Photochemical hazes have frequently been used to interpret exoplanet transmission spectra that show an upward slope toward shorter wavelengths and weak molecular features. While previous studies have ...only considered spherical haze particles, photochemical hazes composed of hydrocarbon aggregate particles are common throughout the solar system. We use an aerosol microphysics model to investigate the effect of aggregate photochemical haze particles on the transmission spectra of warm exoplanets. We find that the wavelength dependence of the optical depth of aggregate particle hazes is flatter than for spheres because aggregates grow to larger radii. Consequently, while spherical haze opacity displays a scattering slope toward shorter wavelengths, aggregate haze opacity can be gray in the optical and near-infrared, similar to those assumed for condensate cloud decks. We further find that haze opacity increases with increasing production rate, decreasing eddy diffusivity, and increasing monomer size, although the magnitude of the latter effect is dependent on production rate and the atmospheric pressure levels probed. We generate synthetic exoplanet transmission spectra to investigate the effect of these hazes on spectral features. For high haze opacity cases, aggregate hazes lead to flat, nearly featureless spectra, while spherical hazes produce sloped spectra with clear spectral features at long wavelengths. Finally, we generate synthetic transmission spectra of GJ 1214b for aggregate and spherical hazes and compare them to space-based observations. We find that aggregate hazes can reproduce the data significantly better than spherical hazes, assuming a production rate that is limited by delivery of methane to the upper atmosphere.
We present spatially resolved millimeter maps of Neptune between 95 and 242 GHz taken with the Atacama Large Millimeter/submillimeter Array (ALMA) in 2016-2017. The millimeter weighting functions ...peak between 1 and 10 bar on Neptune, lying in between the altitudes probed at visible/infrared and centimeter wavelengths. Thus, these observations provide important constraints on the atmospheric structure and dynamics of Neptune. We identify seven well-resolved latitudinal bands of discrete brightness temperature variations, on the order of 0.5-3 K in all three observed ALMA spectral bands. We model Neptune's brightness temperature using the radiative-transfer code Radio-BEAR and compare how various H2S, CH4, and ortho-/para-H2 abundance profiles can fit the observed temperature variations across the disk. We find that observed variations in brightness temperature with latitude can be explained by variations in the H2S profile that range from sub- to supersaturations at altitudes above the 10 bar pressure level, while variations in CH4 improve the quality of fit near the equator. At the south polar cap, our best-fit model has a depleted deep atmospheric abundance of H2S from 30 to only 1.5 times the protosolar value, while simultaneously depleting the CH4 abundance. This pattern of enhancement and depletion of condensible species is consistent with a global circulation structure where enriched air rises at the midlatitudes (32°-12°S) and north of the equator (2°-20°N), and dry air descends at the poles (90°-66°S) and just south of the equator (12°S-2°N). Our analysis finds more complex structure near the equator than accounted for in previous circulation models.
•Downloadable text tables give 5 zonal wind profiles from HST/WFC3 images.•Epochs of data are: 2016.95 (Juno PJ3), 2016.11, 2015.05, 2012.72, and 2009.72.•Average global uncertainties are 5–6 m/s, ...smaller than in past similar studies.•The 24°N jet peak speed decreases following plume outbreaks in 2012 and 2016.•The brightness of the NTB also decreases following these plume outbreaks.
We present five epochs of WFC3 HST Jupiter observations taken between 2009–2016 and extract global zonal wind profiles for each epoch. Jupiter’s zonal wind field is globally stable throughout these years, but significant variations in certain latitude regions persist. We find that the largest uncertainties in the wind field are due to vortices or hot-spots, and show residual maps which identify the strongest vortex flows. The strongest year-to-year variation in the zonal wind profiles is the 24°N jet peak. Numerous plume outbreaks have been observed in the Northern Temperate Belt and are associated with decreases in the zonal velocity and brightness. We show that the 24°N jet peak velocity and brightness decreased in 2012 and again in late 2016, following outbreaks during these years. Our February 2016 zonal wind profile was the last highly spatially resolved measurement prior to Juno's first science observations. The final 2016 data were taken in conjunction with Juno’s perijove 3 pass on 11 December 2016, and show the zonal wind profile following the plume outbreak at 24°N in October 2016.
We measured the horizontal winds in Jupiter's Great Red Spot (GRS) using data from the WFC3/UVIS instrument on board the Hubble Space Telescope (HST). The data cover 11 epochs from 2009 to 2020. ...Long-term monotonic trends in size and shape previously noted from the visible cloud appearance are paralleled by changes in the high-speed ring around the vortex. The circularization of the GRS cannot be explained by changes in the horizontal wind shear of the surrounding environment. The velocity fields suggest no long-term trend in the static stability inside or outside the vortex. Instead, the changes are accompanied by a 4%–8% increase in the mean wind speeds of the high-speed ring from 2009 to 2020. Changes in the wind field coincided with the South Equatorial Belt Outbreak storms of 2016–2017, but not with 2019 "flaking" events involving detachment of red material from the main oval.
