Publications by Patrick Irwin


From spectra to atmospheres: Solving the underconstrained retrieval problem for exoplanets

Proceedings of the International Astronomical Union 8 (2013) 275-276

JK Barstow, S Aigrain, PGJ Irwin, N Bowles, LN Fletcher, JM Lee

Spectroscopic observations of transiting exoplanets have provided the first indications of their atmospheric structure and composition. Optimal estimation retrievals have been successfully applied to solar system planets to determine the temperature, composition and aerosol properties of their atmospheres, and have recently been applied to exoplanets. We show the effectiveness of the technique when combined with simulated observations from the proposed space telescope EChO, and also discuss the difficulty of constraining a complex system with sparse data and large uncertainties, using the super-Earth GJ 1214b as an example. Copyright © 2013, International Astronomical Union.


Climatology and first-order composition estimates of mesospheric clouds from Mars Climate Sounder limb spectra

Icarus 222 (2013) 342-356

E Sefton-Nash, NA Teanby, L Montabone, PGJ Irvin, J Hurley, SB Calcutt


Uranus' cloud particle properties and latitudinal methane variation from IRTF SpeX observations

Icarus 223 (2013) 684-698

DS Tice, PGJ Irwin, LN Fletcher, NA Teanby, J Hurley, GS Orton, GR Davis

The Uranian atmosphere was observed in August 2009 from 0.8 to 1.8. μm using the near-infrared spectrometer, SpeX, at NASA's Infrared Telescope Facility. The observations had a spectral resolution of R=. 1200 and an average seeing of between 0.5' in the H-Band (1.4-1.8. μm) and 0.6' in the I-Band (0.8-0.9. μm). The reduced data were analyzed with a multiple-scattering retrieval code. We were able to reproduce observations when using a vertically-compact cloud in the upper troposphere and a vertically-extended, optically-thin haze above the 1-bar level. The existence of these two clouds is consistent with previous studies.The sub-micron portion of the data are most sensitive to very small scattering particles, allowing more insight into particle size than other portions of the infrared spectrum. This portion of the spectrum was therefore of particular interest and was not available in most previous studies of the planet. We assumed the particles in both clouds to be relatively strong forward scatterers (with a Henyey-Greenstein asymmetry factor of g=. 0.7). Given this assumption, we found single-scattering albedos in the tropospheric cloud particles to be ω̄=0.7 at wavelengths above 1.4. μm and to gradually increase to ω̄=1.0 at wavelengths shortward of 1.0. μm. In the upper haze, we found single-scattering albedos to be ω̄=1.0 with the exception of a narrow drop at 1.0. μm to ω̄=0.6. We found a preference for upper haze particle radii at r=. 0.10. μm. Retrievals of base pressure, fractional scale height, and optical depth in both cloud layers showed the best agreement with data when the base pressure of the upper haze was fixed just above the tropospheric clouds, rather than at or above the tropopausal cold trap. We found that these same retrievals strongly preferred tropospheric cloud particles of 1.35-μm radii, and observed cloud top height to increase away from the equator in the case of latitudinally invariant methane abundance.Latitudinal methane variability was also considered, both through a reflectivity study at the 825-nm collision-induced hydrogen absorption feature, as well as through radiative transfer analysis, using forward modeling and retrievals of cloud properties and methane abundance. The data suggested that methane abundance above the tropospheric clouds increased when moving from the midlatitudes towards the equator by at least 9%. The peak of this equatorial methane enrichment was determined to be at 4. ±. 2° S latitude, having moved nearly 15° northward since a reflectance study of 2002 data (Karkoschka and Tomasko, 2009). © 2013 Elsevier Inc.


