Heavy ion escape from Martian wake enhanced by magnetic reconnection

How ion escape from the near-Mars space is one of the biggest puzzles for understanding the atmospheric evolution of Mars. Ions in the plasma wake region continuously escape from the unmagnetized planet. Although the average ion escape rate in the wake region is relatively low, observations also have revealed the presence of events that contribute bursty and enhanced ion escape fluxes. Boundary instabilities and magnetic reconnection are suggested to be the candidate mechanisms. However, there is a lack of evaluation of ion escape caused by reconnection and comparison of the two mechanisms under a similar plasma environment. Here, we show an exciting reconnection event in the Martian wake. Two types of flux ropes are observed during the event. One was generated by reconnection, while others were produced by dayside boundary instability and convected to tail. The escape rate of oxygen ions in the reconnection region was estimated to be about 53–72% of the total tailward escape. Furthermore, the escape flux in the flux rope produced by reconnection was over twice that caused by dayside instabilities.

Atmospheres of lava exoplanets. Where and what to look for.

<p>Strongly irradiated rocky planets will have sufficiently high surface temperatures to sustain dayside magma lakes and oceans that will outgas low-pressure days-side silicate atmospheres. The surface magma will preferentially outgas its most volatile components and eventually equilibrate itself with the formed atmosphere. However, the volatile constituents may be depleted due to transport and condensation to the cooler night side or escape to space. If the atmospheric evolution is slower than the surface-interior exchange, the melt composition will remain coupled with the planet's interior. In such a case the atmospheric component will be continuously replenished. If atmospheric evolution is fast, the magma pool will lose its volatile component and the atmospheric composition will be irreversibly changed. We employ numerical models of outgassing, atmospheric chemistry, and radiative transfer to model the possible observable emission features of evolving magma atmospheres for all confirmed rocky lava worlds. . Our results highlight the best possible observable features in the atmospheres of hot rocky exoplanets that may give insight in their interior and atmospheric evolution. We also propose a set of ideal targets for JWST and ARIEL missions.<br><br><br></p>

Characterization of secondary organic aerosol from heated-cooking-oil emissions: evolution in composition and volatility

Cooking emissions account for a major fraction of urban organic aerosol. It is therefore important to understand the atmospheric evolution in the physical and chemical properties of organic compounds emitted from cooking activities. In this work, we investigate the formation of secondary organic aerosol (SOA) from oxidation of gas-phase organic compounds from heated cooking oil. The chemical composition of cooking SOA is analyzed using thermal desorption–gas chromatography–mass spectrometry (TD–GC–MS). While the particle-phase composition of SOA is a highly complex mixture, we adopt a new method to achieve molecular speciation of the SOA. All the GC-elutable material is classified by the constituent functional groups, allowing us to provide a molecular description of its chemical evolution upon oxidative aging. Our results demonstrate an increase in average oxidation state (from −0.6 to −0.24) and decrease in average carbon number (from 5.2 to 4.9) with increasing photochemical aging of cooking oil, suggesting that fragmentation reactions are key processes in the oxidative aging of cooking emissions within 2 d equivalent of ambient oxidant exposure. Moreover, we estimate that aldehyde precursors from cooking emissions account for a majority of the SOA formation and oxidation products. Overall, our results provide insights into the atmospheric evolution of cooking SOA, a majority of which is derived from gas-phase oxidation of aldehydes.