•A thermal wind equation applicable at the equator is derived, named the: Equatorial Thermal Wind Equation (EQTWE)•We apply the EQTWE to Galileo Probe data on Jupiter.•We relate these results to CIRS ...temperature retrievals to determine the speed of an equatorial stratospheric jet.•We compare our findings to Juno MWR retrievals of an equatorial ammonia plume.
To relate the vertical wind shear to horizontal temperature gradients at and near the equator, we derive an “Equatorial Thermal Wind Equation” (EQTWE) using a minimum set of assumptions that are easily satisfied for the atmospheres of all the giant planets and Earth. Similar to the textbook Thermal Wind Equation (TWE), the EQTWE requires a small Rossby number, but the relevant Rossby number for the EQTWE depends on the velocity and length scales of the equatorial flows, and on the Coriolis parameter at the north pole (which is large), rather than the Coriolis parameter at the equator (which goes to zero). Unlike the TWE, the EQTWE is valid only for the east-west component of the wind. We apply the EQTWE to the Jovian wind measured by the Galileo probe Doppler wind experiment at jovicentric latitude 6.53°N (7.46°N jovigraphic), which is valid because the EQTWE is accurate at latitudes θ < 18°. Assuming that this wind profile holds at all longitudes, the EQTWE shows that near the equator at altitudes at 0.8 bar < P < 5 bar, the atmosphere is anomalously cool with respect to the surrounding flow, and at 5 bar ≤ P < 13 bar, it is warm. These anomalies imply adiabatic up-welling (down-welling) at 0.8 bar < P < 5 bar (at 5 bar ≤ P ≤ 13 bar), which suggests a Jovian global circulation model with two layers of Hadley cells, with an upper layer like the one on Earth, and the lower has cells with the opposite rotation. Applying the EQTWE to CIRS temperatures at altitudes above 330 mbar, shows that the large vertical wind shears measured by the Galileo probe extend to higher altitudes, and at 3 mbar create a stratospheric equatorial jet with a velocity of 205 m/s (almost 50% faster than the speed that had been obtained earlier with the TWE).
We present observations of Uranus in northern spring with the Very Large Array from 0.7 to 5 cm. These observations reveal details in thermal emission from Uranus' north pole at 10s of bars, ...including a dark collar near 80°N and a bright spot at the polar center. The bright central spot resembles observations of polar emission on Saturn and Neptune at shallower pressures. We constrain the variations in temperature and NH3/H2S abundances which could explain these features. We find that the brightness temperature of the polar spot can be recreated through 5 K temperature gradients and/or 10× depletion of NH3 or H2S vapor between 10 and 20 bars, both consistent with the presence of a cyclonic polar vortex. The contrast of the polar spot may have increased since 2015, which would suggest seasonal evolution of Uranus' polar circulation at depth.
Plain Language Summary
New radio telescope observations of Uranus several interesting features, including a compact feature at the center of the North Pole which appears warmer than its surroundings. This feature likely indicates the presence of a polar cyclone and shows similarities to polar features observed on other giant planets in the solar system.
Key Points
Very Large Array observations in 2021 and 2022 reveal a bright, compact spot centered at Uranus' pole at several wavelengths
Constraints on horizontal temperature and composition gradients necessary to explain the polar emission structure are derived
Inferred patterns in temperature, zonal wind speed and trace gas variations are consistent with the presence of a compact cyclonic vortex
We present a case study of plasma and magnetic field observations in the Martian magnetotail using data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission during an orbit when the ...spacecraft was in the optical shadow, past the dusk terminator and downstream of the strongest crustal magnetic fields. In this region, we observed multiple magnetic field rotations (a signature of currents) closely associated with energized (up to 100 eV) electron populations. Several transitions between closed and draped magnetic topologies also occur in this region, which are likely to be caused by magnetic reconnection between the interplanetary magnetic field (IMF) and crustal magnetic fields. We also observe two regions of energized, counter‐streaming electrons, which are rare in the magnetotail, but twice as likely to occur downstream of strong crustal magnetic fields when they are near the evening terminator. Together, the multiple magnetic field rotations, topological changes, and counter streaming electrons suggest the presence of an electric potential structure similar to those observed above the auroral arc regions at Earth.
Plain Language Summary
Mars does not have a global dipole magnetic field, but a fraction of the planet's crust carries strong remanent magnetization, which affects the plasma environment. When the magnetized solar wind encounters Mars, its magnetic field drapes around the conducting ionosphere and crustal magnetic fields and forms a magnetotail downstream of the planet. Reconnection between the solar wind magnetic field and crustal magnetic fields occurs often, resulting in a complex and changing configuration of open, closed, and draped magnetic field lines. Reconnection and reconfiguration of the magnetic field releases stored energy and drives currents, which energize the plasma. The energized plasma can precipitate onto the atmosphere and cause aurora. In this paper, we present a case study of plasma and magnetic field observations when the MAVEN spacecraft was downstream of strong crustal magnetic fields located near the evening terminator. Multiple magnetic field rotations, topological changes, and energized plasma indicate that reconnection gives rise to a complex magnetic environment downstream of the crustal fields. We found counter‐streaming electrons in the magnetotail, an observation which is thought to be a signature of the potential structure that energizes the auroral electrons, and are commonly observed at high altitudes above auroral regions at Earth. Therefore, we consider the observations in this study to be possible precursors to discrete auroral activity in Mars' crustal magnetic field regions.
