Mapping the zonal structure of Titan's northern polar vortex

Icarus 337 (2020)

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

© 2019 Elsevier Inc. Saturn exhibits an obliquity of 26.7° such that the largest moon, Titan, experiences seasonal variations including the formation of a polar vortex in the winter hemisphere. Titan's polar vortex is characterised by cold stratospheric temperatures due to the lack of insolation over the winter pole, and an increase in trace gas abundance as a result of complex organic chemistry in the upper atmosphere combined with polar subsidence. Meridional variations in temperature and gas abundance across the vortex have previously been investigated, but there has not yet been any in-depth study of the zonal variations in the temperature or composition of the northern vortex. Here we present the first comprehensive two-dimensional seasonal mapping of Titan's northern winter vortex. Using 18 nadir mapping sequences observed by the Composite InfraRed Spectrometer (CIRS) instrument on-board Cassini, we investigate the evolution of the vortex over almost half a Titan year, from late winter through to mid summer (Ls = 326 − 86°, 2007–2017). We find the stratospheric symmetry axis to be tilted from the solid body rotation axis by around 3.5°, although our results for the azimuthal orientation of the tilt are inconclusive. We find that the northern vortex appears to remain zonally uniform in both temperature and composition at all times. A comparison with vortices observed on Earth, Mars, and Venus shows that large-scale wave mechanisms that are important on other terrestrial planets are not as significant in Titan's atmosphere. This allows the northern vortex to be more symmetrical and persist longer throughout the annual cycle compared to other terrestrial planets.

Exoplanetary Monte Carlo radiative transfer with correlated-k - I. Benchmarking transit and emission observables


GKH Lee, J Taylor, SL Grimm, J-L Baudino, R Garland, PGJ Irwin, K Wood

The atmospheric circulation of ultra-hot jupiters

Astrophysical Journal 886 (2019)

X Tan, TD Komacek

© 2019. The American Astronomical Society. All rights reserved Recent observations of ultra-hot Jupiters with dayside temperatures in excess of 2500 K have found evidence for new physical processes at play in their atmospheres. In this work, we investigate the effects of the dissociation of molecular hydrogen and recombination of atomic hydrogen on the atmospheric circulation of ultra-hot Jupiters. To do so, we incorporate these effects into a general circulation model (GCM) for hot Jupiter atmospheres and run a large suite of models varying the incident stellar flux, rotation period, and strength of frictional drag. We find that including hydrogen dissociation and recombination reduces the fractional day-to-night temperature contrast of ultra-hot Jupiter atmospheres and causes the speed of the equatorial jet to decrease in simulations with fixed rotation. This is because the large energy input required for hydrogen dissociation cools the dayside of the planet, and the energy released due to hydrogen recombination warms the nightside. The resulting decrease in the day-to-night temperature contrast reduces the day-to-night pressure gradient that drives the circulation, resulting in weaker wind speeds. The results from our GCM experiments qualitatively agree with previous theory that found that the fractional day-night temperature contrast of ultra-hot Jupiters should decrease with increasing equilibrium temperature owing to hydrogen dissociation and recombination. Lastly, we compute full-phase light curves from our suite of GCM experiments, finding that the reduced day-to-night temperature contrast in ultra-hot Jupiter atmospheres causes a smaller phase curve amplitude. The reduction in phase curve amplitude due to hydrogen dissociation and recombination could explain the relatively small phase curve amplitudes of observed ultra-hot Jupiters.

Oxygen isotopic ratios in Martian water vapour observed by ACS MIR on board the ExoMars Trace Gas Orbiter

Astronomy & Astrophysics EDP Sciences 630 (2019) A91-A91

J Alday, CF Wilson, PGJ Irwin, KS Olsen, L Baggio, F Montmessin, A Trokhimovskiy, O Korablev, AA Fedorova, DA Belyaev, A Grigoriev, A Patrakeev, A Shakun

<jats:p>Oxygen isotope ratios provide important constraints on the history of the Martian volatile system, revealing the impact of several processes that might fractionate them, such as atmospheric loss into space or interaction with the surface. We report infrared measurements of the Martian atmosphere obtained with the mid-infrared channel (MIR) of the Atmospheric Chemistry Suite (ACS), onboard the ExoMars Trace Gas Orbiter. Absorption lines of the three main oxygen isotopologues of water vapour (H <jats:sub>2</jats:sub><jats:sup>16</jats:sup> O, H <jats:sub>2</jats:sub><jats:sup>18</jats:sup> O, and H <jats:sub>2</jats:sub><jats:sup>17</jats:sup> O) observed in the transmission spectra allow, for the first time, the measurement of vertical profiles of the <jats:sup>18</jats:sup>O/<jats:sup>16</jats:sup>O and <jats:sup>17</jats:sup>O/<jats:sup>16</jats:sup>O ratios in atmospheric water vapour. The observed ratios are enriched with respect to Earth-like values (<jats:italic>δ</jats:italic><jats:sup>18</jats:sup>O = 200 ± 80‰ and <jats:italic>δ</jats:italic><jats:sup>17</jats:sup>O = 230 ± 110‰ corresponding to the Vienna Standard Mean Ocean Water). The vertical structure of these ratios does not appear to show significant evidence of altitudinal variations.</jats:p>

