Titan's shape is characterized by a difference between the long equatorial radius and the polar radius that is several hundred meters larger than that predicted by the flattening due to its spin ...rate. The North polar region is covered by large mare filled with hydrocarbons, including ethane. Moreover global circulation models predict ethane precipitation on the polar areas. This study shows that the shape of Titan can be explained by the subsidence associated with the substitution of methane with ethane‐rich liquids percolating into the crust which, as suggested by evolution models, may be composed of methane clathrate hydrates. Such substitutions have been observed in laboratory experiments. This process would provide an additional methane source as required for sustaining the presence of this constituent in Titan's atmosphere through its history. A 270 m subsidence of the polar caps is explained by the circulation of 1.5 to 6 × 1018kg of ethane in the top three kilometers of an initially methane‐clathrate crust. This process would have operated during the last 300–1200 Myr at the present ethane production rate.
Key Points
Ethane can substitute methane in clathrate hydrates on Titan
The densification of clathrates by this exchange induces subsidence
The subsidence is consistent with Titan's shape as measured by the Cassini probe
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 H 216 $_2^{16}$ 216 O, H 218 $_2^{18}$218 O, H 217 $_2^{17}$217 O, 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 $r_{\textrm{h}}^{-6}$rh−6 , 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 H 218 $_2^{18}$218 O/H 217 $_2^{17}$217 O ratio of 5.6 ± 0.8.
The Rosetta mission orbiter science overview: the comet phase Taylor, M. G. G. T.; Altobelli, N.; Buratti, B. J. ...
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
07/2017, Letnik:
375, Številka:
2097
Journal Article
Recenzirano
Odprti dostop
The international Rosetta mission was launched in 2004 and consists of the orbiter spacecraft Rosetta and the lander Philae. The aim of the mission is to map the comet 67P/Churyumov-Gerasimenko by ...remote sensing, and to examine its environment in situ and its evolution in the inner Solar System. Rosetta was the first spacecraft to rendezvous with and orbit a comet, accompanying it as it passes through the inner Solar System, and to deploy a lander, Philae, and perform in situ science on the comet's surface. The primary goals of the mission were to: characterize the comet's nucleus; examine the chemical, mineralogical and isotopic composition of volatiles and refractories; examine the physical properties and interrelation of volatiles and refractories in a cometary nucleus; study the development of cometary activity and the processes in the surface layer of the nucleus and in the coma; detail the origin of comets, the relationship between cometary and interstellar material and the implications for the origin of the Solar System; and characterize asteroids 2867 Steins and 21 Lutetia. This paper presents a summary of mission operations and science, focusing on the Rosetta orbiter component of the mission during its comet phase, from early 2014 up to September 2016.
This article is part of the themed issue ‘Cometary science after Rosetta’.
Aims. We present the spatial and diurnal variation of water outgassing on comet 67P/Churyumov-Gerasimenko using the H216O rotational transition line at 556.936 GHz observed from Rosetta/MIRO in ...August 2014. Methods. The water line was analyzed with a non-LTE radiative transfer model and an optimal estimation method to retrieve the H216O outgassing intensity, expansion velocity, and gas kinetic temperature. On August 7−9, 2014 and August 18−19, 2014, MIRO performed long steady nadir-pointing observations of the nucleus while it was rotating around its spin axis. The ground track of the MIRO beam during the observation was mostly on the northern hemisphere of comet 67P, covering its three distinct parts: the so-called head, body, and neck areas. Results. The MIRO spectral observation data show that the water-outgassing intensity varies by a factor of 30, from 0.1 × 1025 molecules s-1 sr-1 to 3.0 × 1025 molecules s-1 sr-1, the terminal gas expansion velocity varies by 0.17 km s-1 from 0.61 km s-1 to 0.78 km s-1, and the terminal gas temperature varies by 27 K from 47 K to 74 K. The retrieved coma parameters are co-registered with local environment variables such as the subsurface temperatures, measured in the MIRO continuum bands, the local solar time, illumination condition, and beam location on nucleus. The spatial variation of the outgassing activity is very noticeable, and the largest outgassing activity in August 2014 occurs near the neck region of the nucleus. The outgassing activity in the neck region is also found to be correlated with the local solar hour, which is related to the local illumination condition.
Liquid ethane is present in the lakes and seas observed on Titan's surface by the Cassini‐Huygens mission. While interaction between liquid hydrocarbons and water ice is expected to result in the ...formation of clathrate hydrates, such reaction (and its kinetics) has not yet been demonstrated for cryogenic liquids under relevant planetary conditions. In this paper, we report the first experimental evidence for rapid formation of clathrates upon direct contact of liquid ethane with water ice at 1 bar using micro‐Raman spectroscopy. Kinetics experiments conducted in the temperature range 150–173 K yield an activation energy of 14.8 ± 2.2 kJ/mol, which is suggestive of a diffusion‐controlled mechanism for clathrate formation. This implies that a clathrate reservoir can form within seasonal time scales on Titan if liquid ethane comes into contact with a pre‐existing icy bedrock, which may hold important implications for the structure and dynamics of Titan's crust and its global evolution.
