Herschel-PACS measurements of the rotational R(0) and R(1) HD lines in the atmospheres of Uranus and Neptune are analyzed to derive a D/H ratio with improved precision for the two planets. The derivation of the D/H ratio also includes previous measurements of the R(2) line with the Short Wavelength Spectrometer on board the Infrared Space Observatory (ISO). The available spectroscopic line information of the three rotational transitions is discussed and applied in the radiative transfer calculations. The best simultaneous fit of all three lines requires only a minor departure from the Spitzer temperature profile of Uranus and a departure limited to 2K from the Voyager temperature profile of Neptune (each time around the tropopause). The resulting and remarkably similar D/H ratios for Uranus and Neptune are found to be (4.4 ± 0.4) × 10 −5 and (4.1 ± 0.4) × 10 −5 , respectively. Although the deuterium enrichment in the two atmospheres compared to the protosolar value is confirmed, it is found to be lower compared to previous analyses. Using interior models from the literature and assuming that complete mixing of the atmosphere and interior occured during the planets' history, we derive a D/H in protoplanetary ices between (5.75-7.0) × 10 −5 for Uranus and between (5.1-7.7) × 10 −5 for Neptune. Conversely, adopting a cometary D/H for the protoplanetary ices between (15-30) × 10 −5 , we constrain the interior models of the two planets to have an ice mass fraction of 14-32%, i.e., the two planets are rock-dominated.

International audienceThe European Space Agency Rosetta Spacecraft, launched on March 2, 2004 toward Comet 67P/Churyumov-Gerasimenko, carries a relatively small and lightweight millimeter-submillimeter spectrometer instrument, the first of its kind launched into deep space. The instrument will be used to study the evolution of outgassing water and other molecules from the target comet as a function of heliocentric distance. During flybys of the asteroids (2867) Steins and (21) Lutetia in 2008 and 2010 respectively, the instrument will measure thermal emission and search for water vapor in the vicinity of these asteroids. The instrument, named MIRO (Microwave Instrument for the Rosetta Orbiter), consists of a 30-cm diameter, offset parabolic reflector telescope followed by two heterodyne receivers. Center-band operating frequencies of the receivers are near 190 GHz (1.6 mm) and 562 GHz (0.5 mm). Broadband continuum channels are implemented in both frequency bands for the measurement of near surface temperatures and temperature gradients in Comet 67P/Churyumov-Gerasimenko and the asteroids (2867) Steins and (21) Lutetia. A 4096 channel CTS (Chirp Transform Spectrometer) spectrometer having 180 MHz total bandwidth and 44 kHz resolution is, in addition to the continuum channel, connected to the submillimeter receiver. The submillimeter radiometer/spectrometer is fixed tuned to measure four volatile species – CO, CH3OH, NH3 and three, oxygen-related isotopologues of water, H2 16O, H2 17O and H2 18O. The basic quantities measured with the MIRO instrument are surface temperature, gas production rates and relative abundances, and velocity and excitation temperature of each species, along with their spatial and temporal variability. This paper provides a short discussion of the scientific objectives of the investigation, and a detailed discussion of the MIRO instrument system

Cryovolcanic activity near the south pole of Saturn's moon Enceladus produces plumes of H 2 O-dominated gases and ice particles, which escape and populate a torus-shaped cloud. Using submillimeter spectroscopy with Herschel, we report the direct detection of the Enceladus water vapor torus in four rotational lines of water at 557, 987, 1113, and 1670 GHz, and probe its physical conditions and structure. We determine line-of-sight H 2 O column densities of ∼4 × 10 13 cm −2 near the equatorial plane, with a ∼50 000 km vertical scale height. The water torus appears to be rotationally cold (e.g. an excitation temperature of 16 K is measured for the 1113 GHz line) but dynamically excited, with non-Keplerian dispersion velocities of ∼2 km s −1 , and appears to be largely shaped by molecular collisions. From estimates of the influx rates of torus material into Saturn and Titan, we infer that Enceladus' activity is likely to be the ultimate source of water in the upper atmosphere of Saturn, but not in Titan's.

Aims. We present the spatial and diurnal variation of water outgassing on comet 67P/Churyumov-Gerasimenko using the H 16 2 O 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 H 16 2 O 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 × 10 25 molecules s −1 sr −1 to 3.0 × 10 25 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.

[1] The spatiotemporal behavior of the ozone mixing ratio in the upper mesosphere/ mesopause region under nearly polar night conditions is one of the phenomena not completely understood and reproduced by models thus far. We examine this issue using an advanced three-dimensional model of the dynamics and chemistry of the middle atmosphere (0-150 km) particularly designed to investigate the spatiotemporal structure of this phenomenon in the extended mesopause region. The most marked features of the modeling results are a pronounced ozone maximum around 72 km occurring close to the polar night terminator and a strong drop of the mixing ratio above $80 km. These features were also found by means of ground-based microwave measurements in high latitude at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR, 69.29°N, 16.03°E) and even at the moderate latitude of Lindau (51.66°N, 10.13°E) during the night in the winter season but less marked there. They were absent at both stations during the daytime hours. The calculations suggest that the stronger enhanced ozone values occur in a latitudinal band of approximately 15°in the vicinity of the polar night terminator. During nighttime, enhanced values reach into midlatitudes. The effect is confined both to a height interval approximately between 66 and 76 km and to a certain latitudinal range which alters with season according to the change of the polar night terminator. We discuss the model results in terms of chemistry for nearly grazing incidence conditions of the solar insolation and in context with advective transport.

