Last year, the ESA-led Rosetta mission made history by being the first spacecraft to orbit a comet and deploy a lander to its surface. The robotic probe has spent the last 12 months orbiting a comet – officially named Comet 67P/Churyumov-Gerasimenko, or Comet 67P for short – from varying altitudes, beaming back stunning images and unprecedented science data. Comet 67P looked like a typical short-period comet when it was selected as Rosetta’s target, but as the spacecraft made its approach, it was soon apparent this comet was anything but typical.
Comets are chunks of rock and ice, harboring significant amounts of volatile material that sublimates as it’s heated. Due to its weak gravity, the comet is unable to hang onto the sublimated gasses, and they escape into space to form an atmosphere (also called a coma) and a tail. Comets have undergone minimal heating (cosmically speaking) and their chemistry is thought to be primordial, and virtually unchanged over their lifetime.
We know that Comet 67P contains such volatiles as carbon monoxide and molecular nitrogen. We also know that the comet resembles a rubber duck as it features two lobes – a smaller “head” and a larger “body” – connected by a slender neck. Presumed to be leftover bits from the Solar System’s early planet-forming days, comets are essentially cosmic time capsules that can tell us a lot about the conditions when they formed. The Solar System’s early days resembled a cosmic Crash-a-Rama of sorts, with comets and other planetary bodies smashing into one another, constantly forming and reshaping worlds while leaving debris in their wake. With such a fragile neck, the odds that Comet 67P could have survived intact for billions of years are slim. So there must be another explanation.
In addition to its unique shape, Comet 67P’s activity or outgassing has been a source of intrigue for scientists. A new paper published in the Astrophysical Journal offers an explanation for both.
Image: Locations of rapid temperature change on Comet 67P. Credit: V. Alí-Lagoa, M. Delbo, and G. Libourel / Astrophysical Letters
Prior to Rosetta’s arrival, mission scientists extensively studied thermal maps of the comet to determine the best possible landing site for a lander. They needed to find an area with maximum sunlight but minimum activity. In their search, the team noted that the hottest regions were on the lobes – with one hot spot on each. However, they were surprised to find minimal outgassing in these regions. Most of the activity occurred around the comet’s neck.
For the study, a trio of scientists used thermophysical models to examine how the comet’s shape affects surface temperatures in different regions. The team used a rotating comet model to measure thermal changes as regions were exposed to sunlight and then cast in shadow. Their results showed that the neck region experienced the most extreme changes in temperature – as much as 30 kelvin per minute – which correspond with areas of observed outgassing.
In their paper, the authors explain, “Our work suggests that these fast temperature changes are correlated to the early activity of the comet, and we put forward the hypothesis that erosion related to thermal cracking is operating at a high rate on the neck region due to these rapid temperature variations. This may explain why the neck contains some ice.”
Thermal changes can lead to surface cracking, with more dramatic changes resulting in more rapid cracking. When a comet’s surface cracks, subsurface volatiles can be exposed, which would lead to sublimation and even more cracking, especially as the comet approaches the Sun. If true, this means that any small concavity or crater could eventually evolve into a neck like we see on Comet 67P, given enough time.
The authors suggest that this process may be common on small bodies throughout the Solar System. By studying these enigmatic, icy worlds and the processes that shape them, scientists hope to better understand the origin and evolution of comets, and ultimately the Solar System.
[H/T: The Planetary Society]