Publications


Potential vorticity structure of Titan’s polar vortices from Cassini CIRS observations

Icarus Elsevier BV (2020) 114030

J Sharkey, NA Teanby, M Sylvestre, DM Mitchell, WJM Seviour, CA Nixon, PGJ Irwin


Introduction to Icarus special issue “From Mars Express to ExoMars”

Icarus Elsevier BV (2020) 114118

MA Lopez-Valverde, DV Titov, CF Wilson


Colour and tropospheric cloud structure of Jupiter from MUSE/VLT: retrieving a universal chromophore

Icarus Elsevier 338 (2020) 113589

AS Braude, P Irwin, GS Orton, LN Fletcher

Recent work by Sromovsky et al. (2017, Icarus 291, 232-244) suggested that all red colour in Jupiter’s atmosphere could be explained by a single colour-carrying compound, a so-called ‘universal chromophore’. We tested this hypothesis on ground-based spectroscopic observations in the visible and near-infrared (480- 930 nm) from the VLT/MUSE instrument between 2014 and 2018, retrieving a chromophore absorption spectrum directly from the North Equatorial Belt, and applying it to model spatial variations in colour, tropospheric cloud and haze structure on Jupiter. We found that we could model both the belts and the Great Red Spot of Jupiter using the same chromophore compound, but that this chromophore must exhibit a steeper blue-absorption gradient than the proposed chromophore of Carlson et al. (2016, Icarus 274, 106–115). We retrieved this chromophore to be located no deeper than 0.2±0.1 bars in the Great Red Spot and 0.7±0.1 bars elsewhere on Jupiter. However, we also identified some spectral variability between 510 nm and 540 nm that could not be accounted for by a universal chromophore. In addition, we retrieved a thick, global cloud layer at 1.4 ± 0.3 bars that was relatively spatially invariant in altitude across Jupiter. We found that this cloud layer was best characterised by a real refractive index close to that of ammonia ice in the belts and the Great Red Spot, and poorly characterised by a real refractive index of 1.6 or greater. This may be the result of ammonia cloud at higher altitude obscuring a deeper cloud layer of unknown composition.


Color and aerosol changes in Jupiter after a North Temperate Belt disturbance

Icarus Elsevier BV 352 (2020) 114031

S Pérez-Hoyos, A Sánchez-Lavega, J Sanz-Requena, N Barrado-Izagirre, O Carrión-González, A Anguiano-Arteaga, P Irwin, A Braude


The turbulent dynamics of Jupiter's and Saturn's weather layers: order out of chaos?

GEOSCIENCE LETTERS 7 (2020) ARTN 10

PL Read, RMB Young, D Kennedy


Detection of CH3C3N in Titan’s Atmosphere

The Astrophysical Journal American Astronomical Society 903 (2020) L22-L22

AE Thelen, MA Cordiner, CA Nixon, V Vuitton, Z Kisiel, SB Charnley, MY Palmer, NA Teanby, PGJ Irwin


Detection of Cyclopropenylidene on Titan with ALMA

The Astronomical Journal American Astronomical Society 160 (2020) 205-205

CA Nixon, AE Thelen, MA Cordiner, Z Kisiel, SB Charnley, EM Molter, J Serigano, PGJ Irwin, NA Teanby, Y-J Kuan


Constraints on Neptune’s haze structure and formation from VLT observations in the H-band

Icarus Elsevier (2020)

