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Transmission spectroscopy is one of the most successful methods of learning about exoplanet atmospheres. The process of retrievals using transmission spectroscopy consists of creating numerous forward models and comparing them to observations to solve the inverse problem of constraining the atmospheric properties of exoplanets. We explore the impact of one simplifying assumption commonly employed by forward models of transiting exoplanets: namely that the planet can be treated as an isolated, non-rotating spherical body. The centrifugal acceleration due to a planet's rotation opposes the gravitational pull on a planet's atmosphere and increases its scale height. Conventional forward models used for retrievals generally do not include this effect. We find that atmospheric retrievals produce significantly different results for close-in planets with low gravity when this assumption is removed, e.g. differences between true and retrieved values of gas abundances greater than 1σ for a simulated planet analogous to WASP-19b. We recommend that the correction to the atmospheric scale height due to this effect be taken into account for the analysis of high-precision transmission spectra of exoplanets in the future, most immediately JWST Cycle 1 targets WASP-19b and WASP-121b. [ABSTRACT FROM AUTHOR]

The vertical opacity structure of the martian atmosphere is important for understanding the distribution of ice (water and carbon dioxide) and dust. We present a new data set of extinction opacity profiles from the NOMAD/UVIS spectrometer aboard the ExoMars Trace Gas Orbiter, covering one and a half Mars Years (MY) including the MY 34 Global Dust Storm and several regional dust storms. We discuss specific mesospheric cloud features and compare with existing literature and a Mars Global Climate Model (MGCM) run with data assimilation. Mesospheric opacity features, interpreted to be water ice, were present during the global and regional dust events and correlate with an elevated hygropause in the MGCM, providing evidence that regional dust storms can boost transport of vapor to mesospheric altitudes (with potential implications for atmospheric escape). The season of the dust storms also had an apparent impact on the resulting lifetime of the cloud features, with events earlier in the dusty season correlating with longer‐lasting mesospheric cloud layers. Mesospheric opacity features were also present during the dusty season even in the absence of regional dust storms, and interpreted to be water ice based on previous literature. The assimilated MGCM temperature structure agreed well with the UVIS opacities, but the MGCM opacity field struggled to reproduce mesospheric ice features, suggesting a need for further development of water ice parameterizations. The UVIS opacity data set offers opportunities for further research into the vertical aerosol structure of the martian atmosphere, and for validation of how this is represented in numerical models. Plain Language Summary: The martian atmosphere contains several kinds of aerosol, including dust, water ice, and carbon dioxide ice. The NOMAD/UVIS spectrometer aboard the ExoMars Trace Gas orbiter is able to measure the opacity of the atmosphere at different heights by looking through the atmosphere at the Sun, and these opacities provide information on the vertical aerosol distribution. We present a new data set of these opacities for one and a half Mars Years, a period including one Global Dust Storm and several smaller regional‐scale dust storms. By comparing with a martian climate model and previous work we discuss some noteable features in the UVIS data. We see the presence of high‐altitude water ice clouds associated with the Global Dust Storm and several regional dust storms, and find that the lifetime of these cloud layers seems to depend on the seasonal timing of the relevant dust event. These layers also correlate with an elevated higher‐altitude water vapor presence, showing that large dust storms boost the upwards transport of water vapor; this could enhance atmospheric escape of hydrogen. The model temperature structure is consistent with the UVIS results, but the model aerosol field shows a need for improvement in its representation of high‐altitude water ice. Key Points: Mesospheric water ice clouds were present in ExoMars/UVIS opacity profiles during a global and three regional dust stormsMesospheric water ice correlated with elevated hygropause, showing storms boost vertical water transport with hydrogen escape implicationsPerihelion season mesospheric water ice was also present even in the absence of global/regional dust storm activity [ABSTRACT FROM AUTHOR]

