An iron-clad asteroid

A team from Japan and Jena has now succeeded in coaxing a previously undiscovered secret from some of these tiny sample particles: the surface of the dust grains is covered with tiny wafer-thin crystals of iron. This observation surprised Prof. Falko Langenhorst and Dr Dennis Harries of Friedrich Schiller University in Jena. After all, over the last 10 years, research teams all over the world have exhaustively studied the structure and chemical composition of the dust particles from Itokawa, and no one had noticed the iron 'whiskers'. It was only when Japanese researcher Dr Toru Matsumoto, who is spending a year as a visiting scientist with the Analytical Mineralogy group at the Institute of Geosciences in Jena, examined the particles with a transmission electron microscope that he was able to locate the crystals using high-resolution images.

Solar wind weathers celestial bodies

This discovery is exciting not only because the tiny iron 'whiskers' -- which have since been shown on other particles from the asteroid as well -- had previously been missed. Of particular interest is how they were formed. "These structures are the consequence of cosmic influences on the surface of the asteroid," explains Falko Langenhorst. In addition to rocks, high-energy particles from the solar wind also strike the asteroid's surface, thus weathering it. An important constituent of the asteroid is the mineral troilite, in which iron and sulphur are bound. "As a result of space weathering, the iron is released from the troilite and deposited on the surface in the form of the needles that have now been discovered," says the mineralogist Langenhorst. The sulphur from the iron sulphide then evaporates into the surrounding vacuum in the form of gaseous sulphur compounds.

From the size and number of the ice crystals detected, the researchers can also estimate how quickly the asteroid loses sulphur. "The process is incredibly fast from a cosmic perspective," explains Toru Matsumoto. The crystals he analysed are up to two-and-a-half micrometres long, which is around one-fiftieth of the thickness of a human hair. "The tiny whiskers have already reached these sizes after around 1,000 years," adds the researcher from Kyushu University in Fukuoka. Over the long term, the analysis of the ice crystals can be used to gain a better understanding of weathering processes on other celestial bodies as well, and to determine their age.

To this end, the researchers already have specific asteroids in their sights. NASA's OSIRIS-REx probe is currently preparing to take samples from asteroid Bennu, while JAXA's Hayabusa2 is already on its way back to Earth. The Japanese probe visited the Ryugu asteroid last year and, as with Itokawa, it collected dust particles. The samples should land on Earth at the end of 2020 and the international team of Jena mineralogists and Toru Matsumoto are awaiting them with anticipation.