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Lander Images Yield Clues on Asteroid Ryugu’s Makeup and Origins

surface of Ryugu asteroid
The first image acquired by MASCOT during Hayabusa2's descent. A block near the South Pole, which stands out above the horizon line, is up to 100 meters tall. | Credit: Jaumann et al., Science (2019)

New images taken during a prominent mission to the near-Earth asteroid Ryugu offer clues into the composition and origins of its surface rock, according to a new study published in the August 23 issue of Science.

The latest photographs from Japan's Hayabusa2 mission show the asteroid's rocks bear strong similarities to rare and primitive meteorites called carbonaceous chondrites, which suggests the asteroid may have formed during the early stages of the solar system in cataclysmic events.

The lander's images could also give scientists much-needed context when they begin to study samples of the asteroid gathered by the Hayabusa2 spacecraft, which was launched 2014 and is projected to return to Earth in 2020.

Ryugu is classified by astronomers as a near-Earth object, meaning its orbit occasionally brings it close to Earth. Although the asteroid appears barren, scientists believe it could still harbor organic matter and water created during the solar system's formation, according to the JAXA website.

By analyzing samples brought back from Ryugu, researchers could get a clearer picture of the origins of water on Earth and the source of organic matter that made life possible on our planet. The samples could also help scientists understand how other planets were created during collisions between smaller, planet-like objects.

"Asteroids are pristine material in the solar system that provide a window into the beginning of planetary formation," said Ralf Jaumann, a planetary scientist at the Institute of Planetary Research in Berlin, Germany and lead author of the new study.

The Hayabusa2 spacecraft carried a lander called the Mobile Asteroid Surface Scout (MASCOT), which was dropped onto Ryugu's surface in October 2018. As the lander approached the asteroid, its camera took 20 images of the surface, according to the study.

Curious about MASCOT's exact trajectory, Jaumann's team analyzed these images to reconstruct how the lander made its final descent. They traced how MASCOT initially bounced off the surface after the first impact, before settling at a second site 17 meters (or 55 feet) away.

The images acquired during the descent and bouncing show that the rocks and boulders on the surface fell into one of two categories: dark and rough, or bright and smooth.

Notably, both rock types were almost evenly distributed on the surface of the asteroid, the authors said. This observation supports the theory that Ryugu formed from rubble that reaccumulated after an impact onto a parent body.

Upon closer inspection, many of the rocks in the images appeared to harbor small, bright materials called inclusions that had either blue or red tints. Using light analysis techniques, the research team found the inclusions likely contained the mineral olivine and were similar to those found in carbonaceous chondrites, one of the oldest and most diverse categories of meteorites.

"The MASCam observation of abundant multicolor, millimeter-sized inclusions suggests that C-type asteroids [like Ryugu] are linked to carbonaceous chondrites," said Jaumann. "In addition, the MASCOT images provide geological context for samples being returned to Earth."

Unexpectedly, the images showed no fine particles or dust on the surface, which would normally accumulate due to the weathering that affects all objects in the harsh environs of space. The authors suspect there must be an unidentified physical mechanism that efficiently removes dust from the asteroid's surface.

"We expected dust as a result of cosmic weathering," said Jaumann. "The lack of fine particles suggests that there are specific erosional processes taking place on these bodies," which might include impacts by tiny meteorites, seismic shaking and the migration of boulders.

In the future, the scientists plan to compare their images with measurements taken from different carbonaceous chondrites on Earth, using a spare model of the camera. These comparisons could allow them to improve their database when they use MASCOT in future missions, said Jaumann.

[Credit for associated image: Jaumann et al., Science (2019)]