Astrobiological Potential of Venus Atmosphere Chemical Anomalies and Other Unexplained Cloud Properties

Long-standing unexplained Venus atmosphere observations and chemical anomalies point to unknown chemistry but also leave room for the possibility of life. The unexplained observations include several gases out of thermodynamic equilibrium (e.g., tens of ppm O2, the possible presence of PH3 and NH3, SO2 and H2O vertical abundance profiles), an unknown composition of large, lower cloud particles, and the "unknown absorber(s)." Here we first review relevant properties of the venusian atmosphere and then describe the atmospheric chemical anomalies and how they motivate future astrobiology missions to Venus.

Depleted carbon isotope compositions observed at Gale crater, Mars

Carbon isotopic analysis is among the most pervasive geochemical approaches because the fractionation of carbon isotopes produces a natural tracer of biological and chemical processes. Rover-based carbon isotopic analyses of sedimentary rocks on Mars have the potential to reveal modes of Martian carbon cycling. We report carbon isotopic values of the methane released during pyrolysis of samples obtained at Gale crater. The values show remarkable variation indicating different origins for the carbon evolved from different samples. Samples from multiple locations within Gale crater evolved methane with highly fractionated carbon isotopes. We suggest three routes by which highly fractionated carbon could be deposited on Mars, with each suggesting that Martian carbon cycling is quite distinct from that of the present Earth.

Obtaining carbon isotopic information for organic carbon from Martian sediments has long been a goal of planetary science, as it has the potential to elucidate the origin of such carbon and aspects of Martian carbon cycling. Carbon isotopic values (δ¹³C_VPDB) of the methane released during pyrolysis of 24 powder samples at Gale crater, Mars, show a high degree of variation (−137 ± 8‰ to +22 ± 10‰) when measured by the tunable laser spectrometer portion of the Sample Analysis at Mars instrument suite during evolved gas analysis. Included in these data are 10 measured δ¹³C values less than −70‰ found for six different sampling locations, all potentially associated with a possible paleosurface. There are multiple plausible explanations for the anomalously depleted ¹³C observed in evolved methane, but no single explanation can be accepted without further research. Three possible explanations are the photolysis of biological methane released from the subsurface, photoreduction of atmospheric CO₂, and deposition of cosmic dust during passage through a galactic molecular cloud. All three of these scenarios are unconventional, unlike processes common on Earth.

Spectroscopic Characterization of HSO2• and HOSO• Intermediates Involved in SO2 Geoengineering

Sulfur-containing radicals HSO₂^• and HOSO^• are key intermediates involved in stratospheric sulfur geoengineering by SO₂ injection. The spectroscopic characterization and photochemistry of both radicals are crucial to understanding the chemical impact of SO₂ chemistry in the stratosphere. On the basis of the efficient generation of HOSO^• by flash pyrolysis of gaseous sulfinic acid, CHF₂S(O)OH, a strong absorption is observed at 270 nm along with a shoulder up to 350 nm for HOSO^• isolated in low-temperature noble gas matrixes (Ar and Ne). These mainly arise from the excitations from the ground state (X²A) to the C²A/D²A and A²A/B²A states, respectively. Upon a 266 nm laser irradiation, the broad absorption band in the range 320-500 nm for HSO₂^• appears, and it corresponds to the combination of three excitations from the X²A state to the first (A²A), second (B²A), and third (C²A) excited states. Assignment of the UV-vis spectra is consistent with the photochemistry of HOSO^• and HSO₂^• as observed by matrix-isolation IR spectroscopy and also by the agreement with high-level ab initio calculations.

Spectroscopic Characterization of the First and Second Excited States of the HOSO Radical

The spectroscopic properties of the ground and first two excited states of the HOSO radical are investigated using the internally contracted multireference configuration interaction method, including the Davidson correction (MRCI+Q) and explicit treatment of the electron correlation (MRCI-F12). The vertical and adiabatic excitation energies are also determined. The results reveal that both the 1 ²A and 2 ²A electronic states contain minima in their potential energy surfaces. The first excited state 1 ²A possesses a nonplanar structure and has an adiabatic excitation energy of 1.45 eV (855 nm), lying in the near-infrared region. The second excited state 2 ²A has a planar geometry and an adiabatic excitation energy of 2.91 eV (426 nm) existing in the visible region. The calculated oscillator strengths for the vertical electronic excitations to the 1 ²A (327 nm) and 2 ²A (270 nm) states are 0.003 and 0.022, respectively, indicating experimental intensity should be observed. The small but non-negligible Franck-Condon factors for excitations ∼300 nm, and the broad and intense absorption feature in the 225-275 nm region suggest that detection of the HOSO radical with electronic spectroscopy may be feasible.

Valence Bond Alternative Yielding Compact and Accurate Wave Functions for Challenging Excited States. Application to Ozone and Sulfur Dioxide

A novel state-averaged version of ab initio nonorthogonal valence bond method is described, for the sake of accurate theoretical studies of excited states in the valence bond framework. With respect to standard calculations in the molecular orbital framework, the state-averaged breathing-orbital valence bond (BOVB) method has the advantage to be free from the penalizing constraint for the ground and excited state(s) to share the same unique set of orbitals. The ability of the BOVB method to faithfully describe excited states and to compute accurate transition energies from the ground state is tested on the five lowest-lying singlet electronic states of ozone and sulfur dioxide, among which 1¹B₂ and 2¹A₁ are the challenging ones. As the 1¹A₂, 1¹B₁, and 1¹B₂ states are of different symmetries than the ground state, they can be calculated at the state-specific BOVB level. On the other hand, the 2¹A₁ states and the 1¹A₁ ground states, which are of like symmetry, are calculated with the state-averaged BOVB technique. In all cases, the calculated vertical energies are close to the experimental values when available, and at par with the most sophisticated calculations in the molecular framework, despite the extreme compactness of the BOVB wave functions, made of no more than 5-9 valence bond structures in all cases. The features that allow the combination of compactness and accuracy in challenging cases are analyzed. For the "ionic" 1¹B₂ states, which are the site of important charge fluctuations, it is because of the built-in dynamic correlation inherent to the BOVB method. For the 2¹A₁ ones, this is the fact that these states have the degree of freedom of having different orbitals than the ground states, even though they are of like symmetry and calculated simultaneously using the newly implemented state-average BOVB algorithm. Finally, the description of the excited states in terms of Lewis structures is insightful, rationalizing the fast ring closure for the 2¹A₁ state of ozone and predicting some diradical character in the so-called "ionic" 1¹B₂ states.