ALBERT

Description: Aims. We use four observational data sets, mainly from the Rosetta mission, to constrain the activity pattern of the nucleus of comet 67P/Churyumov-Gerasimenko (67P). Methods. We developed a numerical model that computes the production rate and non-gravitational acceleration of the nucleus of comet 67P as a function of time, taking into account its complex shape with a shape model reconstructed from OSIRIS imagery. We used this model to fit three observational data sets: the trajectory data from flight dynamics; the rotation state as reconstructed from OSIRIS imagery; and the water production measurements from ROSINA of 67P. The two key parameters of our model, adjusted to fit the three data sets all together, are the activity pattern and the momentum transfer efficiency (i.e., the so-called η parameter of the non-gravitational forces). Results. We find an activity pattern that can successfully reproduce the three data sets simultaneously. The fitted activity pattern exhibits two main features: a higher effective active fraction in two southern super-regions (~10%) outside perihelion compared to the northern regions (

Description: Context. The MIRO instrument’s remote sensing capability is integral to constraining water density, temperature, and velocity fields in the coma of 67P/Churyumov-Gersimenko. Aims. Our aim is to quantify how much water density originates from the facets of the shape model within the field of view of MIRO versus the water contribution from all the other facets. This information is crucial to understanding the MIRO derived coma production rates and their relation to the nucleus characteristics, and to understanding the spatial resolution of the measurements. Methods. This study relies on a detailed 3D nucleus shape model, illumination conditions, and the pointing information of the viewing geometry. With these parameters we can evaluate the relative contribution of water density originating from facets directly inside the MIRO beam and outside the beam as a function of distance along the MIRO line of sight. We also calculate the ratio of in-beam versus out-of-beam water gas number density. Results. We demonstrate that despite the rather small MIRO field of view there is only a small fraction of molecules that originate from facets within the MIRO beam. This is true for the nadir, but a similar conclusion can also be applied to the limb observing geometry. Conclusions. The MIRO instrument cannot discriminate active from inactive regions directly from observations. This study also suggests that the beam averaged solar incidence angle, local time, and mean normal vectors are not necessarily related to molecules within the MIRO beam. These results also illustrate why the 1D spherical Haser model can be applied with relative success to analyzing the MIRO data (and generally any Rosetta measurements). The future possibilities of constraining gas activity distribution on the surface should use 3D codes extracting information from the MIRO spectral line shapes which contain additional information. The results presented here are applicable to remote sensing instruments on board Rosetta.

Description: Aims. We investigate the influence of three basic factors on water production rate as a function of heliocentric distance: nucleus shape, the spin axis orientation, and the distribution of activity on a comet’s surface. Methods. We used a basic water sublimation model driven by solar insolation to derive total production rates for different nuclei shapes and spin axis orientations using the orbital parameters of 67P/Churyumov-Gerasimenko. We used known shape models derived from prior missions to the Jupiter Family and short period comets. The slopes of production rates versus heliocentric distance were calculated for the different model setups. Results. The standard (homogeneous) outgassing model confirms the well-known result regarding the heliocentric dependence of water production rate that remains invariant for different nuclei shapes as long as the rotation axis is perpendicular to the orbital plane. When the rotation axis is not perpendicular, the nucleus shape becomes a critically important factor in determining the water production curves as the illuminated cross section of the nucleus changes with heliocentric distance. Shape and obliquity can produce changes in the illuminated cross section of up to 50% over an orbit. In addition, different spin axis orientations for a given shape can dramatically alter the pre- and post-perihelion production curves, as do assumptions about the activity distribution on the surface. If, however, the illuminated cross section of the nucleus is invariant, then the dependence on the above parameters is weak, as demonstrated here with the 67P/Churyumov-Gerasimenko shape. The comets Hartley 2 and Wild 2 are shown to yield significantly different production curve shapes for the same orbit and orientation as 67P/CG, varying by as much as a factor of three as a result of only changing the nucleus shape. Finally, we show that varying just three basic parameters, shape, spin axis orientation, and active spots distribution on the surface can lead to arbitrary deviations from the expected inverse square law dependence of water production rates near 1 au. Conclusions. With the results obtained, we cannot avoid the conclusion that, without prior knowledge of basic parameters (shape, spin axis orientation, activity locations), it is difficult to reveal the nature of cometary outgassing from the heliocentric water production rates. Similarly, the inter-comparison of water production curves of two such comets may not be meaningful.