Atmospheric Evolution due to impacts during the final stage of planet formation

<p>We investigate how the bombardment of terrestrial planets by populations of planetesimals left over from the planet formation process, asteroids from the main belt and comets affects the evolution of their atmospheres, through both impact induced atmospheric mass loss and volatile delivery. This work builds on previous studies of this topic by combining prescriptions for the atmosphere loss and mass delivery derived from hydrodynamic simulations with results from dynamical modelling of a realistic population of impactors.</p><p> </p><p>The effect on the atmosphere predicted by the hydrodynamical simulations performed by Shuvalov (2009) as a function of the impactor and system properties are incorporated into a stochastic numerical model for the atmospheric evolution. The effects of rare but destructive giant impacts, that can cause non-local atmosphere loss, are also included using the prescription from Schlichting et al. (2015). The effects of aerial bursts and fragmentation of impactors in the atmosphere are included using a prescription based on the work of Shuvalov (2014). These effects are found to be relevant for hot and dense atmospheres analogous to the present day conditions on Venus.</p><p> </p><p>We compare the impact induced atmosphere evolution of Earth, Venus and Mars using impact velocities and probabilities inferred from the results of dynamical models of the population of left over planetesimals in the early solar system from Morbidelli et al. (2018), the population of asteroids from Nesvorny et al. (2017a) and comets from Nesvorny et al. (2017b). We use realistic size distributions for these populations based on the main belt asteroids and trans-Neptunian objects. The effect of the variation in the distribution of the impactor material through their bulk density and volatile fraction is investigated, as is the effect of varying the initial conditions assumed for the atmospheres of Earth, Venus and Mars.</p><p> </p><p>Our results for the Earth are discussed in light of observational constraints regarding the composition of the material delivered as the late veneer. The results for Venus and Mars are compared to those for the Earth and considered in comparison to observational evidence regarding the past climate of these worlds. A holistic view of the results for all three planets allows constraints on the past atmospheres to be inferred, in the absence of other atmospheric effects.</p>

Interpreting exoplanet biosignatures with a coupled atmosphere-interior-geochemical evolution model

<p>The atmospheric evolution of rocky planets is shaped by a range of astrophysical, geophysical, and geochemical processes. Interpreting observations of potentially habitable exoplanets will require an improved understanding of how these competing influences interact on long timescales. In particular, the interpretation of biosignature gases, such as oxygen, is contingent upon understanding the probable redox evolution of lifeless worlds. Here, we develop a generalized model of terrestrial planet atmospheric evolution to anticipate and interpret future observations of habitable worlds. The model connects early magma ocean evolution to subsequent, temperate geochemical cycling. The thermal evolution of the interior, cycling of carbon-hydrogen-oxygen bearing volatiles, surface climate, crustal production, and atmospheric escape are explicitly coupled throughout this evolution. The redox evolution of the atmosphere is controlled by net planetary oxidation via the escape of hydrogen to space, the loss of atmospheric oxygen to the magma ocean, and oxygen consumption via crustal sinks such as outgassing of reduced species, serpentinization reactions, and direct “dry” oxidation of fresh crust.</p><p>The model can successfully reproduce the atmospheric evolution of a lifeless Earth: it consistently predicts an anoxic atmosphere and temperate surface conditions after 4.5 Gyrs of evolution. This result is insensitive to model uncertainties such as the details of atmospheric escape, mantle convection parameterizations, initial radiogenic inventories, mantle redox, the efficiency of crustal oxygen sinks, and unknown carbon cycle and deep-water cycle parameters. This suggests abundant oxygen is a reliable biosignature for literal Earth twins, defined as Earth-sized planets at 1 AU around sunlike stars with 1-10 Earth oceans and less initial carbon dioxide than water.</p><p>However, if initial volatile inventories are permitted to vary outside these “Earth-like” ranges, then dramatically different redox evolution trajectories are permitted. We identify three scenarios whereby Earth-sized planets in the habitable zones of sunlike stars could accumulate oxygen rich atmospheres (0.01 - 10 bar) in the absence of life. Specifically, (i) high initial CO<sub>2</sub>:H<sub>2</sub>O endowments, (ii), >50 Earth ocean water inventories, or (iii) extremely volatile poor initial inventories, could all result in oxygen-rich atmospheres after 4.5 Gyrs of evolution. These false positives arise despite the assumption that there is always sufficient non-condensible atmospheric gases, N<sub>2</sub>, to maintain an effective cold trap. Fortunately, all three oxygen false positive scenarios could potentially be identified by thorough characterization of the planetary context, such as from using time resolved photometry to constrain surface water inventories.</p><p>The model also sheds light on the atmospheric evolution of Venus and Venus-like exoplanets. We can successfully recover the modern state of Venus’ atmosphere, including a dense CO<sub>2</sub>-dominated atmosphere with negligible water vapor and molecular oxygen. Moreover, there is a clear dichotomy in the evolutionary scenarios that recover modern Venus conditions, one in which Venus was never habitable and perpetually in runaway greenhouse since formation, and another whereby Venus experienced ~1-2 Gyr of surface habitability with a ~100 m deep ocean. We explore the likelihood of each scenario and suggest future in situ observations that could help discriminate between these two alternative histories.</p>