D Toledo Carrasco, P Irwin, P Rannou, L Fletcher, N Teanby, M Wong, G Orton

A 1-dimensional microphysics model has been used to constrain the structure and formation of haze in Neptune's atmosphere. These simulations were coupled to a radiative-transfer and retrieval code (NEMESIS) to model spectral observations of Neptune in the H-band performed by the SINFONI Integral Field Unit Spectrometer on the Very Large Telescope (VLT) in 2013. It was found that observations in the H-band and with emission angles ≤60° are largely unaffected by the imaginary refractive index of haze particles, allowing a notable reduction of the free parameters required to fit the observations. Our analysis shows a total haze production rate of (2.61 ± 0.18) × 10−14 kg m−2 s−1, about 10 times larger than that found in Uranus's atmosphere, and a particle electric charge of q = 8.6 ± 1.1 electrons per μm radius at latitudes between 5 and 15° S. This haze production rate in Neptune results in haze optical depths about 10 times greater than those in Uranus. The effective radius reff was found to be 0.22 ± 0.01 and 0.26 ± 0.02 μm at the 0.1 and 1-bar levels, respectively, with haze number densities of 8.48+1.78−1.31 and 9.31+2.52−1.91 particles per cm3. The fit at weak methane-absorbing wavelengths reveals also the presence of a tropospheric cloud with a total optical depth >10 at 1.46 μm. The tropospheric cloud base altitude was found near the 2.5-bar level, although this estimation may be only representative of the top of a thicker and deeper cloud. Our analysis leads to haze opacities about 3.5 times larger than that derived from Voyager-2 observations (Moses et al., 1995). This larger opacity indicates a haze production rate 2 times larger at least. To study this difference haze opacity or production rate, we performed a timescale analysis with our microphysical model to estimate the time required for haze particles to grow and settle out. Although this analysis shows haze timescales (∼15 years) shorter than the time lapsed between Voyager-2 observations and 2013, the solar illumination at the top of the atmosphere has not varied significantly during this period (at the studied latitudes) to explain the increase in haze production. This difference in haze production rate derived for these two periods may arise from: a) the fact that in our analysis we employed spectral observations in the infrared (H-band), while Moses et al. (1995) used photometric images taken at 5 different filters in the visible. While high-phase-angle Voyager observations are more sensitive to small haze particles and at altitudes above the 0.1-bar level, the haze constraints derived from VLT spectra in H-band are limited to pressures greater than 0.1 bar. As a result of the different phase angles of the two set of observations, differences in the estimation of M0 may arise from the use of Mie phase functions as well. b) our 1-dimensional model does not account for latitudinal redistributions of the haze by dynamics. A possible meridional transport of haze with wind velocities greater than ∼0.03 m s−1 would result in dynamics timescales shorter than 15 years and thus might explain the observed variations in the haze production rate during this period. Compared with our estimations, photochemical models point to even larger production rates on Neptune (by a factor of 2.4). Assuming that the photochemical simulations are correct, we found that this discrepancy can be explained if haze particles evaporate before reaching the tropospheric-cloud levels. This scenario would decrease the cumulative haze opacity above the 1-bar level, and thus a larger haze production rate would be required to fit our observations. However, to validate this haze vertical structure future microphysical simulations that include the evaporation rates of haze particles are required.


Atmospheric circulation of tidally locked gas giants with increasing rotation and implications for white dwarf–Brown dwarf systems

Astrophysical Journal 902 (2020)

X Tan, AP Showman

© 2020. The American Astronomical Society. All rights reserved. Tidally locked gas giants, which exhibit a novel regime of day–night thermal forcing and extreme stellar irradiation, are typically in several-day orbits, implying a modest role for rotation in the atmospheric circulation. Nevertheless, there exist a class of gas-giant, highly irradiated objects—brown dwarfs orbiting white dwarfs in extremely tight orbits—whose orbital and hence rotation periods are as short as 1–2 hr. Phase curves and other observations have already been obtained for this class of objects, raising fundamental questions about the role of an increasing planetary rotation rate in controlling the circulation. So far, most modeling studies have investigated rotation periods exceeding a day, as appropriate for typical hot Jupiters. In this work, we investigate atmospheric circulation of tidally locked atmospheres with decreasing rotation periods (increasing rotation rate) down to 2.5 hr. With a decreasing rotation period, we show that the width of the equatorial eastward jet decreases, consistent with the narrowing of the equatorial waveguide due to a decrease of the equatorial deformation radius. The eastward-shifted equatorial hot-spot offset decreases accordingly, and the off-equatorial westward-shifted hot areas become increasingly distinctive. At high latitudes, winds become weaker and more rotationally dominated. The day–night temperature contrast becomes larger due to the stronger influence of rotation. Our simulated atmospheres exhibit variability, presumably caused by instabilities and wave interactions. Unlike typical hot Jupiter models, the thermal phase curves of rapidly rotating models show a near alignment of peak flux to secondary eclipse. This result helps to explain why, unlike hot Jupiters, brown dwarfs closely orbiting white dwarfs tend to exhibit IR flux peaks nearly aligned with secondary eclipse. Our results have important implications for understanding fast-rotating, tidally locked atmospheres.