Description: Context. The origin of water in the stratospheres of giant planets has been an outstanding question ever since its first detection by the Infrared Space Observatory some 20 years ago. Water can originate from interplanetary dust particles, icy rings and satellites, and large comet impacts. Analyses of Herschel Space Observatory observations have proven that the bulk of Jupiter’s stratospheric water was delivered by the Shoemaker-Levy 9 impacts in 1994. In 2006, the Cassini mission detected water plumes at the South Pole of Enceladus, which made the moon a serious candidate for Saturn’s stratospheric water. Further evidence was found in 2011 when Herschel demonstrated the presence of a water torus at the orbital distance of Enceladus that was fed by the moon’s plumes. Finally, water falling from the rings onto Saturn’s uppermost atmospheric layers at low latitudes was detected during the final orbits of Cassini’s end-of-mission plunge into the atmosphere. Aims. In this paper, we use Herschel mapping observations of water in Saturn’s stratosphere to identify its source. Methods. We tested several empirical models against the Herschel-HIFI and -PACS observations, which were collected on December 30, 2010, and January 2, 2011, respectively. Results. We demonstrate that Saturn’s stratospheric water is not uniformly mixed as a function of latitude, but peaks at the equator and decreases poleward with a Gaussian distribution. We obtain our best fit with an equatorial mole fraction 1.1 ppb and a half width at half maximum of 25°, when accounting for a temperature increase in the two warm stratospheric vortices produced by Saturn’s Great Storm of 2010–2011. Conclusions. This work demonstrates that Enceladus is the main source of Saturn’s stratospheric water.

Description: We present the analysis of ≈100 molecular maps of the coma of comet 67P/Churyumov-Gerasimenko that were obtained with the MIRO submillimeter radiotelescope on board the Rosetta spacecraft. From the spectral line mapping of H216O, H218O, H217O, CH3OH, NH3, and CO and some fixed nadir pointings, we retrieved the outgassing pattern and total production rates for these species. The analysis covers the period from July 2014, inbound to perihelion, to June 2016, outbound, and heliocentric distances rh = 1.24–3.65 AU. A steep evolution of the outgassing rates with heliocentric distance is observed, typically in rh−16, with significant differences between molecules (e.g. steeper variation for H2O post-perihelion than for methanol). As a consequence, the abundances relative to water in the coma vary. The CH3OH and CO abundances increase after perihelion, while the NH3 abundance peaks around perihelion and then decreases. Outgassing patterns have been modeled as 2D Gaussian jets. The width of these jets is maximum around the equinoxes when the bulk of the outgassing is located near the equator. From July 2014 to February 2015, the outgassing is mostly restricted to a narrower jet (full width at half-maximum ≈80°) originating from high northern latitudes, while around perihelion, most of the gaseous production comes from the southernmost regions ( − 80 ± 5° cometocentric latitude) and forms a 100°–130° (full width at half-maximum) wide fan. We find a peak production of water of 0.8 × 1028 molec. s−1, 2.5 times lower than measured by the ROSINA experiment, and place an upper limit to a 50% additional production that could come from the sublimation of icy grains. We estimate the total loss of ices during this perihelion passage to be 4.18 ± 0.18 × 109 kg. We derive a dust-to-gas ratio in the lost material of 0.7–2.3 (including all sources of errors) based on the nucleus mass loss of 10.5 ± 3.4 × 109 kg estimated by the RSI experiment. We also obtain an estimate of the H218O/H217O ratio of 5.6 ± 0.8.

Description: Our aim is to investigate early activity (2014 July) of 67P/C–G with 3D coma and radiative transfer modeling of Microwave Instrument on the Rosetta Orbiter (MIRO) measurements, accounting for nucleus shape, illumination, and orientation of the comet. We investigate MIRO line shape information for spatial distribution of water activity on the nucleus during the onset of activity. During this period we show that MIRO line shape have enough information to clearly isolate contribution from ‘neck’ (Hapi) and bottom of large lobe (Imhotep), and compare it to the nominal case of activity from the entire illuminated surface. We also demonstrate that spectral line shapes differ from the 1D model for different viewing geometries and coma conditions relevant to this study. Specifically, line shapes are sensitive to the location of the terminator in the coma. At last, fitting the MIRO observations we show that the Imhotep region (possible distributed source of H2O sublimating from the icy grains in the coma lifted due to CO2 activities) contributes negligible fraction of the total number of water molecules into MIRO beam in the early activity. On the other hand, a strong enhancement of water activity from the ‘neck’ region seems required to fit the MIRO line shapes. This is consistent with earlier analysis of Rosetta results. Nevertheless, within the assumption of our coma and surface boundary conditions we cannot get a reasonable fit to all MIRO mapping observations in 2014 July. We provide discussion on how to enhance these results and resolve the found issues in the future.