated over time; 300 million years ago, it was much more abundant, and enabled interesting evolutionary marvels like enormous dragonflies.
It’s not clear how much we could learn about this history from Apollo-era lunar samples, says Craig Hardgrove, a planetary scientist at Arizona State University: Not only is it hard to tell Earth oxygen from solar oxygen, it’s hard to tell when a given sample of Earth oxygen arrived on the moon.
Hardgrove also pointed out that some of the oldest oxygen might have already escaped the moon, or may be buried beneath its surface. The moon is constantly bombarded by meteors, so its surface isn’t static but instead tilled over time, in a process astronomers call gardening. A recent study suggested this is happening even faster than previously thought. So the oxygen might be mixed into the lunar soil, and buried deeper and deeper. To study that oxygen, we would have to go back to the moon and dig it up.
“For me, it was cool that a moon of a planet can preserve information about the planet, just from the solar wind. How you get the information out, that’s a different paper,” Hardgrove says.
Kaguya didn’t see any Earth-derived nitrogen, which is surprising because it’s by far the largest component of our atmosphere. Terada says this is because high-energy nitrogen is less abundant in the highest parts of Earth’s atmosphere.
Though it doesn’t have any oxygen now, a similar process is happening on Mars, which was once a lot more like the Earth than it is today. The Mars Atmosphere and Volatile Evolution spacecraft—or MAVEN—arrived there in 2014 to measure the escape of Martian gases into space. MAVEN has noticed oxygen leaving Mars behind, and some of it may have wound up on one of Mars’s small moons. JAXA is planning to send a spacecraft there in 2021, and may be able to find out.
That this is still happening on Mars suggests it could happen on any world with an atmosphere. And that means any planet or moon with an envelope of gas could be shedding, and swapping, material all the time—even the signatures of life.