There is no Plan B for dealing with the climate crisis


R Pierrehumbert

Detection of Propadiene on Titan


NA Lombardo, CA Nixon, TK Greathouse, B Bezard, A Jolly, S Vinatier, NA Teanby, MJ Richter, PJG Irwm, A Coustenis, FM Flasar



EC Brown, KLD Hanna, NE Bowles, VE Hamilton, BE Clark, AD Rogers, DS Lauretta, O-R Team

Constraints on Uranus's haze structure, formation and transport

Icarus Elsevier BV 333 (2019) 1-11

D Toledo, PGJ Irwin, P Rannou, NA Teanby, AA Simon, MH Wong, GS Orton

Measurement of CH3D on Titan at Submillimeter Wavelengths


AE Thelen, CA Nixon, MA Cordiner, SB Charnley, PGJ Irwin, Z Kisiel

A brightening of Jupiter's auroral 7.8-mu m CH4 emission during a solar-wind compression

NATURE ASTRONOMY 3 (2019) 607-613

JA Sinclair, GS Orton, J Fernandes, Y Kasaba, TM Sato, T Fujiyoshi, C Tao, MF Vogt, D Grodent, B Bonfond, JI Moses, TK Greathouse, W Dunn, RS Giles, F Tabataba-Vakili, LN Fletcher, PGJ Irwin

Small bodies science with the Twinkle space telescope


B Edwards, S Lindsay, G Savini, G Tinetti, C Arena, N Bowles, M Tessenyi

Simulating Jupiter's weather layer. Part I: Jet spin-up in a dry atmosphere

ICARUS 326 (2019) 225-252

RMB Young, PL Read, Y Wang

Hazes and clouds in a singular triple vortex in Saturn's atmosphere from HST/WFC3 multispectral imaging

ICARUS 333 (2019) 22-36

JF Sanz-Requena, S Perez-Hoyos, A Sanchez-Lavega, T Del Rio-Gaztelurrutia, PGJ Irwin

Evidence for ultra-cold traps and surface water ice in the lunar south polar crater Amundsen

ICARUS 332 (2019) 1-13

E Sefton-Nash, J-P Williams, BT Greenhagen, TJ Warren, JL Bandfield, K-M Aye, F Leader, MA Siegler, PO Hayne, N Bowles, DA Paige

Simulating Jupiter's weather layer. Part II: Passive ammonia and water cycles

ICARUS 326 (2019) 253-268

RMB Young, PL Read, Y Wang

Linking mineralogy and spectroscopy of highly aqueously altered CM and CI carbonaceous chondrites in preparation for primitive asteroid sample return


HC Bates, AJ King, KL Donaldson Hanna, NE Bowles, SS Russell

The Yamato-type (CY) carbonaceous chondrite group: Analogues for the surface of asteroid Ryugu?

GEOCHEMISTRY 79 (2019) UNSP 125531

AJ King, HC Bates, D Krietsch, H Busemann, PL Clay, PF Schofield, SS Russell

Latitudinal variation in the abundance of methane (CH4) above the clouds in Neptune's atmosphere from VLT/MUSE Narrow Field Mode Observations

ICARUS 331 (2019) 69-82

PGJ Irwin, D Toledo, AS Braude, R Bacon, PM Weilbacher, NA Teanby, LN Fletcher, GS Orton

Investigating the semiannual oscillation on Mars using data assimilation

ICARUS 333 (2019) 404-414

T Ruan, NT Lewis, SR Lewis, L Montabone, PL Read

Towards the analysis of JWST exoplanet spectra: the effective temperature in the context of direct imaging

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) 490 (2019) 2086-2090

J-L Baudino, J Taylor, PGJ Irwin, R Garland

<jats:title>ABSTRACT</jats:title> <jats:p>The current sparse wavelength range coverage of exoplanet direct imaging observations, and the fact that models are defined using a finite wavelength range, lead both to uncertainties on effective temperature determination. We study these effects using blackbodies and atmospheric models and we detail how to infer this parameter. Through highlighting the key wavelength coverage that allows for a more accurate representation of the effective temperature, our analysis can be used to mitigate or manage extra uncertainties being added in the analysis from the models. We find that the wavelength range coverage will soon no longer be a problem. An effective temperature computed by integrating the spectroscopic observations of the James Webb Space Telescope will give uncertainties similar to, or better than, the current state–of–the–art, which is to fit models to data. Accurately calculating the effective temperature will help to improve current modelling approaches. Obtaining an independent and precise estimation of this crucial parameter will help the benchmarking process to identify the best practice to model exoplanet atmospheres.</jats:p>