| dc.description.abstracteng | In August 2014, the Rosetta spacecraft completed its ten year journey when it arrived at its target destination, the comet 67P/Churyumov-Gerasimenko. The Rosetta mission was a flagship endeavour for the European Space Agency as it was the first time that any spacecraft had rendezvoused with a small solar system body for long period of time (two years) and also the first time that a lander had been placed onto the surface of a comet. In September 2016, the mission came to an end when Rosetta descended onto the surface for one final close up look at the surface.
This thesis uses data from one of the instruments on this ground-breaking mission: the Microwave Instrument for the Rosetta Orbiter (MIRO). MIRO was a spectrometer operating at millimetre and submillimetre frequencies and enabled the detection of several volatile species including water. Using the spectroscopic observations of the water lines, I investigated the behaviour of comet 67P/Churyumov-Gerasimenko relating to the gas coma, mass loss, spatial outgassing and the nucleus surface. Since comets are thought to be pristine building blocks left over from the formation of the solar system, we hope that by studying them, we can learn about the conditions from which other solar system bodies originate. Firstly, I used the line areas of the H216O and H218O spectral lines to measure the change in the local water production rate from August 2014 to April 2016. Lookup tables made from a Haser model show how the measured Doppler shift velocity, the continuum temperature and the line area ratio can give the column density for each observation and thus the water production rate. The maximum production rate calculated from the MIRO observations was (1.42 +- 0.51) x 10e28 molec/s, found on August 29, 2015, and integrating under all the data points gave a total water mass loss of (2.4 +- 1.1) x 10e9 kg for this apparition. By making assumptions about the dust-to-gas ratio and the comet mass, the total mass loss was estimated to be (1.2 +- 0.6) x 10e10 kg, or 0.12 +- 0.06 \%. The spatial resolution of MIRO allowed for each measurement to be assigned to a region on the nucleus. The regions on the southern hemisphere appeared to be the origins of the highest production rates, in particular the regions Neith, Wosret and Bes. Finally, the data show that the production rate peak is offset by 22-46 days after perihelion and that the pre- and post-perihelion slopes followed power laws of Q ~ rh^-3.8 +- 0.2 and Q ~ rh^-4.3 +- 0.2, respectively. Following this, I performed numerical modelling to investigate how nucleus shape, spin axis orientation and activity distribution affect the water production rate curves. I found that it is impossible to disentangle these effects from each other by only looking at the change in the production rate and that the pre- and post-perihelion slopes are also functions of heliocentric distance. It is therefore difficult to derive quantitative constraints on surface area ice fraction and active regions from the water production rate curve unless the shape, orientation and activity of the nucleus are well established. In addition, it may not be meaningful to compare the water production rate curves of different comets at different heliocentric distances. I used the measured continuum temperatures from MIRO to derive properties of the nucleus in the third part of this work. I utilised an insolation driven thermal model to derive the temperature gradient in the upper layers of the comet surface and a radiative transfer model to reproduce the MIRO continuum measurements. In conclusion, a low value was derived for the thermal inertia in the surface layers of 67P with an upper bound estimated to be 80 JK^-1m^-2s^-0.5 for most of the MIRO measurements. A low value for the average thermal inertia over the entire surface would be consistent with the majority of reported calculated values for 67P.
In the future, the retrieval of coma parameters from the MIRO spectra will become an important avenue of investigation. Using inverse methods, the behaviour of the gas temperature, expansion velocity and molecular number density profiles can be extracted from the spectral lines. This will be important for assessing and characterising the activity around the nucleus which is observed but not so well understood. In addition, we may learn more about the physics of the coma from the inversion solutions, such as the presence of a Knudsen layer, photolytic heating or extended sources. At the end of my thesis, the application of inverse methods to the MIRO data is described. It is still uncertain to what extent 67P is pristine and primordial but the results of my thesis imply that the surface layer over a couple of metres must be processed by the sun during perihelion passage. In this region, icy volatiles sublimate as the thermal wave propagates into the surface, expelling dust and refractory material away from the nucleus along with the gas. In the most productive regions on the south, the erosion is expected to reach down to a few metres in depth. The proposed CEASAR sample return mission would revisit 67P and the work for my thesis suggests that the drill must go beyond the surface layer, perhaps as much as 10 times the diurnal skin depth, in order to reach any potentially pristine material. | de |