The phase-curve signature of condensible water-rich atmospheres on slowly rotating tidally locked exoplanets

Astrophysical Journal Letters 901 (2020)

F Ding, RT Pierrehumbert

© 2020. The American Astronomical Society. We use an idealized three-dimensional general circulation model to study condensible-rich atmospheres with an ineffective cold trap on slowly rotating tidally locked terrestrial planets. In particular, we show the climate dynamics in a thin and temperate atmosphere with condensible water vapor. The similarities between our thin and temperate atmosphere and the warm and thick atmosphere approaching the water vapor runaway greenhouse in previous works are discussed, including the reversal of the thermal emission between the day and night hemispheres. Different from the transit spectroscopy of water vapor that depends on the absolute amount of atmospheric water vapor, the contrast between the dayside and nightside thermal emission provides information regarding the relative ratio of water vapor to the background atmosphere as well as the atmospheric pressure near the substellar tropopause and the emission level on the nightside on potentially habitable worlds.


Long-duration Venus lander for seismic and atmospheric science

PLANETARY AND SPACE SCIENCE 190 (2020) ARTN 104961

T Kremic, R Ghail, M Gilmore, G Hunter, W Kiefer, S Limaye, M Pauken, C Tolbert, C Wilson

© 2020 An exciting and novel science mission concept called Seismic and Atmospheric Exploration of Venus (SAEVe) has been developed which uses high-temperature electronics to enable a three-order magnitude increase in expected surface life (120 Earth days) over what has been achieved to date. This enables study of long-term, variable phenomena such as the seismicity of Venus and near surface weather, near surface energy balance, and atmospheric chemical composition. SAEVe also serves as a critical pathfinder for more sophisticated landers in the future. For example, first order seismic measurements by SAEVe will allow future missions to deliver better seismometers and systems to support the yet unknown frequency and magnitude of Venus events. SAEVe is focused on science that can be realized with low data volume instruments and will most benefit from temporal operations. The entire mission architecture and operations maximize science while minimizing energy usage and physical size and mass. The entire SAEVe system including its protective entry system is estimated to be around 45 ​kg and approximately 0.6 ​m diameter. These features allow SAEVe to be relatively cost effective and be easily integrated onto a Venus orbiter mission. The technologies needed to implement SAEVe are currently in development by several funded activities. Component and system level work is ongoing under NASA's Long Lived Insitu Solar System Explorer (LLISSE) project and the HOTTech program. LLISSE, is a NASA project to develop a small Venus lander that will operate on the surface of Venus for 60 days and measure variations in meteorology, radiance, and atmospheric chemistry. LLISSE is developing a full-function engineering model of a Venus lander that contains essentially all the core capabilities of SAEVe thus greatly reducing the technology risk to SAEVe. The SAEVe long duration Venus lander promises exciting new science and is an ideal complimentary element to many future Venus orbiter missions being proposed or planned today.


The Equatorial Jet Speed on Tidally Locked Planets. I. Terrestrial Planets

ASTROPHYSICAL JOURNAL 901 (2020) ARTN 78

M Hammond, S-M Tsai, RT Pierrehumbert


Studying the Composition and Mineralogy of the Hermean Surface with the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) for the BepiColombo Mission: An Update

Space Science Reviews 216 (2020)

H Hiesinger, J Helbert, G Alemanno, KE Bauch, M D’Amore, A Maturilli, A Morlok, MP Reitze, C Stangarone, AN Stojic, I Varatharajan, I Weber, G Arnold, M Banaszkiewicz, K Bauch, J Benkhoff, A Bischoff, M Blecka, N Bowles, S Calcutt, L Colangeli, S Erard, S Fonti, BT Greenhagen, O Groussain, J Helbert, H Hiesinger, H Hirsch, J Jahn, R Killen, J Knollenberg, E Kührt, E Lorenz, I Mann, U Mall, A Maturilli, A Morlok, L Moroz, G Peter, M Rataj, M Robinson, W Skrbek, T Spohn, A Sprague, D Stöffler, A Stojic, F Taylor, I Varatharajan, H Venus, J Warrell, I Walter, I Weber, A Witzke, C Wöhler

