Exclusive: We Have Collected The First-Ever Actual Pebbles From An Asteroid

Last September, NASA’s OSIRIS-REx brought back to Earth the biggest haul of asteroid material in the history of humanity. And among that, there are the largest physical fragments of an asteroid: pebbles and other small rocks from the surface of asteroid Bennu. By contrast, the Hayabusa probes that collected samples from Itokawa and Ryugu, respectively, brought back only grains from the two space rocks.

OSIRIS-REx managed to collect so much more both in terms of mass and size. The total amount of material is 121.6 grams (4.29 ounces), double the mission goal. Roughly 70.3 grams (2.48 ounces) were accessed very soon after the capsule landed. For the remaining material, a problem with the fasteners of the Touch-and-Go Sample Acquisition Mechanism (TAGSAM), meant some delays and creative solutions to get them budging.

“Once we got TAGSAM fully open we saw the glorious 121.6 grams. It's an interesting number. It's more than twice what we are required to bring back, but it's less than half of what I thought we had. It's exciting because it's more than we promised, but also a little bit like ‘oh, I thought I had more’,” Professor Dante Lauretta, the principal investigator for OSIRIS-REX, told IFLScience. He then jokingly added: “I try not to be greedy!”

"Huge achievement" is almost an understatement for this endeavor. The collection from Hayabusa-2 of about 5.4 grams (less than 1 ounce) has been already revolutionary, delivering phenomenal discoveries such as the presence of amino acids and water-bearing minerals. Of the accessible 70 grams from Bennu, 1 gram has been distributed to research centers across the US and internationally. From that preliminary analysis alone, 58 presentations with findings will happen at the upcoming Lunar and Planetary Science Conference. And there is much more to come.

“We have stones up to three and a half centimeters [1.4 inches] in their longest dimension, and a lot of stones in the centimeter size range,” Professor Lauretta told IFLScience. “Currently, what we've been doing is characterizing those stones. We are doing a lot of work in Houston, in the curation lab, to understand the nature of that material. Those are the rarer parts of the collection and are scientifically really valuable because you get the whole rock texture at a larger scale. And that's going to be important for the processes that we want to study.”

One of those processes is about the origin of asteroid Bennu itself. The team is extremely excited about the possibility that Bennu might have formed from an ocean world – a much larger body that had liquid water – possibly under an icy or rocky exterior like the icy moons of Jupiter or Saturn. Enceladus is a good example, but this parent body would be half its size, so about 250 kilometers across (155 miles) across.

“We still have work to do to test that hypothesis. I would say there's really three lines of evidence right now that's making me think about ocean world,” Professor Lauretta, who is the director of the Arizona Astrobiology Center, told IFLScience.

The first one is evidence of serpentinite, a type of rock that forms when hot igneous or metamorphic rocks meet water. On Earth, that happens at mid-ocean ridges and similar places. 

A second line of evidence is about the bulk composition. Some analyses have shown an abundance of elements soluble in water, such as sodium, potassium, uranium, thorium, and barium. On top of that, the work suggests they were shifted there by a fluid.

The third piece of evidence is the presence of a phosphate crust on some of the samples that have been analyzed. Lauretta describes it as a sort of coating on the rocks and it looks like something that was left behind as water evaporated. The oceans of Enceladus are abundant in phosphates.

“All those three things support the hypothesis. And I do want to emphasize it's just a hypothesis right now. We're still coming up with ideas on how to test it. But to me, it's the leading candidate for the geologic environment that these rocks formed in,” Professor Lauretta told IFLScience.

With barely a few months of analysis, the Bennu sample is already making us giddy with possibilities. It is a window into the early times of the Solar System and will provide new insights into asteroid and planetary formation. And, it might even help us explain how water came to our planet, and maybe about the building blocks of life as well.