Key Points
The Martian magnetotail exhibits complex morphology and topology downstream of strong crustal magnetic field sources
Multiple magnetic field rotations on spatial scales that are small compared to the magnetotail dimensions suggest multiple current regions
Rare counter‐streaming electrons, are twice as likely to occur downstream of crustal fields when they are located near the dusk terminator
We present photometric properties of six small (radii <100 km) satellites of Uranus based on 32 H-band (1.49-1.78 m) images taken on 2015 August 29 from the Keck II Telescope on Maunakea, Hawaii with ...the near-infrared camera NIRC2 coupled to the adaptive optics system. The sub-observer latitude of our observations was 32°, i.e., we view much of the satellites' north poles, in contrast to the 1986 Voyager measurements. We derive reflectivities based on mean-stacking measurements of these six minor moons of Uranus. We find that the small satellites are significantly brighter than in previous observations, which we attribute to albedo variations between hemispheres. We also search for Mab, a small satellite with an unknown surface composition, orbiting between Puck and Miranda. Despite the significantly improved signal-to-noise ratio we achieved, we could not detect Mab. We suggest that Mab is more similar to Miranda, an icy body, than to the inner rocky moons. Assuming Mab is spherical with a radius of 6 km, as derived from Hubble Space Telescope (HST) observations if its reflectivity is ∼0.46, we derive a 3 upper limit to its reflectivity I/F of 0.14 at 1.6 m.
Motivated by the editor's request to write a Radio Science paper as part of the AGU Grand Challenge series, we decided to use our work on radio observations of the 1994 impact of comet ...D/Shoemaker‐Levy 9 (SL9) on Jupiter. Our intention is to relate this work to potential consequences for Earth. The scenario being considered is an asteroid on a direct impact path to Earth, and the consequences of shattering it into pieces small enough to not hit Earth's surface. Such fragments would explode in the atmosphere, and because there are numerous such explosions, the ionosphere and trapped radiation (van Allen) belts might be significantly modified, as occurred with SL9.
Key Points
CA connection is made between radio observations of SL9 and Earth
A large number of impacts can trigger significant modifications of Earth's ionosphere and radiation belts
•Neptune’s zonal wind profile is derived from near-infrared Keck observations.•Evidence of vertical wind shear at Neptune’s equator is seen.•The observed vertical wind shear is modeled with a ...modified thermal wind equation.•This model may imply a stacked-celled circulation pattern in Neptune’s atmosphere.
We present observations of Neptune taken in H-(1.4–1.8 µm) and K’-(2.0–2.4 µm) bands on the nights of July 3, 2013 and August 20, 2014 from the 10-m W.M. Keck II Telescope using NIRC2 coupled to the Adaptive Optics (AO) system. We track the positions of ∼100 bright atmospheric features over a 4–5 h window on each night to derive zonal velocities and wind profiles.
Our results deviate from the smooth Voyager zonal wind profile from Sromovsky et al. (1993), often by 100–200 m/s, and often by 3–10 times their estimated uncertainties. Besides what appears to be a random dispersion, probably due to a mix of unaccounted for measurement errors, eddy motions, vertical wind shear, and wave-generated features that do not follow the mass flow, there is also a systematic deviation that is wavelength dependent. The H-band profile is best described with a 73–106 m/s shift towards the east for a retrograde flow (i.e., a lessening of the retrograding velocities) from the Voyager profile at the equator. The K’-band profile is consistent with Voyager on both nights.
Comparing H and K’ contribution functions and K’/H intensities suggests equatorial H-band features are, on average, deeper than K’-band features. The H-band equatorial features also have greater eastward (less negative) velocities than K’-band features. Differences in zonal wind speed with depth at constant latitude and time imply vertical wind shear. Assuming the average variations in the zonal wind profiles result from wind shear over 3–5 scale heights, we predict vertical wind shears between −1.0 and −2.2 m/(s km) at the equator (increasing with height).
The standard thermal wind equation and meridional thermal profile for Neptune given by Voyager/IRIS spectra predict wind shear of the wrong sign relative to the observations. We consider two effects that reconcile this inconsistency. First, we calculate the meridional temperature gradients at pressures outside the Voyager/IRIS narrow sensitivity window required to match our predicted wind shears. Second, we generalize to a thermal wind equation that considers global methane variations and re-derive the temperature structure needed to match the observed wind shear. If methane is uniformly distributed or weakly varying, the equator must be 2–15 K cooler than the mid latitudes below 1 bar. If methane is strongly varying, the equator can be 2–3 K warmer than the mid latitudes below 1 bar, qualitatively consistent with observed temperature contrasts. These findings may imply a stacked-celled circulation pattern in Neptune’s troposphere and lower stratosphere.
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