Constraints on Titan's middle atmosphere ammonia abundance from Herschel/SPIRE sub-millimetre spectra

Planetary and Space Science 75 (2013) 136-147

NA Teanby, PGJ Irwin, CA Nixon, R Courtin, BM Swinyard, R Moreno, E Lellouch, M Rengel, P Hartogh

Sub-millimetre spectra measured with Herschel's SPIRE Fourier Transform Spectrometer were used to search for ammonia (NH3) in Titan's stratosphere. Observations were taken during 2010 and 2011, just after Titan's northern spring equinox, which occurred in mid-2009. In our analysis we used high spectral resolution data (0.074 cm-1 apodised) from the SPIRE shortwave spectrometer array (SSW), which provided the best possible signal-to-noise ratio for detecting any NH3 emission features. These data have the most sensitivity to NH3 spectral emission of any currently available observations, although despite this we did not detect any significant emission features above the noise. However, we can place an improved 3-sigma upper limit on NH3 abundance of <0.19ppb for altitudes 65-110 km (75 km peak sensitivity), or alternatively a column abundance of <1.23×1015molecules/cm2. These observations provide modest constraint for future photochemical models and are consistent with most current stratospheric predictions. Scaling of photochemical model profiles, in order to fit elevated abundances observed at 1100 km by Cassini's INMS instrument, are for the most part also consistent with our observations. © 2012 Elsevier Ltd.


Seasonal variations of temperature, acetylene and ethane in Saturn's atmosphere from 2005 to 2010, as observed by Cassini-CIRS

Icarus 225 (2013) 257-271

JA Sinclair, PGJ Irwin, LN Fletcher, JI Moses, TK Greathouse, AJ Friedson, B Hesman, J Hurley, C Merlet

Acetylene (C2H2) and ethane (C2H6) are by-products of complex photochemistry in the stratosphere of Saturn. Both hydrocarbons are important to the thermal balance of Saturn's stratosphere and serve as tracers of vertical motion in the lower stratosphere. Earlier studies of Saturn's hydrocarbons using Cassini-CIRS observations have provided only a snapshot of their behaviour. Following the vernal equinox in August 2009, Saturn's northern and southern hemispheres have entered spring and autumn, respectively, however the response of Saturn's hydrocarbons to this seasonal shift remains to be determined. In this paper, we investigate how the thermal structure and concentrations of acetylene and ethane have evolved with the changing season on Saturn. We retrieve the vertical temperature profiles and acetylene and ethane volume mixing ratios from δν̃=15.5cm-1 Cassini-CIRS observations. In comparing 2005 (solar longitude, Ls~308°), 2009 (Ls~3°) and 2010 (Ls~15°) results, we observe the disappearance of Saturn's warm southern polar hood with cooling of up to 17.1K±0.8K at 1.1mbar at high-southern latitudes. Comparison of the derived temperature trend in this region with a radiative climate model (Section 4 of Fletcher et al., 2010 and Greathouse et al. (2013, in preparation)) indicates that this cooling is radiative although dynamical changes in this region cannot be ruled out. We observe a21±12% enrichment of acetylene and a 29±11% enrichment of ethane at 25°N from 2005 to 2009, suggesting downwelling at this latitude. At 15°S, both acetylene and ethane exhibit a decrease in concentration of 6±11% and 17±9% from 2005 to 2010, respectively, which suggests upwelling at this latitude (though a statistically significant change is only exhibited by ethane). These implied vertical motions at 15°S and 25°N are consistent with a recently-developed global circulation model of Saturn's tropopause and stratosphere(Friedson and Moses, 2012), which predicts this pattern of upwelling and downwelling as a result of a seasonally-reversing Hadley circulation. Ethane exhibits a general enrichment at mid-northern latitudes from 2005 to 2009. As the northern hemisphere approaches summer solstice in 2017, this feature might indicate an onset of a meridional enrichment of ethane, as has been observed in the southern hemisphere during/after southern summer solstice. © 2013 Elsevier Inc.


