T. Cavalié

In-orbit performance of Herschel-HIFI

Aims. In this paper the calibration and in-orbit performance of the Heterodyne Instrument for the Far-Infrared (HIFI) is described. Methods. The calibration of HIFI is based on a combination of ground and in-flight tests. Dedicated ground tests to determine those instrument parameters that can only be measured accurately using controlled laboratory stimuli were carried out in the instrument level test (ILT) campaign. Special in-flight tests during the commissioning phase (CoP) and performance verification (PV) allowed the determination of the remaining instrument parameters. The various instrument observing modes, as specified in astronomical observation templates (AOTs), were validated in parallel during PV by observing selected celestial sources. Results. The initial calibration and in-orbit performance of HIFI has been established. A first estimate of the calibration budget is given. The overall in-flight instrument performance agrees with the original specification. Issues remain at only a few frequencies.

First results of Herschel-PACS observations of Neptune

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) Neptunes 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 Neptunes 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.

Herschel/HIFI observations of Mars: first detection of O2 at submillimetre wavelengths and upper limits on HCl and H2O2

We report on an initial analysis of Herschel/HIFI observations of hydrogen chloride (HCl), hydrogen peroxide (H_2O_2), and molecular oxygen (O_2) in the Martian atmosphere performed on 13 and 16 April 2010 (L_s ~ 77°). We derived a constant volume mixing ratio of 1400 ± 120 ppm for O_2 and determined upper limits of 200 ppt for HCl and 2 ppb for H_2O_2. Radiative transfer model calculations indicate that the vertical profile of O_2 may not be constant. Photochemical models determine the lowest values of H_2O_2 to be around L_s ~ 75° but overestimate the volume mixing ratio compared to our measurements.

A cometary origin for CO in the stratosphere of Saturn

Context. The CO(3–2) line has been observed in the atmosphere of Saturn. The CO(3–2) observation proves that an external source of CO exists in the stratosphere of the planet. Aims. We attempt to constrain the type and magnitude of the external source of CO in the atmosphere of Saturn, by observing the emission core of the CO(6–5) line. Methods. We observed the CO(6–5) line at the limbs of Saturn. We analysed the observations by means of a 1-D transport model of the atmosphere of Saturn, coupled with a radiative transfer model. Results. We obtained a high signal-to-noise ratio spectrum that confirms the existence of an external source of CO in the stratosphere of Saturn. We demonstrated that a cometary origin of CO is the most probable, an impact occurring 220 ± 30 years ago and depositing (2.1 ± 0.4) × 10 15 g of CO above 0.1 mbar. However, we cannot totally reject the possibility of CO originating (at least partially) in a steady source. Conclusions. Complete photochemical modelling of the oxygen compounds is required to determine realistic error bars of the inferred quantities and to conclude on the origin of CO.

HIFI observations of water in the atmosphere of comet C/2008 Q3 (Garradd)

High-resolution far-infrared and sub-millimetre spectroscopy of water lines is an important tool to understand the physical and chemical properties of cometary atmospheres. We present observations of several rotational ortho- and para-water transitions in comet C/2008 Q3 (Garradd) performed with HIFI on Herschel. These observations have provided the first detection of the 2(12)-1(01) (1669 GHz) ortho and 1(11)-0(00) (1113 GHz) para transitions of water in a cometary spectrum. In addition, the ground-state transition 1(10)-1(01) at 557 GHz is detected and mapped. By detecting several water lines quasi-simultaneously and mapping their emission we can constrain the excitation parameters in the coma. Synthetic line profiles are computed using excitation models which include excitation by collisions, solar infrared radiation, and radiation trapping. We obtain the gas kinetic temperature, constrain the electron density profile, and estimate the coma expansion velocity by analyzing the map and line shapes. We derive water production rates of 1.7-2.8 x 10(28) s(-1) over the range r(h) = 1.83-1.85 AU.