© 2020, The Author(s). Launched onboard the BepiColombo Mercury Planetary Orbiter (MPO) in October 2018, the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) is on its way to planet Mercury. MERTIS consists of a push-broom IR-spectrometer (TIS) and a radiometer (TIR), which operate in the wavelength regions of 7-14 μm and 7-40 μm, respectively. This wavelength region is characterized by several diagnostic spectral signatures: the Christiansen feature (CF), Reststrahlen bands (RB), and the Transparency feature (TF), which will allow us to identify and map rock-forming silicates, sulfides as well as other minerals. Thus, the instrument is particularly well-suited to study the mineralogy and composition of the hermean surface at a spatial resolution of about 500 m globally and better than 500 m for approximately 5-10% of the surface. The instrument is fully functional onboard the BepiColombo spacecraft and exceeds all requirements (e.g., mass, power, performance). To prepare for the science phase at Mercury, the team developed an innovative operations plan to maximize the scientific output while at the same time saving spacecraft resources (e.g., data downlink). The upcoming fly-bys will be excellent opportunities to further test and adapt our software and operational procedures. In summary, the team is undertaking action at multiple levels, including performing a comprehensive suite of spectroscopic measurements in our laboratories on relevant analog materials, performing extensive spectral modeling, examining space weathering effects, and modeling the thermal behavior of the hermean surface.


First detection of ozone in the mid-infrared at Mars: implications for methane detection

Astronomy & Astrophysics EDP Sciences 639 (2020) A141

K Olsen, F Lefèvre, F Montmessin, A Trokhimovskiy, L Baggio, A Fedorova​, J Alday​, A Lomakin​, D Belyaev, A Patrakeev, A Shakun​, O Korablev

<br><strong>Aims: </strong>The ExoMars Trace Gas Orbiter (TGO) was sent to Mars in March 2016 to search for trace gases diagnostic of active geological or biogenic processes.</br> <br><strong>Methods: </strong>We report the first observation of the spectral features of Martian ozone (O3) in the mid-infrared range using the Atmospheric Chemistry Suite (ACS) Mid-InfaRed (MIR) channel, a cross-dispersion spectrometer operating in solar occultation mode with the finest spectral resolution of any remote sensing mission to Mars.</br> <br><strong>Results: </strong>Observations of ozone were made at high northern latitudes (> 65◦N) prior to the onset of the 2018 global dust storm (Ls = 163–193◦). During this fast transition phase between summer and winter ozone distribution, the O3 volume mixing ratio observed is 100–200 ppbv near 20 km. These amounts are consistent with past observations made at the edge of the southern polar vortex in the ultraviolet range. The observed spectral signature of ozone at 3000–3060 cm−1 directly overlaps with the spectral range of the methane (CH4) ν3 vibration-rotation band, and it, along with a newly discovered CO2 band in the same region, may interfere with measurements of methane abundance.</br>


First observation of the magnetic dipole CO2 main isotopologue absorption band at 3.3 µm in the atmosphere of Mars by the ExoMars Trace Gas Orbiter ACS instrument

Astronomy & Astrophysics EDP Sciences (2020)

A Trokhimovskiy, V Perevalov, O Korablev, A Fedorova, K Olsen, J Bertaux, A Patrakeev, A Shakun, F Montmessin, F Lefèvre


Spatial structure in Neptune’s 7.90-m stratospheric CH emission, as measured by VLT-VISIR

Icarus Elsevier 345 (2020) 113748

J Sinclair, G Orton, L Fletcher, M Roman, I de Pater, T Encrenaz, H Hammel, R Giles, T Velusamy, J Moses, P Irwin, T Momary, N Rowe-Gurney, F Tabataba-Vakili

We present a comparison of VLT-VISIR images and Keck-NIRC2 images of Neptune, which highlight the coupling between its troposphere and stratosphere. VLT-VISIR images were obtained on September 16th 2008 (UT) at 7.90 μm and 12.27 μm, which are primarily sensitive to 1-mbar CH4 and C2H6 emission, respectively. NIRC2 images in the H band were obtained on October 5th, 6th and 9th 2008 (UT) and sense clouds and haze in the upper troposphere and lower stratosphere (from approximately 600 to 20 mbar). At 7.90 μm, we observe enhancements of CH4 emission in latitude bands centered at approximately 25∘S and 48∘S (planetocentric). Within these zonal bands, tentative detections (&lt;2σ) of discrete hotspots of CH4 emission are also evident at 24∘S, 181∘W and 42∘S, 170∘W. The longitudinal-mean enhancements in the CH4 emission are also latitudinally-coincident with bands of bright (presumably CH4 ice) clouds in the upper troposphere and lower stratosphere evidenced in the H-band images. This suggests the Neptunian troposphere and stratosphere are coupled in these specific regions. This could be in the form of (1) ‘overshoot’ of strong, upwelling plumes and advection of CH4 ice into the lower stratosphere, which subsequently sublimates into CH4 gas and/or (2) generation of waves by plumes impinging from the tropopause below, which impart their energy and heat the lower stratosphere. We favor the former process since there is no evidence of similar smaller-scale morphology in the C2H6 emission, which probes a similar atmospheric level. However, we cannot exclude temperature variations as the source of the morphology observed in CH4 emission. Future, near-infrared imaging of Neptune performed near-simultaneously with future mid-infrared spectral observations of Neptune by the James Webb Space Telescope would allow the coupling of Neptune's troposphere and stratosphere to be confirmed and studied in greater detail.