Upper limits for PH3 and H2S in Titan's atmosphere from Cassini CIRS

Icarus (2013)

CA Nixon, NA Teanby, PGJ Irwin, SM Hörst


On the potential of the EChO mission to characterize gas giant atmospheres

MNRAS 430 (2013) 1188-1207-1188-1207

JK Barstow, S Aigrain, PGJ Irwin, N Bowles, LN Fletcher, J-M Lee


Constraining the atmosphere of GJ 1214b using an optimal estimation technique

Monthly Notices of the Royal Astronomical Society 434 (2013) 2616-2628

JK Barstow, S Aigrain, PGJ Irwin, LN Fletcher, JM Lee

We explore cloudy, extended H2-He atmosphere scenarios for the warm super-Earth GJ 1214b using an optimal estimation retrieval technique. This planet, orbiting an M4.5 star only 13 pc from the Earth, is of particular interest because it lies between the Earth and Neptune in size and may be a member of a new class of planet that is neither terrestrial nor gas giant. Its relatively flat transmission spectrum has so far made atmospheric characterization difficult. The Non-linear optimal Estimator for MultivariateE spectral analySIS (NEMESIS) algorithm is used to explore the degenerate model parameter space for a cloudy, H2-He-dominated atmosphere scenario. Optimal estimation is a data-led approach that allows solutions beyond the range permitted by ab initio equilibrium model atmosphere calculations, and as such prevents restriction from prior expectations. We show that optimal estimation retrieval is a powerful tool for this kind of study, and present an exploration of the degenerate atmospheric scenarios for GJ 1214b. Whilst we find a family of solutions that provide a very good fit to the data, the quality and coverage of these data are insufficient for us to more precisely determine the abundances of cloud and trace gases given an H2-He atmosphere, and we also cannot rule out the possibility of a high molecular weight atmosphere. Future ground- and space-based observations will provide the opportunity to confirm or rule out an extended H2-He atmosphere, but more precise constraints will be limited by intrinsic degeneracies in the retrieval problem, such as variations in cloud top pressure and temperature. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Radiative forcing of the stratosphere of Jupiter, Part I: Atmospheric cooling rates from Voyager to Cassini

Planetary and Space Science 88 (2013) 3-25

X Zhang, CA Nixon, RL Shia, RA West, PGJ Irwin, RV Yelle, MA Allen, YL Yung

We developed a line-by-line heating and cooling rate model for the stratosphere of Jupiter, based on two complete sets of global maps of temperature, C2H2 and C2H6, retrieved from the Cassini and Voyager observations in the latitude and vertical plane, with a careful error analysis. The non-LTE effect is found unimportant on the thermal cooling rate below the 0.01 mbar pressure level. The most important coolants are molecular hydrogen between 10 and 100 mbar, and hydrocarbons, including ethane (C2H6), acetylene (C2H2)and methane(CH4), in the region above. The two-dimensional cooling rate maps are influenced primarily by the temperature structure, and also by the meridional distributions of C2H2 and C2H6.The temperature anomalies at the 1 mbar pressure level in the Cassini data and the strong C2H6 latitudinal contrast in the Voyager epoch are the two most prominent features influencing the cooling rate patterns, with the effect from the 'quasi-quadrennial oscillation (QQO)' thermal structures at ~20 mbar. The globally averaged CH4 heating and cooling rates are not balanced, clearly in the lower stratosphere under 10 mbar, and possibly in the upper stratosphere above the 1 mbar pressure level. Possible heating sources from the gravity wave breaking and aerosols are discussed. The radiative relaxation timescale in the lower stratosphere implies that the temperature profile might not be purely radiatively controlled. © 2013 Elsevier Ltd.


A Gemini ground-based transmission spectrum of WASP-29b: a featureless spectrum from 515 to 720 nm

Monthly Notices of the Royal Astronomical Society 428 (2013) 3680-3692-3680-3692

NP Gibson, S Aigrain, JK Barstow, TM Evans, LN Fletcher, PGJ Irwin


High resolution in three dimensions with SWIFT and PALM3K

3rd AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes (2013)

F Clarke, N Thatte, M Tecza, K O'Brien, R Houghton, D Tice, L Fletcher, P Irwin, A Verma, R Dekany, R Buruss, J Roberts

SWIFT is a visible light (650-1000nm) integral field spectorgaph fed by the Palomar extreme adaptive optics system PALM3K. With a subaperture spacing of 8cm, PALM3K is capable of delivering diffraction limited performance even in the visible. With SWIFT providing spatially resolved spectroscopy at R=4000, this provides a truly unique facility for high resolution science in three dimensions. We present here some results from the first year of PALM3K+SWIFT science. We also report on our experience of operating a small field of view instrument (1"x0.5") with a high performance AO system, and hope the lessons learned will provide valuable input to designing successful and productive AO plus Instrument combinations for ELTs.


