Cosmos Editorial
Comet 67P is shining bright
Data from the European Space Agency’s Rosetta spacecraft have revealed auroral emissions in the far ultraviolet around a comet for the first time, astronomers say.
The auroras we see on Earth are formed when charged particles from the Sun follow the planet’s magnetic field lines to the north and south poles. Solar particles strike atoms and molecules in the atmosphere, creating shimmering curtains of coloured light.
Similar phenomena have been seen around planets, moons and even a distant star in our Solar System – and now at Comet 67P/Churyumov-Gerasimenko, according to a paper in the journal Nature Astronomy.
“Initially, we thought the ultraviolet emissions… were phenomena known as dayglow, a process caused by solar photons interacting with cometary gas,” says Joel Parker, from the Southwest Research Institute (SWRI), US.
“We were amazed to discover that the UV emissions are aurora, driven not by photons, but by electrons in the solar wind that break apart water and other molecules in the coma and have been accelerated in the comet’s nearby environment. The resulting excited atoms make this distinctive light.”
A team led by Marina Galand from Imperial College London, UK, used a physics-based model to integrate measurements made by the instruments aboard Rosetta and “unambiguously identify” how the ultraviolet atomic emissions form and reveal their auroral nature.”
“I’ve been studying the Earth’s auroras for five decades,” says SWRI’s Jim Burch. “Finding auroras around 67P, which lacks a magnetic field, is surprising and fascinating.”
New ice on Enceladus
We also have new and detailed views of Saturn’s moon Enceladus, courtesy of NASA’s Cassini spacecraft. And the data used to build them provides strong evidence the northern hemisphere has been resurfaced with ice from its interior.
Cassini’s Visible and Infrared Mapping Spectrometer (VIMS) collected light reflected off Saturn, its rings and its 10 major icy moons – light that is visible to humans as well as infrared light. VIMS then separated the light into its various wavelengths, information that tells scientists more about the makeup of the material reflecting it.
The data, combined with detailed images captured by Cassini’s Imaging Science Subsystem, were used to make the new global spectral map of Enceladus.
“The infrared shows us that the surface of the south pole is young, which is not a surprise because we knew about the jets that blast icy material there,” says Gabriel Tobie, from the University of Nantes, France, co-author of a paper in the journal Icarus.
“Now, thanks to these infrared eyes, you can go back in time and say that one large region in the northern hemisphere appears also young and was probably active not that long ago, in geologic timelines.”
The rocks on Ryugu
As researchers eagerly await the arrival of rocks from Ryugu (they’re on board Hayabusa2 and due in Australia on 6 December), a Japanese team studying the spectral properties of the asteroid’s surface features has reported identifying two kinds of bright boulders.
Writing in Nature Astronomy, Seiji Sugita from the University of Tokyo and colleagues suggest that the nature and distribution of the two, and the subtle differences between them, may reveal how the asteroid formed.
“Ryugu is considered a C-type or carbonaceous asteroid, meaning it’s primarily composed of rock that contains a lot of carbon and water,” says co-author Eri Tatsumi.
“As expected, most of the surface boulders are also C-type; however, there are a large number of S-type or siliceous rocks as well. These are silicate-rich, lack water-rich minerals and are more often found in the inner, rather than outer, Solar System.”
This leads the team to believe Ryugu likely formed from the collision between a small S-type asteroid and a larger C-type parent asteroid. If the nature of this collision had been the other way around, they say, the ratio of C- to S-type material in Ryugu would be reversed.
“We used the optical navigation camera on Hayabusa2 to observe Ryugu’s surface in different wavelengths of light, and this is how we discovered the variation in rock types, Tatsumi says.
“Among the bright boulders, C and S types have different albedos, or reflective properties.”