Uranus in Northern Mid-spring: Persistent Atmospheric Temperatures and Circulations Inferred from Thermal Imaging (vol 159, 45, 2020)

ASTRONOMICAL JOURNAL American Astronomical Society 160 (2020) ARTN 56

MT Roman, LN Fletcher, GS Orton, N Rowe-Gurney, PGJ Irwin


Thermodynamic and energetic limits on continental silicate weathering strongly impact the climate and habitability of wet, rocky worlds

Astrophysical Journal American Astronomical Society 896 (2020) 115

R Graham, R Pierrehumbert

The “liquid water habitable zone” (HZ) concept is predicated on the ability of the silicate weathering feedback to stabilize climate across a wide range of instellations. However, representations of silicate weathering used in current estimates of the effective outer edge of the HZ do not account for the thermodynamic limit on concentration of weathering products in runoff set by clay precipitation, nor for the energetic limit on precipitation set by planetary instellation. We find that when the thermodynamic limit is included in an idealized coupled climate/weathering model, steady-state planetary climate loses sensitivity to silicate dissolution kinetics, becoming sensitive to temperature primarily through the effect of temperature on runoff and to pCO2 through an effect on solute concentration mediated by pH. This increases sensitivity to land fraction, CO2 outgassing, and geological factors such as soil age and lithology, all of which are found to have a profound effect on the position of the effective outer edge of the HZ. The interplay between runoff sensitivity and the energetic limit on precipitation leads to novel warm states in the outer reaches of the HZ, owing to the decoupling of temperature and precipitation. We discuss strategies for detecting the signature of silicate weathering feedback through exoplanet observations in light of insights derived from the revised picture of weathering.


Jupiter in the Ultraviolet: Acetylene and Ethane Abundances in the Stratosphere of Jupiter from Cassini Observations between 0.15 and 0.19 mu m

ASTRONOMICAL JOURNAL American Astronomical Society 159 (2020) ARTN 291

H Melin, L Fletcher, P Irwin, S Edgington

&#xA9; 2020. The American Astronomical Society. All rights reserved. At wavelengths between 0.15 and 0.19 &#x3BC;m, the far-ultraviolet spectrum of Jupiter is dominated by the scattered solar spectrum, attenuated by molecular absorptions primarily by acetylene and ethane, and to a lesser extent ammonia and phosphine. We describe the development of our radiative transfer code that enables the retrieval of abundances of these molecular species from ultraviolet reflectance spectra. As a proof-of-concept we present an analysis of Cassini Ultraviolet Imaging Spectrograph (UVIS) observations of the disk of Jupiter during the 2000/2001 flyby. The ultraviolet-retrieved acetylene abundances in the upper stratosphere are lower than those predicted by models based solely on infrared thermal emission from the mid-stratosphere observed by the Composite Infrared Spectrometer (CIRS), requiring an adjustment to the vertical profiles above 1 mbar. We produce a vertical acetylene abundance profile that is compatible with both CIRS and UVIS, with reduced abundances at pressures &lt;1 mbar: the 0.1 mbar abundances are 1.21 &#xB1; 0.07 ppm for acetylene and 20.8 &#xB1; 5.1 ppm for ethane. Finally, we perform a sensitivity study for the JUICE ultraviolet spectrograph, which has extended wavelength coverage out to 0.21 &#x3BC;m, enabling the retrieval of ammonia and phosphine abundances, in addition to acetylene and ethane.


Baroclinic and barotropic instabilities in planetary atmospheres: energetics, equilibration and adjustment

NONLINEAR PROCESSES IN GEOPHYSICS 27 (2020) 147-173

P Read, D Kennedy, N Lewis, H Scolan, F Tabataba-Vakili, Y Wang, S Wright, R Young

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