Upper limits for PH<inf>3</inf> and H<inf>2</inf>S in Titan's atmosphere from Cassini CIRS

Icarus 224 (2013) 253-256

CA Nixon, NA Teanby, PGJ Irwin, SM Hörst

We have searched for the presence of simple P and S-bearing molecules in Titan's atmosphere, by looking for the characteristic signatures of phosphine and hydrogen sulfide in infrared spectra obtained by Cassini CIRS. As a result we have placed the first upper limits on the stratospheric abundances, which are 1ppb (PH3) and 330ppb (H2S), at the 2-σ significance level. © 2013.


Radiative forcing of the stratosphere of Jupiter, Part I: Atmospheric cooling rates from Voyager to Cassini

Planetary and Space Science (2013)

X Zhang, RL Shia, MA Allen, YL Yung, CA Nixon, RA West, PGJ Irwin, RV Yelle

We developed a line-by-line heating and cooling rate model for the stratosphere of Jupiter, based on two complete sets of global maps of temperature, CH and CH, retrieved from the Cassini and Voyager observations in the latitude and vertical plane, with a careful error analysis. The non-LTE effect is found unimportant on the thermal cooling rate below the 0.01 mbar pressure level. The most important coolants are molecular hydrogen between 10 and 100 mbar, and hydrocarbons, including ethane (CH), acetylene (CH) and methane (CH), in the region above. The two-dimensional cooling rate maps are influenced primarily by the temperature structure, and also by the meridional distributions of CH and CH. The temperature anomalies at the 1 mbar pressure level in the Cassini data and the strong CH latitudinal contrast in the Voyager epoch are the two most prominent features influencing the cooling rate patterns, with the effect from the 'quasi-quadrennial oscillation (QQO)' thermal structures at ~20 mbar. The globally averaged CH heating and cooling rates are not balanced, clearly in the lower stratosphere under 10 mbar, and possibly in the upper stratosphere above the 1 mbar pressure level. Possible heating sources from the gravity wave breaking and aerosols are discussed. The radiative relaxation timescale in the lower stratosphere implies that the temperature profile might not be purely radiatively controlled. © 2013 Elsevier Ltd.


Latitudinal variation of upper tropospheric NH<inf>3</inf> on Saturn derived from Cassini/CIRS far-infrared measurements

Planetary and Space Science 73 (2012) 347-363

J Hurley, LN Fletcher, PGJ Irwin, SB Calcutt, JA Sinclair, C Merlet

Ammonia (NH3) has been detected both on Saturn and Jupiter, and although its concentration and distribution has been well-studied on Jupiter, it has proven more difficult to do so on Saturn due to higher sensitivity requirements resulting from Saturn's lower atmospheric temperatures and the dominance of Saturn's phosphine which masks the ammonia signal. Using far-infrared measurements of Saturn taken by Cassini/CIRS between February 2005 and December 2010, the latitudinal variations of upper tropospheric ammonia on Saturn are studied. Sensitivity to NH3 in the far-infrared is explored to provide estimates of temperature, para-H2 and PH 3, from 2.5 cm-1 spectral resolution measurements alone, 0.5 cm-1 spectral-resolution measurements alone, and 0.5 cm -1 measurements degraded to 2.5 cm-1 spectral resolution. The estimates of NH3 from these three different datasets largely agree, although there are notable differences using the high emission angle 0.5 cm-1 data, which are asserted to result from a reduction in sensitivity at higher emission angles. For low emission angles, the 0.5 cm -1-retrieved values of NH3 can be used to reproduce the 2.5 cm-1 spectra with similar efficacy as those derived directly from the 2.5 cm-1 resolution data itself, and vice versa. Using low emission angle data, NH3 is observed to have broad peak abundances at ±25°latitude, attributed to result from condensation and/or photolytic processes. Lack of data coverage at equatorial latitudes precludes analysis of NH3 abundance at less than about 10°latitude. Noise levels are not sufficient to distinguish fine zonal features, although it seems that NH3 cannot trace the zonal belt/zone structure in the upper troposphere of Saturn. © 2012 Elsevier Ltd. All rights reserved.


