On the attempts to measure water (and other volatiles) directly at the surface of a comet

Abstract

The Ptolemy instrument on the Philae lander (of the Rosetta space mission) was able to make measurements of the major volatiles, water, carbon monoxide and carbon dioxide, directly at the surface of comet 67P/Churyumov-Gerasimenko. We give some background to the mission and highlight those instruments that have already given insights into the notion of water in comets, and which will continue to do so as more results are either acquired or more fully interpreted. On the basis of our results, we show how comets may in fact be heterogeneous over their surface, and how surface measurements can be used in a quest to comprehend the daily cycles of processes that affect the evolution of comets.This article is part of the themed issue 'The origin, history and role of water in the evolution of the inner Solar System'.

Keywords: Rosetta; comets; space mission; volatiles; water.

Figure 1.

Plots of mass spectra taken by Ptolemy at the Abydos landing site. Each plot shows on the x-axis the mass of the species measured (or, more correctly, the mass/charge ratio, i.e. m/z) versus the relative intensity of the signal at each mass (y-axis). Successive plots from top to bottom were taken at progressively longer intervals after Philae came to rest at Abydos. The plots on the left and right in each case are the same, but with a 10-fold difference in the intensity scale. What can be observed here is that even by the end of the monitoring period there are still peaks at m/z 18 (H₂O⁺) and 44 (CO₂⁺), the measured number of counts being way above anything expected as a background.

Figure 2.

Plot of H₂O/CO₂ ratios of comets versus CO/CO₂ (data taken from [38]). There are a number of additional measurements that have yielded H₂O/CO₂ ratios (spanning a range of 3.5–14.5) but no accompanying CO/CO₂. Filled diamonds are spectroscopic measurements made by remote, telescopic means. The circles represent the equivalent ratios measured at the nucleus of 67P by Ptolemy (data from [34]; note that there are, in fact, seven points plotted—two of the Abydos points plot on top of each other, while the point that shows the highest CO/CO₂ ratio is that measured at Agilkia). The far more extensive datasets acquired by ROSINA are not included since these are influenced by specific regional-scale processes (which are, in turn, the result of daily and seasonal effects) and are complicated by the orientation of the spacecraft with respect to the nucleus at the time the measurements were made. Notwithstanding this, on the basis of data taken from the coma at about 3.5 AU from the Sun, H₂O/CO₂ is typically in the range 5–10 with CO/CO₂ of 1–3 [39]. In extreme cases, H₂O/CO₂ drops to about 0.01.

Figure 3.

Plot of the intensities of mass spectral peaks due to H₂O, CO₂ and CO showing how these varied with time after the arrival of Philae at the surface of 67P. The first data point in each case is from the experiments conducted at Agilkia; the remainder are from the long-term monitoring campaign at Abydos. Best-fit lines though data collected between 881 and 1248 min (four consecutive points in each case) are shown as the dotted lines. Note that the species labelled as CO₂ and CO are not corrected for the effects of cracking of CO₂⁺ (m/z 44) to CO⁺ (m/z 28). This has not been done herein because the exact details of corrections like this are not yet certain and will require further work. However, it is a fact that without correction the values recorded for CO are upper limits. Error bars are 1 s.d. (i.e. the noise associated with ion counting).

Figure 4.

The H₂O data acquired at Abydos plotted in linear format as intensity versus time (the data are the same as those in figure 3). The broken line at the top of the plot shows the times when Philae was illuminated (white) and in darkness (black). Superimposed on the first four points is a freeform shape meant to represent a putative spike of outgassing, as might be envisaged to accompany the daily period of illumination. The profile is assumed to show a sharp, rapid development of outgassing, followed by an exponential decay in intensity as the local site cools during the night. This is certainly consistent with the measured data (i.e. the filled dots joined by a line). The outgassing profile is then reproduced to coincide with the two further cycles of day–night experienced at Abydos. Note that the Agilkia data are not included to avoid confusion. Error bars are 1 s.d. (i.e. the noise associated with ion counting).