Aims. Using spectroscopic and continuum data measured by the MIRO instrument on board Rosetta of comet 67P/Churyumov-Gerasimenko, it is possible to derive and track the change in the water production rate, to learn how the outgassing evolves with heliocentric distance. The MIRO data are well suited to investigate the evolution of 67P, in unprecedented spatial and temporal detail. Methods. To obtain estimates of the local effective Haser production rates we developed an efficient and reliable retrieval approach with precalculated lookup tables. We employed line area ratios (H 16 2 O/H 18 2 O) from pure nadir observations as the key variable, along with the Doppler shift velocity, and continuum temperature. This method was applied to the MIRO data from August 2014 until April 2016. Perihelion occurred on August 13, 2015 when the comet was 1.24 AU from the Sun. Results. During the perihelion approach, the water production rates increased by an order of magnitude, and from the observations, the derived maximum for a single observation on August 29, 2015 is (1.42 ± 0.51) × 10 28. Modeling the data indicates that there is an offset in the peak outgassing, occurring 34 ± 10 days after perihelion. During the pre-perihelion phase, the production rate changes with heliocentric distance as r −3.8±0.2 h ; during post-perihelion, the dependence is r −4.3±0.2 h. The comet is calculated to have lost 0.12 ± 0.06 % of its mass during the perihelion passage, considering only water ice sublimation. Additionally, this method provides well sampled data to determine the spatial distribution of outgassing versus heliocentric distance. The time evolution is definitely not uniform across the surface. Pre-and post-perihelion, the surface temperature on the southern hemisphere changes rapidly, as does the sublimation rate with an exponent of ∼−6. There is a strong latitudinal dependence on the r h exponent with significant variation between northern and southern hemispheres, and so the average over the comet surface may only be of limited importance. We present more detailed regional variation in the outgassing, demonstrating that the highest derived production rates originate from the Wosret, Neith and Bes regions during perihelion.

We report on the initial analysis of a Herschel-PACS full range spectrum of Neptune, covering the 51-220 μm range with a mean resolving power of ∼3000, and complemented by a dedicated observation of CH 4 at 120 μm. Numerous spectral features due to HD (R(0) and R(1)), H 2 O, CH 4 , and CO are present, but so far no new species have been found. Our results indicate that (i) Neptune's mean thermal profile is warmer by ∼3 K than inferred from the Voyager radio-occultation; (ii) the D/H mixing ratio is (4.5 ± 1) × 10 −5 , confirming the enrichment of Neptune in deuterium over the protosolar value (∼2.1 × 10 −5 ); (iii) the CH 4 mixing ratio in the mid stratosphere is (1.5 ± 0.2) × 10 −3 , and CH 4 appears to decrease in the lower stratosphere at a rate consistent with local saturation, in agreement with the scenario of CH 4 stratospheric injection from Neptune's warm south polar region; (iv) the H 2 O stratospheric column is (2.1 ± 0.5) × 10 14 cm −2 but its vertical distribution is still to be determined, so the H 2 O external flux remains uncertain by over an order of magnitude; and (v) the CO stratospheric abundance is about twice the tropospheric value, confirming the dual origin of CO suspected from ground-based millimeter/submillimeter observations.

Dedicated line observations of CH 4 rotational lines performed with Herschel/PACS and HIFI in 2009-2011 provide new inferences of the mean methane profile in the upper tropospheres and stratospheres of Uranus and Neptune. At Uranus, CH 4 is found to be near saturation, with a ∼9 × 10 −4 tropopause/lower stratosphere mole fraction. This is nominally six times larger than inferred from Spitzer in 2007, although reconciliation may be possible if the CH 4 abundance decreases sharply from ∼100 to 2 mbar. This unexpected situation might reflect heterogeneous conditions in Uranus' stratosphere, with local CH 4 depletions and heating associated with downwelling motions. Higher CH 4 abundances compared to values inferred under solstitial conditions by Voyager in 1989 suggest that atmospheric mixing is effectively subdued at high latitudes and/or is time-variable. At Neptune, the mid-stratosphere CH 4 abundance is (1.15 ± 0.10) × 10 −3 , in agreement with earlier determinations and indicative of either leakage through a warmer polar region or upwelling at low or middle latitudes. On both planets, spatially resolved observations of temperature and methane in the stratosphere are needed to further identify the physical processes at work.