Sciency Thoughts: 67P/Churyumov-Gerasimenko

Showing posts with label 67P/Churyumov-Gerasimenko. Show all posts

Thursday, 5 November 2015

Molecular oxygen in the coma of Comet 67P/Churymov-Gerasimenko.

In recent years studies of the comas of numerous active comets (the coma of an active comet is the gas and dust envelope which surrounds it; this is not a true atmosphere as it is continuously replenished by the sublimation of ices from the comets surface as it is lost to space), both by remote sensing and by space probes, has given us a good understanding of the composition of typical comets. In all comets water (H₂O), carbon dioxide (CO₂) and carbon monoxide (CO) together make up about 95% of the coma material, and presumably therefore the ices forming the comet itself. The remaining 5% of the material is predominantly made up of hydrocarbons and sulphur compounds with a broad array of other chemicals present, though to date molecular oxygen (O₂) has never been found in the coma of any comet.

In a paper published in the journal Nature on 29 October 2015, a team of scientists led by Andre Bieler of the Department of Climate and Space Science and Engineering at the University of Michigan and the Physikalisches Institüt at the University of Bonn, describe the discovery of molecular oxygen in the coma of Comet 67P/Churymov-Gerasimenko.

The discovery was made by the ROSINA-DFMS mass spectrometer on the Rosetta spacecraft, which measured the composition of the coma of 67P/Churymov-Gerasimenko from September 2014 until March 2015. While the levels of molecular oxygen varied over this time (as is the case for all molecules in the comas of comets, which are constantly lost to space and replenished by the melting of different patches of ice with differing compositions) the ratio of molecular oxygen to water remained fairly constant, suggesting that the molecular oxygen was trapped within water ice prior to its release.

Image of 67P/Churymov-Gerasimenko taken by the Rosetta spacecraft from a distance of 30 km, showing fine jets of material that make up the cometary coma. European Space Agency.

Molecular oxygen can form within water ice by photolysis (breaking of the bonds in the H₂O molecule by collisions with ultraviolet photons, with the smaller hydrogen atoms subsequently able to escape from the crystal matrix of the ice, while the larger oxygen atoms are trapped) or radiolysis (the same, but with more energetic photons or fast electrons and ions) and molecular oxygen detected on the Jovian moons Europa, Ganymede and Calisto and in the rings of Saturn is thought to have formed in this way.

However in addition to molecular oxygen (O₂₎) photolysis and radiolysis of water ice should form ozone (O₃) molecules; these have been detected on the Jovian moons and in the rings of Saturn, but are apparently absent from the coma of 67P/Churymov-Gerasimenko (or any other comet).

The Rosina DFMS (Double Focusing Mass Spectrometer) instrument that was used to detect molecular oxygen in the coma of 67P/Churymov-Gerasimenko. European Space Agency.

The alternative hypothesis is that molecular oxygen was present in the molecular cloud from which the comet formed. This would help to explain the presence of oxygen in the coma of 67P/Churymov-Gerasimenko but not other comets studied, as 67P/Churymov-Gerasimenko is a relative newcomer to the Inner Solar System (expand) retaining a coating of ices stable in the cooler environment of the outer Solar System but unstable closer to the Sun, which have been lost in most other studied comets, which are longer term residents of the Inner Solar System.

However this does not fit well with our current understanding of how the Solar System formed. To date molecular oxygen has been found in only two interstellar clouds, the ρ Ophiuchi Dense Core and the Orion A Giant Molecular Cloud. Studies of the ρ Ophiuchi Dense Core suggest that it has experienced temperatures of 20-30 K during its lifetime, while most interstellar clouds never experience temperatures of more than 10 K. This suggests that in order for molecular oxygen to have been present in the Outer Solar System to be incorporated into forming comets, the molecular cloud from which the Solar System formed would also have had to have been exceptionally warm.

Image of the ρ Ophiuchi cloud taken by the Wide-field Infrared Explorer (WISE) space telescope. NASA/JPL.

This is not something that has previously been predicted, and would prompt a re-evaluation of our ideas on how the Solar System formed. However one other piece of evidence from 67P/Churymov-Gerasimenko does support this theory. The coma of the comet has been found to contain considerably lower levels of molecular nitrogen (N₂) than predicted, and this lower nitrogen level is also consistent with formation from a warmer gas cloud.

Sunday, 4 October 2015

Sinkholes on Comet 67P/Churyumov-Gerasimenko.

