NASA is about to grab its first taste of Mars. On 30 July, its Perseverance rover is set to launch to the red planet — the first step towards fulfilling a long-standing dream of planetary scientists. If everything goes to plan, Perseverance will arrive in February 2021 and drive around, collecting samples of rock that — one day — other spacecraft will pick up and fly back to Earth. The rocks will become the first samples ever returned from Mars.
They will join a priceless collection of cosmic material brought back from other planetary bodies throughout the space age. From lunar rocks gathered by the Apollo astronauts to shards of a distant asteroid collected by robot spacecraft, these samples of other worlds have reshaped scientific study of the Solar System.
Without planetary missions, the only way scientists can directly study rocks from other worlds is to analyse meteorites that have fallen to Earth. “Just waiting for [material] to arrive here on Earth would be a lot cheaper,” says Queenie Hoi Shan Chan, a planetary scientist at Royal Holloway University of London in Egham, UK. “But we cannot just wait for it to happen, because it’s really rare.”
And so space agencies go to a lot of trouble to collect fragments of the Moon, Mars and other worlds. One advantage is that in well-equipped Earthly laboratories, researchers can apply tools and techniques to understand these samples that they can’t from a small spacecraft, Chan says. Sample-return missions also allow researchers to know the exact geological area their rock comes from. That is “priceless context”, says Jessica Barnes, a planetary scientist at the University of Arizona in Tucson.
With two asteroid-sampling missions under way, and renewed interest in the Moon, the 2020s are shaping up to be a golden age of sample return. Nature looks at the sample-return missions that have been carried out so far — and how Perseverance’s goal to bring back rocks from Mars fits into these efforts.
The first and largest collection of samples comes from the Moon. Between 1969 and 1972, a dozen astronauts on NASA’s Apollo programme flew to the Moon, walked around on it, and picked up and brought back 382 kilograms of lunar rocks (see ‘Sampling the Solar System’). Studies of those samples have rewritten scientific understanding of the history of the Solar System.
“When Apollo 11 landed on the Moon, many considered that our small moon had formed cold ,” says Donald Brownlee, an astronomer at the University of Washington in Seattle. “This turned out to be spectacularly wrong.” Studies of the Moon rocks showed instead that the Moon was hot at its birth, more than 4.5 billion years ago, and covered with an ocean of molten rock.
Researchers are still learning from the Apollo samples. Last year, to mark the 50th anniversary of the Apollo 11 landing, NASA began to open some Apollo samples that had been sealed since they came back to Earth, to see what new science they might yield. Those studies are under way, although progressing slowly because of the COVID-19 pandemic.
Three Soviet Luna missions, all involving robots, also brought back small amounts of Moon dust between 1970 and 1976. And China plans to retrieve some lunar samples with its upcoming Chang’e 5 mission, which could launch by the end of this year and would deliver the first lunar sample return since the 1970s.
NASA is looking to bring back many more Moon rocks as part of its Artemis programme, which aims to send astronauts back to the lunar surface by the end of 2024.
The Japan Aerospace Exploration Agency (JAXA) is the only space agency so far to have brought back material from an asteroid. In 2010, the Hayabusa spacecraft returned from a visit to the potato-shaped asteroid Itokawa, although it wasn’t clear whether it had managed to collect any samples during a series of mishaps at the asteroid. But when the spacecraft returned to Earth, JAXA researchers opened it and found more than 1,500 precious asteroid grains there.
“These are tiny particles that are even smaller than the diameter of a human hair,” says Chan. “On Earth we can do a great deal of detailed analysis, even with that.” That includes determining the isotopic composition of water in the Itokawa material, an analysis that isn’t possible in space because the instrument required takes up most of a room. Studies of the Itokawa grains confirmed, among other things, that the most common type of meteorite that falls to Earth, called an ordinary chondrite, comes from silicate-rich asteroids such as Itokawa1. The Itokawa particles had also been heated and shocked at some point in the past, suggesting that they had experienced cosmic collisions in the asteroid belt.
Two other asteroid samples should arrive on Earth soon, if all goes well. JAXA’s second asteroid-sampling mission, Hayabusa2, is due to land in Australia in December. It should be carrying a couple of grams of material collected from a carbon-rich asteroid called Ryugu, which lies between the orbits of Earth and Mars. And NASA’s OSIRIS-REx spacecraft is currently orbiting its own asteroid, the diamond-shaped Bennu, in the hope of grabbing a sample from it in October and returning to Earth in 2023.
In 2004, NASA’s Stardust spacecraft whizzed through the tail of Comet Wild 2, six times faster than a speeding bullet, and grabbed the only samples of a comet that have ever been brought back to Earth. Those, too, turned up huge surprises.
NASA named the mission Stardust because scientists thought the comet contained ancient dust from other stars, frozen in ice for billions of years. “This idea was also spectacularly wrong,” says Brownlee, the mission’s principal investigator. When scientists got their hands on the cometary dust, they found the grains had formed close to the Sun at incandescently hot temperatures. That showed that hot materials had been transported throughout the early Solar System and somehow become incorporated into the icy body of the comet.
While flying through space, Stardust also scooped up at least seven dust particles from interstellar space. They were surprisingly different from one another, including two that contained crystalline minerals that researchers had not expected to find in the space between the stars2.
The solar wind
Despite Stardust’s success, 2004 wasn’t a great year for sample return. After spending more than two years in space collecting some of the charged particles that stream from the Sun and make up the solar wind, NASA’s Genesis spacecraft crashed into the Utah desert. As it flew back to Earth, its parachute failed to deploy when re-entering the atmosphere, and the spacecraft plummeted into the ground at 300 kilometres per hour, breaking apart.
But engineers salvaged much of the canister containing the precious solar-wind samples. Researchers have used them to make discoveries, including that the solar wind — and thus the Sun — has a higher proportion of the main oxygen isotope than has Earth, contrary to what scientists had expected3.
Returning samples from Mars is a bigger challenge than any other mission so far. Mars is farther away than the Moon and has more gravity than a comet or an asteroid, making it harder to escape the surface and get back to Earth.
NASA wants Perseverance to drill and store at least 30 tubes of Martian rock and soil at its landing site in Jezero Crater. Long-term plans call for NASA and the European Space Agency to collaborate to send a second rover to collect those tubes and launch them into Martian orbit, and a third spacecraft to fetch them from Martian orbit and fly them back to Earth. The aim is for the samples to reach Earth in 2031.
But Japan might well achieve the first sample return from Mars — sort of. JAXA is developing a spacecraft that would fly to Mars’ biggest moon, Phobos, and scoop up some dust there and fly it back to Earth as early as 2029. The mission is called Martian Moons Exploration, or MMX.
MMX would mark the first material from the Mars system ever brought back to Earth. A paper last year4 reported that the surface of Phobos probably contains many particles from Mars, kicked off the surface by meteorite impacts and stuck onto Phobos. If so, then MMX might be able to pick up more than 100 grains on Phobos that originally came from Mars.
Each of those grains could contain minerals that yield information about the material’s age, as well as its magnetic and chemical properties, say Tomohiro Usui and Ryuki Hyodo of JAXA’s Institute of Space and Astronautical Science in Sagamihara. “Each grain has geochemical information about the Martian surface environment from the time that grain was formed.” By analysing as many Mars grains as possible, researchers can build up a picture of how the Martian surface environment changed over time.
And by working first with Mars samples in the lab, the MMX team can help prepare NASA and ESA for what lies ahead with Perseverance.
Nature 584, 16-17 (2020)