The dusty particles on comet 67P/Churyumov-Gerasimenko come in various shapes and sizes - from single grains to large porous clumps - reports a study published in this week’s Nature. These results stem from observations carried out by MIDAS, the first space-borne atomic force microscope, on board comet-orbiter Rosetta, and could provide clues about how the early Solar System formed.

Comets have stored material in an almost pristine state since the birth of the Solar System, when planets and comets were being built by the collision of dust particles. Comets therefore offer a rare opportunity to observe the microstructure of pristine dust particles, which had not been observed in situ until now.

Mark Bentley and colleagues report in situ measurements from MIDAS (the Micro-Imaging Dust Analysis System) of the size, shape, texture and microstructure of cometary dust on comet 67P collected during a period spanning November 2014 to February 2015. The authors show that the particles are aggregates of smaller, elongated grains. The particles were seen to cover a range of sizes - from tens of micrometres down to a few hundreds of nanometres - and a range of shapes - from single grains to larger, porous, aggregated particles. The authors also calculated the elongation of 114 dust grains, and confirmed that grain elongations are similar to the value inferred for interplanetary dust particles, supporting the idea that such grains could represent a fraction of the building blocks of comets.

“The authors’ results enhance our fundamental understanding of cometary dust, and the processes that ultimately gave rise to planetary systems such as the Solar System”, says Ludmilla Kolokolova in a related News & Views article.