What did Rosetta-Philae discover at comet 67P/Churyumov–Gerasimenko?

Monday, September 20, 2021

The Rosetta-Philae spacecraft that visited 67P/Churyumov–Gerasimenko was perhaps ESA’smost ambitious mission. Launched in 2004 and arriving in 2014, it spent 2 informative years around the very interesting-looking comet. Although not everything went to plan, the data that Rosetta and Philae were able to collect about comets has changed our perspective of what we understand about the formation of the solar system and even about our home planet, Earth.

The data will continue to be examined for years to come, but what have we learned so far? we will uncover what the Rosetta-Philae mission discovered around 67P/Churyumov–Gerasimenko.67P is currently a Jupiter family comet, meaning its orbit doesn’t take it much further out than Jupiter anymore, although it was once a Kuiper belt object which means it originated beyond the orbit of Neptune. Arriving at this comet was a revelation to mission planners by itself. So far, nothing in the solar system that has been examined closely looks anything like67P. It’s about 5 km across at its longest point and has two lobes which are joined by a narrow stretch of material in the middle.

This by itself was somewhat unusual, but it is also very jagged unlike a lot of asteroids that we’ve visited. During the course of the mission, it was discovered that the surface of this comet is quite changeable. If we look closely at the neck connecting the two lobes, it becomes apparent that this section is under mechanical stress. If we have a look at these rocks, we can see that there are fracture lines running through them.

Fracture lines are also apparent from a different angle. Scientists have used models based on fracture lines found all around the neck region to determine that these fractures permeate deeply inside the comet, up to 500m below the surface. It seems that as the comet rotates about its axis, the two lobes are pulling away from each other, thinning the neck region gradually over time. Huge 10m boulders were observed being displaced by this mechanical stress, as well as from the volatility on the surface, sometimes by up to 100m due to the comet’s weak gravity.

This also implies that the comet is really quite brittle and porous, which is something that wasn’t known about comets before this mission. As well as fracture lines, layers can also be seen, implying that during its formation, this comet was built up gradually over time. However, even though it is brittle, the surface of the comet is a lot harder than expected. Scientists thought the initial landing site for Philae would almost be soft and fluffy, kind of like dirty snow, but this was not the case. As Philae came to land on 67P to directly interact with the comet, it found that its final resting location was solid, thought to be water ice, with a thin layer of dust. Mission controllers for Philae tried to get a sample of the soil, but as you can see, Philae ended up at an awkward angle and wasn’t able to get its drill into the surface. However, readings were still able to be obtained by examining the material on the craft itself, which had ended up on Philae after the bounces.

Of the surface material examined, it was determined that there were 16 different organic compounds, four of which had never been detected on a comet before. While organic compounds do not mean life, life is based on organic compounds. While Philae wasn’t able to get too many readings from the surface, Rosetta was able to get some samples of the comet by collecting some of the dust “snow” that was ejected away from the comet into space. 

Throughout the mission, Rosetta collected roughly 31,000 dust particles, and interestingly, their composition didn’t change throughout the course of the mission even as the comet became more active, meaning that the whole nucleus of the comet is likely to be consistent throughout. The dust particles consisted of complex, organic, carbonaceous material, mixed in with sodium, magnesium, aluminum, silicon, calcium, and iron. What separates this material from an asteroid’s, however, is the presence of an abundance of hydrogen and oxygen. It is theorized that asteroids have been heated a lot longer than comets have due to their closer proximity to the Sun, which has stripped the hydrogen from their compositions.

Comets, however, have been kept away from the inner solar system for much of their lives, meaning these dust samples are pristine relics from the formation of the solar system, and potentially even the molecular cloud the Sun would have originated from. Oxygen was an unexpected find, as it’s highly reactive, and if there is hydrogen around, it will usually bind together to form H2O. Carbon and hydrogen were also detected in the comet’s tenuous atmosphere by Philae.

The dust particles you see here are tiny, the biggest that was collected was only 2mmacross, but interestingly it is particles just like these ones that light up the sky during a meteor shower. What these views do give us though is an insight into the material that formed the solar system, so we can see where our solar system evolved from. Comets tend to be very dark, only reflecting 3-4% of the sunlight that falls on them, which you wouldn’t expect from something that is considered to be icy. But actually, not a lot of the ice in a comet is exposed to the surface directly, most of the comet is coated in this layer of complex carbonaceous dust, which is darker than asphalt.

The light that isn’t reflected is instead absorbed, heating the volatile material beneath the dust layer, causing outgassing of water and carbon dioxide, which also blasts the tiny dust particles Rosetta picked up into space. These colored sections in this time-lapse show exposed water ice, as you can see, it's not a very big percentage of the comet itself. Zooming out a little bit and looking at comets generally, this is why comets have two tails. One tail follows the orbit of the comet, this tail is the dust tail.The dust tail is illuminated as it reflects sunlight. The other tail consists of the volatile material, the water, and carbon dioxide that outgassed from the comet.

This tail follows the direction of the solar wind, and these particles are illuminated through ionization and interactions with the charged particles from the Sun. It is often hard to see comets with your naked eye on Earth, but every so often a comet will outgas enough material that it is visible. In the northern hemisphere, the last one I saw was Hale Bopp in 1997 when I was just a kid. What an amazing sight it was! You guys in the southern hemisphere have been a bit luckier with comets, you’ve had CometMcNaught in 2007 and Comet Lovejoy in 2011. Going back to 67P, there was one other very big reason why the Rosetta-Philae mission happened in the first place, and that was to see if water on comets is the source of water on Earth. Before this mission, the theory was that Earth was bombarded by comets early in its development, back when the solar system was a lot more chaotic. Considering a large portion of comets are water ice, these could have given the surface of Earth the water we enjoy today. But as it turns out from Philae’s findings, this was not the case. Scientists were able to determine this from the water vapor’s deuterium ratio to hydrogen, which is significantly different from Earth’s. Deuterium is an isotope of hydrogen with an added neutron. The ratio of deuterium to hydrogen in water is key to determining where in the SolarSystem an object originated.

Here’s Earth’s ratio, and here is 67P’s. As you can see, they are very different. Only two comets have had their water vapor measured for deuterium directly, 67P and Halley’scomet, and neither suggest that comets were the source of water on Earth. Instead, this data gives more weight to models that suggest asteroids are the source of water on Earth, even though their water content is generally very low. If this is the case, Earth had a rough time during its formation. Rosetta and Philae were also equipped to detect if the comet had a magnetic field. Initially, scientists thought they discovered the presence of a magnetic field on the comet, the “hum” of which they converted to audio sound. This is what it sounds like! However, it turns out this was not the result of a magnetic field, as Philae could not detect the presence of a magnetic field on the surface, but rather, this sound is the solar wind interaction with the comet’s atmosphere. In fact, because of this interaction, the atmosphere and comet nucleus are completely devoid of any magnetic field, which is called a diamagnetic cavity.

Rosetta finished its mission by crashing into the surface of the comet. As the comet was going further away from the Sun, there was no guarantee Rosetta would have enough power for its heaters, so in order to maximize the science gained, mission controllers commanded it to perform a controlled descent into the comet. During this descent, it took multiple images which you can see in this time-lapse, providing better resolution images of the comet than ever before until it finally hit the surface and all communication was lost. Between Rosetta and Philae, they have opened our eyes to what the solar system was like during its formation, and have provided data that has and will yet lead to many discoveries.

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