The origin and evolution of saturn's 2011-2012 stratospheric vortex

Icarus 221 (2012) 560-586

LN Fletcher, BE Hesman, RK Achterberg, PGJ Irwin, G Bjoraker, N Gorius, J Hurley, J Sinclair, GS Orton, J Legarreta, E García-Melendo, A Sánchez-Lavega, PL Read, AA Simon-Miller, FM Flasar

The planet-encircling springtime storm in Saturn's troposphere (December 2010-July 2011, Fletcher, L.N. et al. [2011]. Science 332, 1413-1414; Sánchez-Lavega, A. et al. [2011]. Nature 475, 71-74; Fischer, G. et al. [2011]. Nature 475, 75-77) produced dramatic perturbations to stratospheric temperatures, winds and composition at mbar pressures that persisted long after the tropospheric disturbance had abated. Thermal infrared (IR) spectroscopy from the Cassini Composite Infrared Spectrometer (CIRS), supported by ground-based IR imaging from the VISIR instrument on the Very Large Telescope and the MIRSI instrument on NASA's IRTF, is used to track the evolution of a large, hot stratospheric anticyclone between January 2011 and March 2012. The evolutionary sequence can be divided into three phases: (I) the formation and intensification of two distinct warm airmasses near 0.5. mbar between 25 and 35°N (B1 and B2) between January-April 2011, moving westward with different zonal velocities, B1 residing directly above the convective tropospheric storm head; (II) the merging of the warm airmasses to form the large single 'stratospheric beacon' near 40°N (B0) between April and June 2011, disassociated from the storm head and at a higher pressure (2 mbar) than the original beacons, a downward shift of 1.4 scale heights (approximately 85. km) post-merger; and (III) the mature phase characterised by slow cooling (0.11. ±. 0.01. K/day) and longitudinal shrinkage of the anticyclone since July 2011. Peak temperatures of 221.6. ±. 1.4. K at 2. mbar were measured on May 5th 2011 immediately after the merger, some 80. K warmer than the quiescent surroundings. From July 2011 to the time of writing, B0 remained as a long-lived stable stratospheric phenomenon at 2. mbar, moving west with a near-constant velocity of 2.70. ±. 0.04. deg/day (-24.5. ±. 0.4. m/s at 40°N relative to System III longitudes). No perturbations to visible clouds and hazes were detected during this period.With no direct tracers of motion in the stratosphere, we use thermal windshear calculations to estimate clockwise peripheral velocities of 200-400m/s at 2mbar around B0. The peripheral velocities of the two original airmasses were smaller (70-140m/s). In August 2011, the size of the vortex as defined by the peripheral collar was 65° longitude (50,000km in diameter) and 25° latitude. Stratospheric acetylene (C 2H 2) was uniformly enhanced by a factor of three within the vortex, whereas ethane (C 2H 6) remained unaffected. The passage of B0 generated a new band of warm stratospheric emission at 0.5mbar at its northern edge, and there are hints of warm stratospheric structures associated with the beacons at higher altitudes (p<0.1mbar) than can be reliably observed by CIRS nadir spectroscopy. Analysis of the zonal windshear suggests that Rossby wave perturbations from the convective storm could have propagated vertically into the stratosphere at this point in Saturn's seasonal cycle, one possible source of energy for the formation of these stratospheric anticyclones. © 2012 Elsevier Inc.