Many comets have been observed to have pitted surfaces. Initially these pits were thought to be the result of collisions with smaller bodies, as with craters on planets and moons, but they have been shown to be far to numerous for this to be the case, as collisions between bodies are thought to be rare in the Outer Solar System and comets in the Inner Solar System have surfaces which are regularly resurfaced by the heat of the Sun. Alternatively it has been suggested that these pits could be the result of sublimation of certain ices on the surface of the comets (comets are made up of different ices, including water, carbon dioxide and carbon monoxide, which sublimate – pass directly from a solid to a gaseous state – at different temperatures). However this should produce wide, shallow pits, whereas many of the of those seen on comets appear to be deep and narrow.

In a paper published in the journal Nature on 2 July 2015, a team of scientists led by Jean-Baptiste Vincent of the Max Planck-Institut fü rSonnensystemforchung discuss observations made of comet 67P/Churyumov-Gerasimenko by the OSIRIS camera system on the Rosetta space probe between July and December 2014, comparing these observations to previous observations of comets 9P/Tempel 1 and 81P/Wild 2, and suggest a theoretical explanation for these phenomena.

Comet 67P/Churyumov-Gerasimenko had a close encounter with Jupiter in 1959, which altered its orbit from one with a perihelion of 2.7 AU (i.e. an orbit which at its closest approach to the Sun was 2.7 times as distant as the Earth) to one with a perihelion of 1.2 AU. This means that the surface of the comet is likely to be relatively 'fresh', with features reflecting a periodic exposure to a new highest temperature regime, rather than one developed from regular exposure to the same conditions over a very long time interval.

Rosetta paid particular attention to an area in the northern hemisphere of 67P/Churyumov-Gerasimenko with eighteen approximately circular pits, clustered in small groups and ranging from tens to hundreds of meters in diameter, with the deepest being a few hundred meters deep.

Image of Comet 67P/Churyumov/Gerasimenko taken on 19 September 2014 by the Rosetta space probe. ESA/Wikipedia.

The walls of these pits have a non-uniform texture, with smooth areas, fractured areas, areas showing terraces and alcoves, and in the deeper parts of the pits, areas with a globular texture. This globular texture extends at least 200 m below the surface, and may reflect the nature of the cometary interior. Jets of material could be seen emerging from the walls of some of these pits, primarily in areas with fractured or globular surfaces. These are thought to be the product of active sublimation of ices on the walls of the pits. However this sublimation cannot account for the formation of the pits, as it would take thousands of years of sublimation at the observed rate to form some of the larger pits.

Comet 9P/Tempel 1, which was visited by the Deep Impact spacecraft in July 2005, had a surface covered by much wider pits than those on 67P/Churyumov-Gerasimenko, with many of these pits having merged together and frequent violent emissions seen on its surface. 9P/Tempel 1 has actually been in its current orbit only slightly longer than 67P/Churyumov-Gerasimenko (it was shifted onto its current orbit by a close encounter with Jupiter in 1953), but this orbit brings it closer to the Sun (at its closest only 1.5 AU from the Sun, or times as far from the Sun as the Earth is) and its surface is therefore considered to be more processed. Comet 81P/Wild 2 also has a more processed surface with larger pits, also having an orbit which brings it much closer to the Sun (1.59 AU), which it switched to after a close encounter with Jupiter in September 1974.

Clearly, therefore, pit formation is a process occurring on comets with 'young' orbital paths, i.e. orbits which they have been switched to fairly recently and which result in more heating than their previous orbits, but cannot be accounted for by surface sublimation.

Vincent et al. suggest that these pits could be formed in a similar way to sinkholes on Earth, with voids within the comets being formed or enlarged by solar heating until material above the voids collapses inwards, leaving a circular or cylindrical pit at the surface. The walls of these pits are then exposed to direct solar heating, leading to further sublimation, which widens the pits, and eventually causes them to merge.

Vincent et al. propose three different (but not mutually exclusive) mechanisms by which such voids might form.

Firstly, they may have formed along with the comet. Comets (and other large bodies in the Solar System) are thought to have formed by the accretion of smaller bodies, and several large protocomets (or cometesimals) coming together in a low gravity environment could conceivably leave voids inside the final structure.