Plain Language Summary
Saturn's moon Titan has a dense atmosphere and liquid lakes (of methane and ethane) on its surface. These organic compounds are the main constituents of natural gas on Earth. Water is completely frozen and comprises most of Titan's bedrock materials. Under the right conditions, it is common for water ice to form new solids that trap natural gas in their structures, called clathrate hydrates. On Earth, clathrate hydrates are often found in the permafrost and deep ocean. However, Titan is so much colder that these natural gases exist as liquids. We do not yet know if clathrates can form under such conditions, and if so, how quickly. In this study, we prepared small grains of water ice, bathed them in liquid ethane, and monitored the rate at which clathrate hydrates appear. Our data confirm that clathrates would indeed form on Titan from an ethane lake or rain event onto water ice, and they would do so very quickly (in just a few Earth years). This suggests that clathrate hydrates can be an important material on Titan that may be encountered by future missions there.
Key Points
Rapid formation of clathrate hydrates from interaction between liquid ethane and water ice at 1 bar is demonstrated for the first time
Kinetics data at 150–173 K yield an activation energy of 14.8 kJ/mol, implying a diffusion‐controlled mechanism for clathrate formation
Extrapolation to Titan's conditions suggests that ethane‐clathrate caps can form on seasonal time scales, which may explain polar depressions
Enceladus and possibly Europa spew materials from their internal ocean into their exosphere, some of which are deposited back onto the surface of those Ocean Worlds. This setting provides a unique ...opportunity to seek traces of past or extant life in ice plume deposits on their surfaces. However, the design of lander missions and surface sampling techniques and the choice of sampling locations rely heavily on strength expectations. Here we present an experimental investigation of the evolution in strength of ice plume deposit analogs at several temperatures, as well as a model that predicts first‐order estimates of the strength of evolved ice plume deposits under geologic timescales relevant to Enceladus and Europa. These results suggest that plume deposits remain weak and poorly consolidated on Enceladus, while they may develop substantial strength (comparable to solid ice) within <100 Myr on Europa.
Plain Language Summary
Enceladus and Europa are Ocean Worlds; they harbor an internal ocean beneath their ice shells. There is proof that a plume emits ocean materials out of Enceladus, similar to geysers on Earth, and some evidence for similar activity on Europa. Based on the composition of the plumes and the surface, both Enceladus and Europa are the leading outer solar system candidates for possibly harboring life. Areas where fresh plume materials are deposited would be the best location to search for traces of life on the surface. A major challenge in preparing mission concepts to explore these locations arises from the need to collect samples of the surface ice, while little is known at present about the mechanical properties of the surface. In this study, we prepared icy plume deposit analogs and let them evolve in the laboratory over extended periods of time to investigate the evolution of their strength. We find that plume deposits are likely to remain loose and exhibit a low strength over geologic timescales under Enceladus's conditions, suggesting that they would be relatively easy to sample. Conversely, under Europa's surface conditions, such plume deposits appear likely to develop a substantial strength.
Key Points
The cone penetration resistance of fine‐grained porous ice held under isothermal conditions increases linearly over time
The temperature dependence of the strengthening rate yields an activation energy similar to self‐diffusion at the surface of ice grains
Plume deposits would remain weak on Enceladus, while they may develop substantial strength within a few million years on Europa
The Sun was an order of magnitude more luminous during the first few hundred thousand years of its existence, due in part to the gravitational energy released by material accreting from the solar ...nebula. If Jupiter was already near its present mass, the planet's tides opened an optically thin gap in the nebula. Using Monte Carlo radiative transfer calculations, we show that sunlight absorbed by the nebula and re-radiated into the gap raised temperatures well above the sublimation threshold for water ice, with potentially drastic consequences for the icy bodies in Jupiter's feeding zone. Bodies up to a meter in size were vaporized within a single orbit if the planet was near its present location during this early epoch. Dust particles lost their ice mantles, and planetesimals were partially to fully devolatilized, depending on their size. Scenarios in which Jupiter formed promptly, such as those involving a gravitational instability of the massive early nebula, must cope with the high temperatures. Enriching Jupiter in the noble gases through delivery trapped in clathrate hydrates will be more difficult, but might be achieved by either forming the planet much farther from the star or capturing planetesimals at later epochs. The hot gap resulting from an early origin for Jupiter also would affect the surface compositions of any primordial Trojan asteroids.
The Microwave Instrument on the Rosetta Orbiter (MIRO) has been observing the coma of comet 67P/Churyumov-Gerasimenko almost continuously since June 2014 at wavelengths near 0.53 mm. We present here ...a map of the water column density in the inner coma (within 3 km from nucleus center) when the comet was at 3.4 AU from the Sun. Based on the analysis of the H216O and H218O (110-101) lines, we find that the column density can vary by two orders of magnitude in this region. The highest column density is observed in a narrow region on the dayside, close to the neck and north pole rotation axis of the nucleus, while the lowest column density is seen against the nightside of the nucleus where outgassing seems to be very low. We estimate that the outgassing pattern can be represented by a Gaussian distribution in a solid angle with FWHM ≈ 80°.
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