Scientific rationale for Saturn׳s in situ exploration

Remote sensing observations meet some limitations when used to study the bulk atmospheric composition of the giant planets of our solar system. A remarkable example of the superiority of in situ probe measurements is illustrated by the exploration of Jupiter, where key measurements such as the determination of the noble gases׳ abundances and the precise measurement of the helium mixing ratio have only been made available through in situ measurements by the Galileo probe. This paper describes the main scientific goals to be addressed by the future in situ exploration of Saturn placing the Galileo probe exploration of Jupiter in a broader context and before the future probe exploration of the more remote ice giants. In situ exploration of Saturn׳s atmosphere addresses two broad themes that are discussed throughout this paper: first, the formation history of our solar system and second, the processes at play in planetary atmospheres. In this context, we detail the reasons why measurements of Saturn׳s bulk elemental and isotopic composition would place important constraints on the volatile reservoirs in the protosolar nebula. We also show that the in situ measurement of CO (or any other disequilibrium species that is depleted by reaction with water) in Saturn׳s upper troposphere may help constraining its bulk O/H ratio. We compare predictions of Jupiter and Saturn׳s bulk compositions from different formation scenarios, and highlight the key measurements required to distinguish competing theories to shed light on giant planet formation as a common process in planetary systems with potential applications to most extrasolar systems. In situ measurements of Saturn׳s stratospheric and tropospheric dynamics, chemistry and cloud-forming processes will provide access to phenomena unreachable to remote sensing studies. Different mission architectures are envisaged, which would benefit from strong international collaborations, all based on an entry probe that would descend through Saturn׳s stratosphere and troposphere under parachute down to a minimum of 10 bar of atmospheric pressure. We finally discuss the science payload required on a Saturn probe to match the measurement requirements.

First results on Martian carbon monoxide from Herschel/HIFI observations

We report on the initial analysis of Herschel/HIFI carbon monoxide (CO) observations of the Martian atmosphere performed between 11 and 16 April 2010. We selected the (7-6) rotational transitions of the isotopes ^{13}CO at 771 GHz and C^{18}O at 768 GHz in order to retrieve the mean vertical profile of temperature and the mean volume mixing ratio of carbon monoxide. The derived temperature profile agrees within less than 5 K with general circulation model (GCM) predictions up to an altitude of 45 km, however, show about 12-15 K lower values at 60 km. The CO mixing ratio was determined as 980 \pm 150 ppm, in agreement with the 900 ppm derived from Herschel/SPIRE observations in November 2009.

Giant Planet Observations with the James Webb Space Telescope

This white paper examines the benefit of the upcoming James Webb Space Telescope for studies of the Solar Systems four giant planets: Jupiter, Saturn, Uranus, and Neptune. JWSTs superior sensitivity, combined with high spatial and spectral resolution, will enable near- and mid-infrared imaging and spectroscopy of these objects with unprecedented quality. In this paper we discuss some of the myriad scientific investigations possible with JWST regarding the giant planets. This discussion is preceded by the specifics of JWST instrumentation most relevant to giant planet observations. We conclude with identification of desired pre-launch testing and operational aspects of JWST that would greatly benefit future studies of the giant planets.

New constraints on the CH4 vertical profile in Uranus and Neptune from Herschel observations

Dedicated line observations of CH4 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, CH4 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 CH4 abundance decreases sharply from 100 to 2 mbar. This unexpected situation might reflect heterogeneous conditions in Uranus’ stratosphere, with local CH4 depletions and heating associated with downwelling motions. Higher CH4 abundances compared to values inferred under solstitial conditions by Voyager in 1989 suggest that atmospheric mixing is e ectively subdued at high latitudes and/or is time-variable. At Neptune, the mid-stratosphere CH4 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.

The Herschel-SPIRE submillimetre spectrum of Mars

We have obtained the first continuous disk averaged spectrum of Mars from 450 to 1550 Ghz using the Herschel-SPIRE Fourier-transform spectrometer. The spectrum was obtained at a constant resolution of 1.4 GHz across the whole band. The flux from the planet is such that the instrument was operated in “bright source” mode to prevent saturation of the detectors. This was the first successful use of this mode and in this work we describe the method used for observing Mars together with a detailed discussion of the data reduction techniques required to calibrate the spectrum. We discuss the calibration accuracy obtained and describe the first comparison with surface and atmospheric models. In addition to a direct photometric measurement of the planet the spectrum contains the characteristic transitions of 12 CO from J 5‐4 to J 13‐12 as well as numerous H2O transitions. Together these allow the comparison to global atmospheric models allowing the mean mixing ratios of water and 12 CO to be investigated. We find that it is possible to match the observed depth of the absorption features in the spectrum with a fixed water mixing ratio of 1 × 10 −4 and a 12 CO mixing ratio of 9 × 10 −4 .