Secondly the voids could be formed by the sublimation pockets of low-temperature ices, such as carbon monoxide or carbon dioxide, within a largely water-ice body. This would require heating of the outer body of the comet combined with sufficient heat conductivity to sublimate these low temperature ices below the surface – conditions which are thought likely to occur in comets.

Finally a subsurface source of heating could trigger the formation of voids. The most likely source of such heat would be the re-crystallization of amorphous water-ice (which only exists at very low temperatures) to crystalline ice. This process requires heat input to begin, but once initiated releases further heat as a bi-product of the re-crystallization process.

Saturday, 9 August 2014

Rosetta Mission rendezvouses with Comet 67P/Churyumov–Gerasimenko.

The Rosetta Spacecraft moved into position alongside Comet 67P/Churyumov–Gerasimenko on Wednesday 6 August 2014, the first spacecraft to reach a cometary target, and ten years after the mission was launched. It will now spend six weeks making a series of close passes around the comet, before attempting to move into a stable orbit about 30 km above its surface. The Probe is equipped with a small lander, named Philae, which will attempt to reach the surface of the comet and take samples, probably in November. The Spacecraft will then remain with and monitor the comet as it moves towards and passes through its perihelion (the closest point on its orbit to the Sun) in August 2015.

The surface of Comet 67P/Churyumov–Gerasimenko. Rosetta/European Space Agency.

Images of the surface of 67P/Churyumov–Gerasimenko produced by Rosetta so far show an asteroid-like body. This is not altogether a surprise, as while asteroids and comets have been thought of as different categories of objects, recent studies have shown the division between them to be much less clear. During the formation of the Solar System the Sun would have been surrounded by a protoplanetary disk, within which the planets and smaller bodies orbiting the Sun would have formed. For a small body the main determining factor in what particles could accumulate in its composition would be temperature. In the inner parts of the Solar System only silica and metal based minerals could crystalize, due to the heat of the Sun, but further out within the protoplanetary disk a variety of ices could also form, with higher temperature ices such as water and carbon dioxide forming closer to the Sun than cooler ices such as nitrogen or cyanide.

Such bodies probably had roughly circular orbits when they formed but many have subsequently been disturbed and adopted more eccentric, elliptical orbits, which bring them closer to and further away from the Sun. When bodies come closer to the Sun they are warmed by the Sun's heat, and any ices in their composition will begin to sublimate (turn from solids directly to gasses - liquids cannot form in a vacuum), releasing clouds of snowflakes and dust in the process to form a visible cometary halo. Exactly how far from the Sun a comet begins to form such a halo depends on what ices are present, with lower temperature ices beginning to sublimate further from the Sun. Bodies have been found within the Main Asteroid Belt which behave as asteroids for most of the time, but which still produce a small halo around their perihelion (closest point on their orbit to the Sun), suggesting that they originated further out in the Solar System and at some point adopted a cometary trajectory, falling deeper into the Solar System then back out, from which they were then nudged into a more asteroid line path; these are thought to be fairly old bodies, still venting the last of their icy material.

67P/Churyumov–Gerasimenko is a short period, Jupiter Family comet (a comet with a period of less than 20 years with an orbit angled at less than 30° to the plane of the Solar System), with a 6.44 year orbital period and an elliptical orbit tilted at 7° to the plane of the Solar System, which takes it from 5.68 AU from the Sun (i.e 5.68 times as far from the Sun as the Earth, slightly outside the orbit of Jupiter) to 1.24 AU from the Sun (i.e. 1.24 times the average distance between the Earth and the Sun, roughly halfway between the orbits of Earth and Mars).

The orbit of 67P/Churyumov–Gerasimenko. JPL Small Body Database Browser.

67P/Churyumov–Gerasimenko was discovered in 1969 by Klim Ivanovych Churyumov and Svetlana Ivanovna Gerasimenko, working at the Kiev Astronomical Observatory. The name 67P/Churyumov–Gerasimenko implies that it was the 67th periodic comet discovered (technically all comets are likely to be periodic, but some have such long periods that they if they are seen again then the two events are unlikely to be connected; for practical purposes comets with periods of less than 200 years are regarded as periodic and designated with the letter P, while longer period comets are regarded as non-periodic and designated with the letter C). Many more comets are discovered today than was the case in the 1960s, due to the advent of automated sky surveys, and more recent comets tend to be named for the year in which they are discovered. To make matters worse, some comets are initially identified as asteroids, and given asteroid designations, then reclassified as comets by appending the letters P or C.