How does the universe hide its solid iron?

A Japanese research team launched a rocket loaded with lasers and a nucleation chamber more than 300 km above Earth’s surface to investigate how the universe might stow its solid iron. Although the mission didn’t discover interstellar iron troves, the researchers did find one form the element is unlikely to take: pure iron granules (Sci. Adv. 2017, DOI: 10.1126/sciadv.1601992).

Iron-rich particles floating in the expanses between stars likely play a role in catalyzing the universe’s chemical evolution, but scientists are uncertain about the forms solid iron takes in interstellar space. Pure iron particles now seem an unlikely option, according to the research team led by Yuki Kimura of Hokkaido University.

The team evaporated iron atoms within a nucleation chamber that allowed researchers to control temperature and pressure. Flying the chamber in parabolic patterns aboard a rocket allowed the team to emulate the microgravity of space. The famed “vomit comet” aircraft used to train astronauts relied on the same patterns, but rockets can counteract gravity’s tug for longer times.

The team observed pure iron granules forming using a laser interferometer, but such events were exceedingly rare. Only about one in every hundred thousand iron collisions resulted in atoms sticking together and nucleating particles, the team reports.

This low sticking probability surprises Bruce T. Draine, an astrophysicist at Princeton University, although he’s not sure how much the finding tells us about the forms interstellar iron does take. Draine and Kimura’s team think that iron atoms are most likely captured by existing particles, such as silicates and carbonaceous grains, floating through space.