ELUSIVE ETHYLENE DETECTED IN SATURN'S NORTHERN STORM REGION

ASTROPHYSICAL JOURNAL 760 (2012) ARTN 24

BE Hesman, GL Bjoraker, PV Sada, RK Achterberg, DE Jennings, PN Romani, AW Lunsford, LN Fletcher, RJ Boyle, AA Simon-Miller, CA Nixon, PGJ Irwin


EChO

EXPERIMENTAL ASTRONOMY 34 (2012) 311-353

G Tinetti, JP Beaulieu, T Henning, M Meyer, G Micela, I Ribas, D Stam, M Swain, O Krause, M Ollivier, E Pace, B Swinyard, A Aylward, R van Boekel, A Coradini, T Encrenaz, I Snellen, MR Zapatero-Osorio, J Bouwman, JY-K Cho, VC du Foresto, T Guillot, M Lopez-Morales, I Mueller-Wodarg, E Palle, F Selsis, A Sozzetti, PAR Ade, N Achilleos, A Adriani, CB Agnor, C Afonso, C Allende Prieto, G Bakos, RJ Barber, M Barlow, V Batista, P Bernath, B Bezard, P Borde, LR Brown, A Cassan, C Cavarroc, A Ciaravella, C Cockell, A Coustenis, C Danielski, L Decin, R De Kok, O Demangeon, P Deroo, P Doel, P Drossart, LN Fletcher, M Focardi, F Forget, S Fossey, P Fouque, J Frith, M Galand, P Gaulme, JI Gonzalez Hernandez, O Grasset, D Grassi, JL Grenfell, MJ Griffin, CA Griffith, U Groezinger, M Guedel, P Guio, O Hainaut, R Hargreaves, PH Hauschildt, K Heng, D Heyrovsky, R Hueso, P Irwin, L Kaltenegger, P Kervella, D Kipping, TT Koskinen, G Kovacs, A La Barbera, H Lammer, E Lellouch, G Leto, MA Lopez Valverde, M Lopez-Puertas, C Lovis, A Maggio, JP Maillard, J Maldonado Prado, JB Marquette, FJ Martin-Torres, P Maxted, S Miller, S Molinari, D Montes, A Moro-Martin, JI Moses, O Mousis, N Nguyen Tuong, R Nelson, GS Orton, E Pantin, E Pascale, S Pezzuto, D Pinfield, E Poretti, R Prinja, L Prisinzano, JM Rees, A Reiners, B Samuel, A Sanchez-Lavega, J Sanz Forcada, D Sasselov, G Savini, B Sicardy, A Smith, L Stixrude, G Strazzulla, J Tennyson, M Tessenyi, G Vasisht, S Vinatier, S Viti, I Waldmann, GJ White, T Widemann, R Wordsworth, R Yelle, Y Yung, SN Yurchenko


EChO: Exoplanet characterisation observatory

Experimental Astronomy 34 (2012) 311-353

G Tinetti, JP Beaulieu, T Henning, M Meyer, G Micela, I Ribas, D Stam, M Swain, O Krause, M Ollivier, E Pace, B Swinyard, A Aylward, R van Boekel, A Coradini, T Encrenaz, I Snellen, MR Zapatero-Osorio, J Bouwman, JYK Cho, VC de Foresto, T Guillot, M Lopez-Morales, I Mueller-Wodarg, E Palle, F Selsis, A Sozzetti, PAR Ade, N Achilleos, A Adriani, CB Agnor, C Afonso, CA Prieto, G Bakos, RJ Barber, M Barlow, V Batista, P Bernath, B Bézard, P Bordé, LR Brown, A Cassan, C Cavarroc, A Ciaravella, C Cockell, A Coustenis, C Danielski, L Decin, R de Kok, O Demangeon, P Deroo, P Doel, P Drossart, LN Fletcher, M Focardi, F Forget, S Fossey, P Fouqué, J Frith, M Galand, P Gaulme, JIG Hernández, O Grasset, D Grassi, JL Grenfell, MJ Griffin, CA Griffith, U Grözinger, M Guedel, P Guio, O Hainaut, R Hargreaves, PH Hauschildt, K Heng, D Heyrovsky, R Hueso, P Irwin, L Kaltenegger, P Kervella, D Kipping, TT Koskinen, G Kovács, A La Barbera, H Lammer, E Lellouch, G Leto, MA Lopez Valverde, M Lopez-Puertas, C Lovis, A Maggio, JP Maillard, J Maldonado Prado, JB Marquette, FJ Martin-Torres, P Maxted, S Miller, S Molinari, D Montes, A Moro-Martin