We present observations of terminator CO2 ice clouds events in three groups: Equatorial dawn, Equatorial dusk (both between 20°S and 20°N) and Southern midlatitudes at dawn (45°S and 55°S east of Hellas Basin) with ESA ExoMars Trace Gas Orbiter's Nadir and Occultation for MArs Discovery instrument. CO2 ice abundance is retrieved simultaneously with water ice, dust, and particle sizes, and rotational temperature and CO2 column profiles in 16 of 26 cases. Small particles (<0.5 μm) prevail at dusk, while water ice likely provides most source nuclei at dawn. Clouds east of Hellas are found to be dominantly nucleated on surface‐lifted dust. CO2 ice is sometimes detected in unsaturated air together with dust nuclei at dawn, suggesting ongoing sublimation. Depending on latitude and local time, the interplay between particle precipitation and the lifetime of temperature minima (i.e., cold pockets) determines CO2 ice properties. Plain Language Summary: The upper atmosphere of Mars is characterized by the seasonal presence of CO2 ice clouds. Their properties have been long studied, as well as their formation mechanisms in relation to the thermal structure of the atmosphere and its variability. In this study, we present the first observations of these clouds at the terminator (dawn and dusk) by the NOMAD spectrometer onboard the Exomars Trace Gas Orbiter. CO2 ice is detected simultaneously with dust, water ice and the temperature profile. Our results agree with previous findings in terms of how clouds are spatially distributed and their temporal occurrence. However, we also explore the sources of condensation nuclei for CO2 ice particles, showing that water ice is a possible source at dawn near the Equator. We also identify surface‐lifted dust below the CO2 ice clouds observed east of Hellas Basin, suggesting that, at that location, dust could provide nuclei for CO2 ice. CO2 ice is also sometimes detected at temperatures higher than the CO2 freezing point, suggesting ongoing sublimation. In this work we explore for the first time the composition of CO2 ice clouds, which is critical to advance our understanding of how CO2 ice clouds form in the mesosphere at Mars. Key Points: Twenty Six mesospheric CO2 ice clouds were detected with NOMAD SO in Mars Year 35, simultaneously with water ice, dust, and CO2 saturation ratiosEquatorial CO2 clouds are observed at 50–80 km altitude at dusk, and 40–60 km at dawn, when water ice likely provides condensation nucleiSix CO2 ice clouds are found east of Hellas basin; their formation is likely sourced by surface‐lifted dust at 40–65 km [ABSTRACT FROM AUTHOR]

Super‐rotation affects—and is affected by—the distribution of dust in the martian atmosphere. We modeled this interaction during the 2018 global dust storm (GDS) of Mars Year 34 using data assimilation. Super‐rotation increased by a factor of two at the peak of the GDS, as compared to the same period in the previous year which did not feature a GDS. A strong westerly jet formed in the tropical lower atmosphere, with strong easterlies above 60 km, as a result of momentum transport by thermal tides. Enhanced super‐rotation is shown to have commenced 40 sols before the onset of the GDS, due to dust lifting in the southern mid‐latitudes and tropics. The uniform distribution of dust in the tropics resulted in a symmetric Hadley cell with a tropical upwelling branch that could efficiently transport dust vertically; this may have significantly contributed to the rapid expansion of the storm. Plain Language Summary: Dust plays a major role in driving the behavior of the atmosphere of Mars. During a global dust storm (GDS), winds lift, and transport dust throughout the atmosphere; in turn, dust affects wind direction and strength by heating and cooling the surrounding air. Using a technique that combines satellite observations with simulations of the martian atmosphere, we demonstrate that winds at tropical latitudes were greatly strengthened during the 2018 GDS as a result of heating by dust. We show that tropical winds were strengthened even before the onset of the storm, as a result of a dust‐driven modification to the tropical circulation pattern. This change in the tropical circulation may have played a role in the formation of the GDS. Key Points: The martian atmosphere was already in a state of enhanced super‐rotation prior to the onset of the Mars Year 34 global dust stormSuper‐rotation doubled during the peak of the storm and tropical easterlies were strongly enhanced above 60 kmDust lifting in the southern hemisphere led to enhanced tropical heating and increased vertical dust transport in the lead‐up to the storm [ABSTRACT FROM AUTHOR]

