This film covers the most recent science news from the Rosetta mission, as well as selected scientific highlights from the last year.
Video: science highlights one year since comet landing
OSIRIS Image of the Day
View Rosetta’s comet
Where is Rosetta?
Rosetta NOW
Once upon a time…
Rosetta's grand finale
Ambition the film
Click image to watch Ambition
Read Rosetta: the ambition to turn science fiction into science fact
Rosetta Art Tribute
A repository of artistic tributes to the Rosetta mission by artists across the world.
ESA website – images
ESA website – videos
Discussion: 6 comments
Emily: In their DPS abstract 413.08, Steffl et al mention an outburst on 10 July observed using Alice: “Beginning around 02:10 UTC, the dust on the anti-sunward side of the nucleus brightened rapidly, increasing by a factor of 21 over pre-outburst levels, when integrated over a 10-minute exposure. A 40s exposure beginning at 02:20 showed an additional factor of two increase in brightness. During the outburst, the dust became significantly brighter than the sunlit nucleus. Concurrent NAVCAM images show a large dust cloud expanding out from the night side of the nucleus.” This was during the time of the “avalanches” on Imhotep and has not been covered on this blog. It would be interesting to see some material. The authors also mention the outburst on 22 August which was covered in Cometwatch.
Harvey: You mentioned that photoionization of O2 by solar radiation is difficult to detect since the photoionization of H2O swamps it, because of its much greater volume. Since O2 is not detected from afar, the question I have is whether this is again because of the swamping effect of other substances, or because it photoionizes/reacts and should be searched for as some other compound.
Kamal
Molecular O2 happens to be rather difficult to detect remotely.
There are several reasons for this.
The molecule is a ‘homonuclear diatomic’ and such molecules (H2,N2, Cl2 etc) have no infrared or microwave/mm region electric dipole transitions due to their symmetry.
O2 is unusual in that it *does* have very weak infrared transitions due to the electron spins, and some microwave lines near 60GHz & higher in the THz region with the same origin. But these are magnetic dipole transitions which are very weak.
So one way to remote sense available for many molecules is blocked.
Then there are the electronic transitions, which for oxygen lie in the UV (the atmosphere is colourless!)
Electronic transitions have very different structure & selection rules compared to infrared vibrational bands. It turns out that for molecular O2 there is really only one feasible band, called the Schumann Runge B-X band. Whilst it spreads over a wide region, it only has resolved line structure over quite a narrow region of the UV mostly around 170-200nm or so. Away from this the band exists but its a smooth continuum, no structure – so you can’t tell from that its O2 you are seeing; you need lines. Unfortunately what intensity there is is spread out over many lines, which makes them hard to spot in noise. Transitions in *atoms* on the other hand tend to produce discrete line (or smal multiplets) with all the intensity ‘in one place’.
If you see electronic transition in atomic O, from ALICE (or earth/satellites etc) you have no way to know where it comes from. LOTS of it from photodissociated water molecules, that from photdissociated O2 is just a small perturbation on that; hard to spot. (IGNORE the EU nonsense about photodissociation being ‘a weak process’, repeatedly claimed. Meaningless. The cross sections have been measured in the lab, we know the solar power/spectrum, this is the dominant process.)
So spectroscopically molecular oxygen is *hard*.
In the mass spectrometer, you need to look at the signal close to 32 with adequate resolution to distinguish it from other things with a mass close to 32 – which Rosetaa can do. So it can ‘see’ low O2 levels in amongst a lot of other stuff. The earlier mass spec results were lower resolution; O2 was lumped in with everything else close in mass.
If you see atomic O in a mass spec, near 16, it could have come from anything with an O in it, & there are other things close to mass 16 (NH2 comes to mind) so again with a low resolution instrument pretty hopeless to infer O2.
S Rosetta, uniquely I think with its high resolution mass spec could unambiguously identify molecular oxygen.
You can look back at old results with 20:20 hind sight & say ‘yes, those results are consistent with some molecular O2’ – but you could never have reliably identified it at the time because of the poor resolution.
Certainly an issue is that no one *expected* molecular O2. So no dedicated instrument was looking for it (hard anyway) & you would need strong, clear evidence to claim it.
I hope that answers the question?
(I got interested in the O2 molecular lines caused by the spin, magnetic dipole transitions & dug it all out; close to my field. Ill mybe write a note for anyone interested over the break.)
Harvey: Thanks very much for the detailed answer.
It raises other questions: could New Horizons, Dawn and Cassini (the last two are there for a longer time) have detected molecular O2 on Pluto, Vesta, Ceres, Saturn and its moons? If it was not looked for because it was not expected, can one go back and check the data now to at least get a “consistency” statement like they have done for Halley’s comet?
Kamal
I think you are suggesting no because Rosetta is “unique”, but just confirming.
Now we have an analysis that O2 was present on Halley in 1986 but not detected:
M. Rubin et al. Molecular oxygen in Oort cloud comet 1P/Halley, Astrophysical Journal Letters (Dec 10, 2015).