The Cosmic Watchdog

Underground pools of liquid brine probably formed 4.6 billion years ago on the parent body of the asteroid Bennu and contained the building blocks of life but not living creatures.

That’s the takeaway from new chemical analyses of samples that NASA’s OSIRIS-REx mission snagged from the roughly 500-meter-long space rock in 2020 and returned to Earth three years later. The findings suggest that not all potentially habitable environments go on to develop life, researchers report in two papers published January 29 in Nature and Nature Astronomy.

Planetary scientists study asteroids and comets because they offer a glimpse of the pristine conditions present in the early solar system. “It’s like trying to understand what Thanksgiving dinner is by looking at the leftover scraps on the compost pile,” says astrobiologist Jason Dworkin, the project scientist for OSIRIS-Rex at NASA’s Goddard Space Flight Center in Greenbelt, Md. “You can look and see: ‘Oh, there’s a little bit of carrot tops. There must have been carrots used somewhere.’”

In this case, investigations of the space rock’s compost pile yielded organic compounds including the five nucleic acids that make up DNA and RNA and 14 of the 20 amino acids that cells use to make proteins, along with 19 other amino acids rarely used by living things. There were also a number of salty minerals that would have formed as liquid water slowly evaporated, similar to those found around dry lakebeds on Earth.

Bennu, which orbits mostly between Earth and Mars, broke off between 700 million and 2 billion years ago from a larger body, an ancient relic of the dawn of the solar system that formed somewhere out past the orbit of Saturn. The decay of radioactive elements present in that earlier body would have produced heat, melting water inside the object and creating room temperature brine pools that might have been a few meters deep and persisted for several thousand years.

While such warm wet ponds appear to have contained the biochemical precursors for life, these pools don’t seem to have proceeded to the life-making stage. Many organic molecules come in two mirror-image versions of one another. Living things on Earth almost exclusively use the left-handed versions, while Bennu’s sample contains the right- and left-handed versions of nucleic and amino acids in equal amounts.

Some potential explanations are that the brine pools didn’t last long enough, or that they had the wrong temperature or pH, or perhaps that Bennu lacked an atmosphere, Dworkin says.