A dedicated mission to investigate exoplanetary atmospheres represents a major milestone in our quest to understand our place in the universe by placing our Solar System in context and by addressing the suitability of planets for the presence of life. EChO-the Exoplanet Characterisation Observatory-is a mission concept specifically geared for this purpose. EChO will provide simultaneous, multi-wavelength spectroscopic observations on a stable platform that will allow very long exposures. The use of passive cooling, few moving parts and well established technology gives a low-risk and potentially long-lived mission. EChO will build on observations by Hubble, Spitzer and ground-based telescopes, which discovered the first molecules and atoms in exoplanetary atmospheres. However, EChO's configuration and specifications are designed to study a number of systems in a consistent manner that will eliminate the ambiguities affecting prior observations. EChO will simultaneously observe a broad enough spectral region-from the visible to the mid-infrared-to constrain from one single spectrum the temperature structure of the atmosphere, the abundances of the major carbon and oxygen bearing species, the expected photochemically-produced species and magnetospheric signatures. The spectral range and resolution are tailored to separate bands belonging to up to 30 molecules and retrieve the composition and temperature structure of planetary atmospheres. The target list for EChO includes planets ranging from Jupiter-sized with equilibrium temperatures Teq up to 2,000 K, to those of a few Earth masses, with Teq \u223c 300 K. The list will include planets with no Solar System analog, such as the recently discovered planets GJ1214b, whose density lies between that of terrestrial and gaseous planets, or the rocky-iron planet 55 Cnc e, with day-side temperature close to 3,000 K. As the number of detected exoplanets is growing rapidly each year, and the mass and radius of those detected steadily decreases, the target list will be constantly adjusted to include the most interesting systems. We have baselined a dispersive spectrograph design covering continuously the 0. 4-16 μm spectral range in 6 channels (1 in the visible, 5 in the InfraRed), which allows the spectral resolution to be adapted from several tens to several hundreds, depending on the target brightness. The instrument will be mounted behind a 1. 5 m class telescope, passively cooled to 50 K, with the instrument structure and optics passively cooled to \u223c45 K. EChO will be placed in a grand halo orbit around L2. This orbit, in combination with an optimised thermal shield design, provides a highly stable thermal environment and a high degree of visibility of the sky to observe repeatedly several tens of targets over the year. Both the baseline and alternative designs have been evaluated and no critical items with Technology Readiness Level (TRL) less than 4-5 have been identified. We have also undertaken a first-order cost and development plan analysis and find that EChO is easily compatible with the ESA M-class mission framework. © 2012 Springer Science+Business Media B.V.


The application of new methane line absorption data to Gemini-N/NIFS and KPNO/FTS observations of Uranus' near-infrared spectrum

Icarus 220 (2012) 369-382

PGJ Irwin, C de Bergh, R Courtin, B Bézard, NA Teanby, GR Davis, LN Fletcher, GS Orton, SB Calcutt, D Tice, J Hurley

