Dry asteroids are actually wet: water molecules found

A team of US astronomers has confirmed the existence of water on the surface of two asteroids.

Using infrared data, the researchers spotted a signal that “unambiguously” comes from water molecules – the first time this signal has been seen on dry asteroids.

They’ve published their findings in The Planetary Science Journal.

“Asteroids are leftovers from the planetary formation process, so their compositions vary depending on where they formed in the solar nebula,” says lead author Dr Anicia Arredondo, a planetary scientist at the Southwest Research Institute, US.

“Of particular interest is the distribution of water on asteroids, because that can shed light on how water was delivered to Earth.”

The researchers used data from SOFIA: the now-retired Stratospheric Observatory for Infrared Astronomy, which was a Boeing 747SP with an infrared telescope attached.

They investigated signals from 4 asteroids, named Iris, Parthenope, Melpomene, and Massalia. These asteroids are anhydrous, or dry, asteroids: they form close to the Sun, unlike their icy counterparts which form far away.

“We detected a feature that is unambiguously attributed to molecular water on the asteroids Iris and Massalia,” says Arredondo.

“We based our research on the success of the team that found molecular water on the sunlit surface of the Moon. We thought we could use SOFIA to find this spectral signature on other bodies.”

One infrared light signal, with a wavelength of 3 micrometres, is commonly used to indicate hydration, because it comes from the O-H bond. But this doesn’t necessarily mean there’s currently water (H₂O) molecules present – it might be from hydroxyl (OH) compounds bound to rocks.

But SOFIA had previously spotted another infrared signal, with a wavelength of 6 micrometres, on the Moon. This signal definitely comes from water, because it’s emitted by an H-O-H bond.

The researchers also saw this signal on 2 of the asteroids.

“Based on the band strength of the spectral features, the abundance of water on the asteroid is consistent with that of the sunlit Moon,” says Arredondo.

The researchers couldn’t get clear enough signals from the other 2 asteroids (Parthenope and Melpomene) to tell whether they housed water.

They’ve now booked time on the James Webb Space Telescope to look more closely at other dry asteroids.

“We have another proposal in for the next cycle to look at another 30 targets. These studies will increase our understanding of the distribution of water in the solar system.”