Interaction of Natural Organic Matter with Layered Minerals: Recent Developments in Computational Methods at the Nanoscale

The role of mineral surfaces in the adsorption, transport, formation, and degradation of natural organic matter (NOM) in the biosphere remains an active research area owing to the difficulties in identifying proper working models of both NOM and mineral phases present in the environment. The variety of aqueous chemistries encountered in the subsurface (e.g., oxic vs. anoxic, variable pH) further complicate this field of study. Recently, the advent of nanoscale probes such as X-ray adsorption spectroscopy and surface vibrational spectroscopy applied to study such complicated interfacial systems have enabled new insight into NOM-mineral interfaces. Additionally, due to increasing capabilities in computational chemistry, it is now possible to simulate molecular processes of NOM at multiple scales, from quantum methods for electron transfer to classical methods for folding and adsorption of macroparticles. In this review, we present recent developments in interfacial properties of NOM adsorbed on mineral surfaces from a computational point of view that is informed by recent experiments

Saturn Atmospheric Structure and Dynamics

2 Saturn inhabits a dynamical regime of rapidly rotating, internally heated atmospheres similar to Jupiter. Zonal winds have remained fairly steady since the time of Voyager except in the equatorial zone and slightly stronger winds occur at deeper levels. Eddies supply energy to the jets at a rate somewhat less than on Jupiter and mix potential vorticity near westward jets. Convective clouds exist preferentially in cyclonic shear regions as on Jupiter but also near jets, including major outbreaks near 35°S associated with Saturn electrostatic discharges, and in sporadic giant equatorial storms perhaps generated from frequent events at depth. The implied meridional circulation at and below the visible cloud tops consists of upwelling (downwelling) at cyclonic (anti-cyclonic) shear latitudes. Thermal winds decay upward above the clouds, implying a reversal of the circulation there. Warm-core vortices with associated cyclonic circulations exist at both poles, including surrounding thick high clouds at the south pole. Disequilibrium gas concentrations in the tropical upper troposphere imply rising motion there. The radiative-convective boundary and tropopause occur at higher pressure in the southern (summer) hemisphere due to greater penetration of solar heating there. A temperature “knee ” of warm air below the tropopause, perhaps due to haze heating, is stronger in the summer hemisphere as well. Saturn’s south polar stratosphere is warmer than predicted by radiative models and enhanced in ethane, suggesting subsidence-driven adiabatic warming there. Recent modeling advances suggest that shallow weather laye

Sensitivity of single membrane-spanning alpha-helical peptides to hydrophobic mismatch with a lipid bilayer: Effects on backbone structure, orientation, and extent of membrane incorporation

The extent of matching of membrane hydrophobic thickness with the hydrophobic length of transmembrane protein segments potentially constitutes a major director of membrane organization. Therefore, the extent of mismatch that can be compensated, and the types of membrane rearrangements that result, can provide valuable insight into membrane functionality. In the present study, a large family of synthetic peptides and lipids is used to investigate a range of mismatch situations. Peptide conformation, orientation, and extent of incorporation are assessed by infrared spectroscopy, tryptophan fluorescence, circular dichroism, and sucrose gradient centrifugation. It is shown that peptide backbone structure is not significantly affected by mismatch, even when the extent of mismatch is large. Instead, this study demonstrates that for tryptophan-flanked peptides the dominant response of a membrane to large mismatch is that the extent of incorporation is reduced, when the peptide is both too short and too long. With increasing mismatch, a smaller fraction of peptide is incorporated into the lipid bilayer, and a larger fraction is present in extramembranous aggregates. Relatively long peptides that remain incorporated in the bilayer have a small tilt angle with respect to the membrane normal. The observed effects depend on the nature of the flanking residues: long tryptophan-flanked peptides do not associate well with thin bilayers, while equisized lysine-flanked peptides associate completely, thus supporting the notion that tryptophan and lysine interact differently with membrane-water interfaces. The different properties that aromatic and charged flanking residues impart on transmembrane protein segments are discussed in relation to protein incorporation in biological systems

From Voyager-IRIS to Cassini-CIRS: Interannual variability in Saturn’s stratosphere?

International audienceWe present an intercomparison of Saturn’s stratosphere between Voyager 1-IRIS observations in 1980 and Cassini-CIRS observations in 2009 and 2010. Over a saturnian year (∼29.5 years) has now passed since the Voyager flybys of Saturn in 1980/1981. Cassini observations in 2009/2010 capture Saturn in the same season as Voyager observations (just after the vernal equinox) but one year later. Any differences in Saturn’s atmospheric properties implied by a comparison of these two datasets could therefore reveal the extent of interannual variability. We retrieve temperature and stratospheric acetylene and ethane concentrations from Voyager 1-IRIS (View the MathML sourceΔν̃=4.3cm-1) observations in 1980 and Cassini-CIRS (View the MathML sourceΔν̃=15.5cm-1) ‘FIRMAP’ observations in 2009 and 2010. We observe a difference in temperature at the equator of 7.1 ± 1.2 K at the 2.1-mbar level that implies that the two datasets have captured Saturn’s semiannual oscillation (SSAO) in a slightly different phase suggesting that its period is more quasi-semiannual. Elevated concentrations of acetylene at 25°S in 1980 with respect to 2010 imply stronger downwelling at the former date which may also be explained by a difference in the phase of the SSAO and its dynamical forcing at low latitudes. At high-southern and high-northern latitudes, stratospheric temperatures and hydrocarbon concentrations appear elevated in 1980 with respect to 2009/2010. This could be an artefact of the low signal-to-noise ratio of the corresponding observations but might also be explained by increased auroral activity during solar maximum in 1980