New line data describing the absorption of CH 4 and CH 3D from 1.26 to 1.71μm (Campargue, A., Wang, L., Mondelain, D., Kassi, S., Bézard, B., Lellouch, E., Coustenis, A., de Bergh, C., Hirtzig, M., Drossart, P. [2012]. Icarus 219, 110-128), building upon previous papers by Campargue et al. (Campargue, A., Wang, L., Kassi, S., Masat, M., Votava, O. [2010]. J. Quant. Spectrosc. Radiat. Transfer 111, 1141-1151; Wang, L., Kassi, S., Campargue, A. [2010]. J. Quant. Spectrosc. Radiat. Transfer 111, 1130-1140; Wang, L., Kassi, S., Liu, A.W., Hu, S.M., Campargue, A. [2011]. J. Quant. Spectrosc. Radiat. Transfer 112, 937-951)) have been applied to the analysis of Gemini-N/NIFS observations of Uranus made in 2010 and compared with earlier disc-averaged observations made by KPNO/FTS in 1982. The new line data are found to improve greatly the fit to the observed spectra and present a huge advance over previous methane absorption tables by allowing us to determine the CH 3D/CH 4 ratio and also start to break the degeneracy between methane abundance and cloud top height. The best fits are obtained if the cloud particles in the main cloud deck at the 2-3bar level become less scattering with wavelength across the 1.4-1.6μm region and we have modelled this variation here by varying the extinction cross-section and single-scattering albedo of the particles.Applying the new line data to the NIFS spectra of Uranus, we determine a new estimate of the CH 3D/CH 4 ratio of 2.9-0.5+0.9×10-4, which is consistent with the estimate of de Bergh et al. (de Bergh, C., Lutz, B.L., Owen, T., Brault, J., Chauville, J. [1986]. Astrophys. J. 311, 501-510) of 3.6-2.8+3.6×10-4, made by fitting a disc-averaged KPNO/FTS spectrum measured in 1982, but much better constrained. The NIFS observations made in 2010 have been disc-averaged and compared with the 1982 KPNO/FTS spectrum and found to be in excellent agreement.Using k-tables fitted to the new line data, the central meridian observations of Uranus' H-band spectrum (1.49-1.64μm) made by Gemini-N/NIFS in 2010 have been reanalyzed. The use of the new methane absorption coefficients and the modified scattering properties of the cloud particles in the main cloud deck appears to break the degeneracy between cloud height and methane abundance immediately above it in this spectral region and we find that both vary with latitude across Uranus' disc. Overall, we find that the main cloud deck becomes higher, but thinner from equator to poles, with a local maximum in cloud top height in the circumpolar zones at 45°N and 45°S. At the same time, using the 'D' temperature pressure profile of Lindal et al. (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987]. J. Geophys. Res. 92, 14987-15001) and a deep methane abundance of 1.6% (Baines, K.H., Mickelson, M.E., Larson, L.E., Ferguson, D.W. [1995]. Icarus 144, 328-340) we find that the relative humidity of methane is high near the equator (~60%) and decreases sharply towards the poles, except near the circumpolar zone at 45°N, which has brightened steadily since 2007, and where there is a local maximum in methane relative humidity. In tests conducted with the warmer 'F1' profile of Sromovsky et al. (2011) we find a similar variation of methane abundance above the main cloud, although for this warmer temperature profile this abundance is dependent mostly on the fitted deep methane mole fraction. © 2012 Elsevier Inc.


Active upper-atmosphere chemistry and dynamics from polar circulation reversal on Titan.

Nature 491 (2012) 732-735

NA Teanby, PGJ Irwin, CA Nixon, R de Kok, S Vinatier, A Coustenis, E Sefton-Nash, SB Calcutt, FM Flasar

Saturn's moon Titan has a nitrogen atmosphere comparable to Earth's, with a surface pressure of 1.4 bar. Numerical models reproduce the tropospheric conditions very well but have trouble explaining the observed middle-atmosphere temperatures, composition and winds. The top of the middle-atmosphere circulation has been thought to lie at an altitude of 450 to 500 kilometres, where there is a layer of haze that appears to be separated from the main haze deck. This 'detached' haze was previously explained as being due to the co-location of peak haze production and the limit of dynamical transport by the circulation's upper branch. Here we report a build-up of trace gases over the south pole approximately two years after observing the 2009 post-equinox circulation reversal, from which we conclude that middle-atmosphere circulation must extend to an altitude of at least 600 kilometres. The primary drivers of this circulation are summer-hemisphere heating of haze by absorption of solar radiation and winter-hemisphere cooling due to infrared emission by haze and trace gases; our results therefore imply that these effects are important well into the thermosphere (altitudes higher than 500 kilometres). This requires both active upper-atmosphere chemistry, consistent with the detection of high-complexity molecules and ions at altitudes greater than 950 kilometres, and an alternative explanation for the detached haze, such as a transition in haze particle growth from monomers to fractal structures.

Pages