A small robot finds an asteroid that is strangely dust-free

In recent months, the Japanese probe Hayabusa2 has been implicated in various acts of interplanetary aggression. She shot the asteroid Ryugu in order to blow free matter for a return to Earth. But Hayabusa's visit2 also featured a variety of less violent activities, as Ryugu's imagery and characterization gave us a new body image, supposed to act as a time capsule for material formed at the very beginning of our solar system.

As part of these studies, Hayabusa2 dropped a Franco-German robot intended to jump on the surface of the asteroid in order to sample some of its rocks. Despite landing backwards, the robot finally took the correct orientation and an article describing his findings was published in Thursday's Science edition.

While hopping, but not like a rabbit

If you are like me, then the image of a small robot jumping to the surface of an asteroid has made me think of something adorable, or even d & # 39; anthropomorphic. You can get rid of these images immediately. MASCOT, the Scout mobile surface asteroid, is a rectangular box. Its ability to jump is provided by a weighted internal device. By quickly rotating this weight, the robot could generate enough speed to overcome Ryugu's tiny gravitational field and launch the box to new locations.

This jump ability proved to be essential to the mission. After Hyabusa2 dropped MASCOT 40 meters above the surface of the asteroid, the robot bounced for 17 meters before settling into an uncomfortable position. An attempt to orient the probe did not go very well: "A raising maneuver at this first measurement position left the undercarriage upside down, with most of the instruments geared towards the sky." The imagery of this position has confirmed the existence of Jupiter, Saturn and star σ Sagittarii.

Thus, the appropriate commands were sent and the box made another jump. This time he landed in the right direction to do some observation. In addition to his weighted arm for jumping, MASCOT had a camera, a magnetometer, a radiometer and an infrared spectrometer. These allowed him to get an idea of ​​the materials that surround them and to associate their properties with specific rocks.

A tale of two rocks

The area around MASCOT had two different types of rocks. The first was darker and rougher, with a surface appearance that researchers describe as "cauliflower-like". The second was brighter and had smoother surfaces and more angular shapes. These two categories of materials are consistent with the idea that Ryugu's "debris" appearance is the result of a collision between two bodies with different properties, with debris slowly re-agglomerating under the body's low gravity. (This also accords with the alternative view that Ryugu is made from a small fraction of the remains of a single body that has formed different geological layers.)

The Hyabusa2 team think that the characteristics of the two different materials are the product of the heating / cooling cycles that occur when the rocks are exposed to sunlight and then in the darkness of space . In some cases, this causes the rock to burst, producing smoother surfaces that are observed between certain materials. In others, the rock slowly disintegrates, losing its internal structure and producing the other rocks observed by MASCOT.

The strange thing about this, however, is that crumbled and broken rocks usually produce dust and sand-like materials. Still, there was no sign of that. "MASCam did not observe any deposition of fine material during the descent," note the researchers, before continuing: "We expect the dust to continually form on the surface of Ryugu by exposure to water." space environment. " They assume that the smallest grains are either lost in space or end up entering the rubble. But the rocks visible to MASCOT all measured tens of centimeters or more (some reached tens of meters).

Ready for my close up

Whatever the case may be, the close-ups of several of the rocks observed by Ryugu have shown that it was composed of composites with many small inclusions embedded in larger matrices of material. This is exactly the kind of things that scientists hoped to see. Indeed, it was thought that the first rocks formed when our solar system began to condense were composites, the inclusions representing the high-melting materials that condensed first. If similar materials are found in the cache returned to Earth by Hayabusa2, then they can help illuminate our ideas about the formation of the solar system.

In any case, MASCOT's work on the asteroid ended in less than a day, as the battery only had the battery capacity for about 17 hours of operation. But there will undoubtedly be additional studies from all the data captured during the brief period of operation.

Science, 2019. DOI: 10.1126 / science.aaw8627 (About DOIs).