Earth Observation Data Group

Abstract:

Recent ground-based observations of Venus have detected a single spectral feature consistent with phosphine (PH₃) in the middle atmosphere, a gas which has been suggested as a biosignature on rocky planets. The presence of PH₃ in the oxidized atmosphere of Venus has not yet been explained by any abiotic process. However, state-of-the-art experimental and theoretical research published in previous works demonstrated a photochemical origin of another potential biosignature-the hydride methane-from carbon dioxide over acidic mineral surfaces on Mars. The production of methane includes formation of the HC · O radical. Our density functional theory (DFT) calculations predict an energetically plausible reaction network leading to PH₃, involving either HC · O or H· radicals. We suggest that, similarly to the photochemical formation of methane over acidic minerals already discussed for Mars, the origin of PH₃ in Venus' atmosphere could be explained by radical chemistry starting with the reaction of ·PO with HC·O, the latter being produced by reduction of CO₂ over acidic dust in upper atmospheric layers of Venus by ultraviolet radiation. HPO, H₂P·O, and H₃P·OH have been identified as key intermediate species in our model pathway for phosphine synthesis.

Abstract:

Abstract A hollow cathode discharge with a CuNi (Cu50Ni50) cathode was operated in an Ar/N$$_2$$ 2 /O$$_2$$ 2 gas mixture. The energy distribution of plasma ions is investigated with the help of energy-resolved mass spectrometry. Formation of singly ionised Ar$$^+$$ + and of Cu$$^+$$ + and Ni$$^+$$ + ions is observed in pure argon. With the addition of N$$_2$$ 2 or O$$_2$$ 2 gas the additional formation of molecular N$$_2^ +$$ 2 + or O$$_2^+$$ 2 + ions is observed. The intensity of these ions is reduced in the Ar+N$$_2$$ 2 +O$$_2$$ 2 gas mixture and molecular NO$$^+$$ + ions become the most abundant ionic species. The formation of neutral NO molecules is confirmed by optical emission spectroscopy. Gas samples collected at the exhaust of the vacuum chamber confirm the formation of NO and, additionally, of NO$$_2$$ 2 molecules. Graphic Abstract Sample taken from the exhaust of the plasma chamber with a liquid-nitrogen-cooled glass container showing solid nitrogen oxide (blue).

Abstract:

Volcanic ash clouds are carefully monitored as they present a significant hazard to humans and aircraft. The primary tool for forecasting the transport of ash from a volcano is dispersion modelling. These models make a number of assumptions about the size, sphericity and density of the ash particles. Few studies have measured the density of ash particles or explored the impact that the assumption of ash density might have on the settling dynamics of ash particles. In this paper, the raw apparent density of 23 samples taken from 15 volcanoes are measured with gas pycnometry, and a negative linear relationship is found between the density and the silica content. For the basaltic ash samples, densities were measured for different particle sizes, showing that the density is approximately constant for particles smaller than 100 µm, beyond which it decreases with size. While this supports the current dispersion model used by the London Volcanic Ash Advisory Centre (VAAC), where the density is held at a constant (2.3 g cm-3), inputting the measured densities into a numerical simulation of settling velocity reveals a primary effect from the silica content changing this constant. The VAAC density overestimates ash removal times by up to 18 %. These density variations, including those varying with size beyond 100 µm, also impact short-range particle-size distribution (PSD) measurements and satellite retrievals of ash.

Abstract: