Exclusive: We’ve collected the very first real pebbles from an asteroid

Last September, NASA’s OSIRIS-REx returned the largest amount of asteroid material in human history to Earth. And among them are the largest physical fragments of an asteroid: pebbles and other small rocks from the surface of asteroid Bennu. In 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 target. About 70.3 grams (2.48 ounces) were recovered very soon after the capsule landed. For the remaining material, a problem with the Touch-and-Go Sample Acquisition Mechanism (TAGSAM) fasteners meant some delays and creative solutions to get them going.

“Once we had TAGSAM fully open, we saw a glorious 121.6 grams. It’s an interesting song. It’s more than double what we need 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 bit like ‘oh, I thought I had more’,” Professor Dante Lauretta, the principal investigator of OSIRIS-REX, told IFLScience. He then jokingly added, “I’m not trying to be greedy!”

‘A huge achievement’ is almost an understatement for this endeavor. Hayabusa-2’s collection of approximately 5.4 grams (less than 1 ounce) has already been revolutionary and has yielded phenomenal discoveries such as the presence of amino acids and hydrous minerals. Of the accessible 70 grams of Bennu, 1 gram has been distributed to research centers in the US and internationally. Based on that preliminary analysis alone, 58 presentations of findings will take place 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 many rocks in the centimeter range,” Professor Lauretta told IFLScience. “What we are currently doing is characterizing those stones. We’re doing a lot of work in Houston, in the curation lab, to understand the nature of that material. These are the rarer parts of the collection and are very valuable scientifically because you get the whole rock texture on a larger scale. And that will be important for the processes we want to investigate.”

One of those processes concerns the origin of asteroid Bennu itself. The team is extremely excited about the possibility that Bennu could have formed from an ocean world – a much larger body with liquid water – possibly beneath 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 as large, so about 250 kilometers in diameter.

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

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

A second line of evidence concerns bulk composition. Some analyzes have shown an abundance of water-soluble elements such as sodium, potassium, uranium, thorium and barium. Furthermore, the work suggests that they were shifted there by a fluid.

The third piece of evidence is the presence of a phosphate crust on some of the samples analyzed. Lauretta describes it as a kind of coating on the rocks and it looks like something left behind when the water evaporated. Enceladus’ oceans are rich in phosphates.

“All three of these things support the hypothesis. And I want to emphasize that it’s just a hypothesis at this point. We’re still coming up with ideas to test it. But for me it is the leading candidate for the geological environment in which these rocks were formed,” Professor Lauretta told IFLScience.

With just a few months of analysis, the Bennu monster is already making us dizzy with possibilities. It is a window into the early times of the solar system and will provide new insights into the formation of asteroids and planets. And it could even help us explain how water came to our planet, and perhaps the building blocks of life.

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