Astroboffins think strangely porous boulders found on asteroid Ryugu may be the stuff of proto-planets

Pumice-like stuff spotted by Japanese Hayabusa2 probe would float on water

Asteroid Ryugu, the rock from which Japan’s Hayabusa2 probe brought home some samples, is home to highly porous boulders that might just have been the stuff of which planets were made.

So says a new paper titled “Anomalously porous boulders on (162173) Ryugu as primordial materials from its parent body” published on May 24th in Nature Astronomy.

The paper is based on observations of the asteroid, not analysis of samples returned by Hayabusa2.

As explained in a Japan Aerospace Exploration Agency (JAXA) summary of the paper, Hayabusa2 used its Thermal Infrared Camera and the infrared radiometer carried by its Mobile Asteroid Surface Scout (MASCOT) lander to get a look at Ryugu’s rocks.

They found that many of the rocks on Ryugu’s surface have low thermal inertia – a measure of how quickly they can be warmed or cool. Low thermal inertia indicates low density or high porosity.

Global map of Ryugu captured during descent
to Ryugu and used in this paper. Click to enlarge

JAXA’s summary of the paper describes images captured by the probe as depicting material “similar to a pumice stone used to remove dead skin cells on your foot.” By doing so it means, dear reader, that you have discovered something about asteroid rocks and astroboffins’ grooming habits.

High porosity has also been observed in the core of comets, which are thought to be relics of the “planetesimals” – the balls of dust and rock that agglomerated and may eventually have grown into planets.

The paper’s many authors suggest that the stuff found on Ryugu are remnants of planetismals that never got big enough for gravity and pressure to squish their component parts into planets. They suggest that the porosity is intrinsic, and not the result of impacts with Ryugu or cracking, and that the asteroid therefore has a lot to teach us about how planets formed in our Solar System.

JAXA’s summary holds out some hope that the samples it holds will be found to contain material with the same porosity observed by other instruments.

Data used by the paper's authors is available here, if you want the chance to draw your own conclusions. ®