Rosetta data reveal an invisible ultraviolet aurora around comet 67P

Comet 67P/Churyumov-Gerasimenko has its own version of the northern lights.

Observations taken by the Rosetta spacecraft reveal the comet’s aurora, which — unlike Earth’s eye-catching light shows — shimmers in invisible ultraviolet light, researchers report online September 21 in Nature Astronomy. Comet 67P joins comet C/Hyakutake 1996 B2, Mars (SN: 3/19/15), Saturn (SN: 4/6/20) and moons of Jupiter as known hosts of extraterrestrial auroras.

Electrons in the solar wind — a stream of charged particles continually flowing from the sun — interact with the gas surrounding 67P to create the auroral glow, planetary scientist Marina Galand of Imperial College London and colleagues report. Solar wind electrons are drawn toward the comet by an electric field surrounding 67P, similar to the way electrons cascade into Earth’s atmosphere to produce the northern and southern lights (SN: 7/25/14).

Electrons strike oxygen in Earth’s atmosphere to paint the sky red and green. But solar wind electrons strike water molecules in 67P’s coma, or shroud of gas. That shatters the water molecules and makes some of the resulting oxygen and hydrogen atoms glow ultraviolet. A similar water-smashing interaction creates auroras on Jupiter’s moons Europa and Ganymede (SN: 3/12/15).

Also unlike Earth, 67P has no magnetic field to steer incoming electrons toward the poles and form auroras with distinct patterns in the sky (SN: 2/7/20). If 67P’s ultraviolet aurora were visible, it would look like a diffuse halo around the comet.

Such cometary auroras could someday be used to probe variations in the solar wind, Galand says. That may lead to better forecasts for space weather, which can mess with satellites and power grids (SN: 7/5/18).