Long-term variability of Jupiter's northern auroral 8-μm CH4 emissions

peer reviewedWe present a study of the long term variability of Jupiter's mid-infrared CH4 auroral emissions. 7.7–7.9 μm images of Jupiter recorded by NASA's Infrared Telescope Facility, Subaru and Gemini-South over the last three decades were collated in order to quantify the magnitude and timescales over which the northern auroral hotspot's CH4 emission varies. These emissions predominantly sound the 10- to 1-mbar pressure range and therefore highlight the temporal variability of lower-stratospheric auroral-related heating. We find that the ratio of the radiance of the poleward northern auroral emissions to a lower-latitude zonal-mean, henceforth ‘Relative Poleward Radiance’ or RPR, exhibits variability over a 37% range and over a range of apparent timescales. We searched for patterns of variability in order to test whether seasonally varying solar insolation, the 11-year solar cycle, or short-term solar wind variability at Jupiter's magnetopause could explain the observed evolution. The variability of the RPR exhibits a weak (r < 0.2) correlation with both the instantaneous and phase-lagged solar insolation received at Jupiter's high-northern latitudes. This rules out the hypothesis suggested in previous work (e.g. Sinclair et al. 2017a, 2018) that shortwave solar heating of aurorally produced haze particles is the dominant auroral-related heating mechanism in the lower stratosphere. We also find the variability exhibits negligible (r < 0.18) correlation with both the instantaneous and phase-lagged monthly-mean sunspot number, which therefore rules out a long-term variability associated with the solar cycle. On shorter timescales, we find moderate correlations of the RPR with solar wind conditions at Jupiter in the preceding days before images were recorded. For example, we find correlations of r = 0.45 and r = 0.51 of the RPR with the mean and standard deviation solar wind dynamical pressure in the preceding 7 days. The moderate correlation suggests that either: (1) only a subset of solar wind compressions lead to brighter, poleward CH4 emissions and/or (2) a subset of CH4 emission brightening events are driven by internal magnetospheric processes (e.g. Io activity) and independent of solar wind enhancements

Spatial Distribution of the Pedersen Conductance in the Jovian Aurora From Juno‐UVS Spectral Images

Ionospheric conductivity perpendicular to the magnetic field plays a crucial role in the electrical coupling between planetary magnetospheres and ionospheres. At Jupiter, it controls the flow of ionospheric current from above and the closure of the magnetosphere‐ionosphere circuit in the ionosphere. We use multispectral images collected with the Ultraviolet Spectral (UVS) imager on board Juno to estimate the two‐dimensional distribution of the electron energy flux and characteristic energy. These values are fed to an ionospheric model describing the generation and loss of different ion species, to calculate the auroral Pedersen conductivity. The vertical distributions of H3+, hydrocarbon ions, and electrons are calculated at steady state for each UVS pixel to characterize the spatial distribution of electrical conductance in the auroral region. We find that the main contribution to the Pedersen conductance stems from collisions of H3+and heavier ions with H2. However, hydrocarbon ions contribute as much as 50% to Σp when the auroral electrons penetrate below the homopause. The largest values are usually associated with the bright main emission, the Io auroral footprint and occasional bright emissions at high latitude. We present examples of maps for both hemispheres based on Juno‐UVS images, with Pedersen conductance ranging from less than 0.1 to a few mhos

Observations of Jupiter’s Hydrogen Airglow by Juno-UVS

While the primary function of the Juno spacecraft’s Ultraviolet Spectrograph (UVS) during perijoves is to observe Jupiter’s auroral features, it is also capable of detecting and measuring Jupiter’s airglow. Jupiter’s airglow is caused, in part, by hydrogen emission signatures. The UV photons are emitted after photoionized gas in Jupiter’s upper atmosphere returns to its ground state. Juno’s low altitude perijove allows for UVS to detect Hydrogen Lyman-alpha emissions as a function of zenith angle. We search for variation in this emission, based on a variety of criteria, including spacecraft latitude, longitude, local time information, solar zenith angle, and location with respect to certain features (e.g., the "Great Blue Spot" magnetic anomaly, the magnetic equator). We will describe attempts to detect and characterize spectrally these emissions with Juno-UVS

Observations of Jupiter’s Hydrogen Airglow by Juno’s UVS

While the primary function of the Juno spacecraft’s Ultraviolet Spectrograph (UVS) during perijove is to observe Jupiter’s auroral features, it is also capable of detecting and measuring Jupiter’s airglow. The perijove location of Juno in Jupiter’s upper atmosphere allows for the UVS to detect Hydrogen Lyman-alpha and H2 emissions as a function of zenith angle. We look at the features of Jupiter’s airglow beginning early in the mission to attempt to determine trends based on a variety of criteria, including spacecraft latitude and local time information, solar zenith angle, and the location of the emissions themselves. Juno-UVS is also well suited to search for “shuttle glow” as the spacecraft moves through Jupiter’s atmosphere. “Shuttle glow” has been observed at Earth as a result of a spacecraft re-entering or orbiting at low altitude within an atmosphere. We will describe attempts to detect and characterize these photon emissions with Juno, which moves through Jupiter’s upper atmosphere at ~60 km/s