Mars possesses dynamical features called polar vortices: regions of cold, isolated air over the poles circumscribed by powerful westerly jets which can act as barriers to transport to dust, water, and chemical species. The 2018 Global Dust Storm was observed by multiple orbiters and offered a valuable opportunity to study the effects of such a storm on polar dynamics. To this end, we assimilate data from the Mars Climate Sounder and Atmospheric Chemistry Suite into a Mars Global Climate Model. We find that the storm had asymmetrical hemispherical impacts, with the northern vortex remaining relatively robust while the southern vortex was substantially diminished in its intensity. We propose that this asymmetry was due both to the storm's latitudinal extent, as it extended further south than north, and to its equinoctial timing, occurring as the southern vortex was already decaying. We show that both polar vortices, in particular the northern, were reduced in ellipticity by the storm. There was a well‐correlated reduction in stationary topographic wave activity at high latitudes in both hemispheres. We demonstrate that the characteristic elliptical Martian polar vortex shape is the pattern of the stationary waves, which was suppressed by the shifting of the polar jet away from regions of high mechanical forcing (north) or reduction of polar jet intensity by a reduced meridional temperature gradient (south). These asymmetric effects suggest increased transport into the southern (but not northern) polar region during global dust storms at northern autumn equinox, and more longitudinally symmetric transport around both poles. Plain Language Summary: Like Earth, Mars has regions of cold air around its winter poles. The temperature contrast creates powerful polar jets, polar vortices, which can block the transport of atmospheric aerosols and chemicals. Unlike Earth, Mars regularly experiences global dust storms which have enormous effects on atmospheric temperatures and winds. The most recent storm occurred at Mars' equinox and was observed by multiple spacecraft. We combined these observations with a numerical model. We find that the northern vortex remained relatively strong and coherent, while the southern vortex was greatly disrupted, showing atmospheric warming and a diminished polar jet. This was because of the greater southward extent of the storm and its seasonal timing. Both vortices normally show a distinct elliptical shape; the storm made both vortices more longitudinally symmetrical. We link this to a corresponding decrease in the amplitude of topographic planetary‐scale waves, showing that the elliptical shape is that of the planetary wave structure. These results suggest that equinoctial storms may enhance transport into the southern pole due to the diminished vortex, while the more robust northern vortex continues to act as an effective barrier. The reduced ellipticity of both vortices may also lead to more longitudinally symmetric transport into the polar regions. Key Points: The 2018 Global Dust Storm had significant and asymmetrical impacts on the morphology and intensity of Mars' two polar vorticesThe southern vortex was substantially diminished while the northern vortex remained robust, but both were reduced in ellipticityThese vortex changes enhance transport into Mars' southern (but not northern) polar region and modify longitudinal transport patterns [ABSTRACT FROM AUTHOR]

The impact of Mars's 2018 Global Dust Storm (GDS) on surface and near‐surface air temperatures was investigated using an assimilation of Mars Climate Sounder observations. Rather than simply resulting in cooling everywhere from solar absorption (average surface radiative flux fell 26 W/m2), the globally averaged result was a 0.9‐K surface warming. These diurnally averaged surface temperature changes had a novel, highly nonuniform spatial structure, with up to 16‐K cooling/19‐K warming. Net warming occurred in low thermal inertia regions, where rapid nighttime radiative cooling was compensated by increased longwave emission and scattering. This caused strong nightside warming, outweighing dayside cooling. The reduced surface‐air temperature gradient closely coupled surface and air temperatures, even causing local dayside air warming. Results show good agreement with Mars Climate Sounder surface temperature retrievals. Comparisons with the 2001 GDS and free‐running simulations show that GDS spatial structure is crucial in determining global surface temperature effects. Plain Language Summary: Martian Global Dust Storms (GDS) are planet‐encircling events which fill the atmosphere with a deep layer of mineral dust. During these events the dayside of the planet cools due to the blocking of sunlight, but the nightside warms from dust scattering back surface emissions in the manner of the greenhouse effect. We combined observations of the most recent (2018) GDS from an orbiting instrument, the Mars Climate Sounder, with a Mars climate model to study the storm's effects on surface and near‐surface temperatures. We found that the net effect was actually an increase in global average surface temperatures. The cause was the significant nighttime warming of regions with low thermal inertia, which normally cool rapidly at night but are provided an atmospheric dust "blanket" by the storm. The magnitude of warming was enough to compensate for the net cooling over areas with higher thermal inertia. Near‐surface air temperatures also rose, as the storm coupled these more closely to the surface. Further simulations showed that these results are valid over various possible storm intensities. The role of thermal inertia suggests that the geographical extent of a GDS, and which regions it covers, plays a significant role in its ultimate effects. Key Points: Mars's 2018 Global Dust Storm caused a 0.9‐K globally averaged surface warming but with local 16‐K cooling/19‐K warmingThe magnitude of dayside cooling was controlled by atmospheric dust, and nightside warming by surface thermal inertiaThe effects were strongly nonuniform, with high dust loading causing net warming (cooling) over low (high) thermal inertia continents [ABSTRACT FROM AUTHOR]