Localized Ionization Hypothesis for Transient Ionospheric Layers

The persistent two‐peaked vertical structure of the Martian ionosphere is created by extreme and far ultraviolet radiation whose energies respectively determine their ionization altitude. A third low‐altitude transient layer (previously referred to as M3 or Mm) has been observed by radio occultation techniques and attributed to meteor ablation. However, recent remote sensing and in‐situ observations disfavor a meteoric origin. Here we propose an alternative hypothesis for these apparent layers associated with impact ionization from penetrating solar wind ions, previously observed as proton aurora. Localized ionization, occurring non‐globally at a given altitude range, breaks the symmetry assumed by the radio occultation technique, and creates electron layers apparently lower in the ionosphere than their true altitude. This may occur when the upstream bowshock is altered by a radial interplanetary magnetic field configuration, which allows the solar wind to penetrate directly into the thermosphere. This localized ionization hypothesis provides an explanation for apparent layers’ wide variation in heights and their transient behavior. Moreover, this hypothesis is testable with new observations by the Mars Atmospheric and Volatile EvolutioN (MAVEN) Radio Occultation Science Experiment (ROSE) in future Mars years. This hypothesis has implications for the ionospheres of Venus and Titan, where similar transient layers have been observed

A Modeling Study of the Seasonal, Latitudinal, and Temporal Distribution of the Meteoroid Mass Input at Mars: Constraining the Deposition of Meteoric Ablated Metals in the Upper Atmosphere

This study provides a comprehensive description of the deposition of meteor-ablated metals in the upper atmosphere of Mars, accounting for the temporal, vertical, latitudinal, and seasonal distribution. For this purpose, the Leeds Chemical Ablation Model is combined with a meteoroid input function to characterize the size and velocity distributions of three distinctive meteoroid populations around Mars—the Jupiter-family comets (JFCs), main-belt asteroids, and Halley-type comets (HTCs). These modeling results show a significant midnight-to-noon enhancement of the total mass influx because of the orbital dynamics of Mars, with meteoroid impacts preferentially distributed around the equator for particles with diameters below 2000 μm. The maximum total mass input occurs between the northern winter and the first crossing of the ecliptic plane with 2.30 tons sol−1, with the JFCs being the main contributor to the overall influx with up to 56% around Mars' equator. Similarly, total ablated atoms mainly arise from the HTCs with a maximum injection rate of 0.71 tons sol−1 spanning from perihelion to the northern winter. In contrast, the minimum mass and ablated inputs occur between the maximum vertical distance above the ecliptic plane and aphelion with 1.50 and 0.42 tons sol−1, respectively. Meteoric ablation occurs approximately in the range altitude between 100 and 60 km with a strong midnight-to-noon enhancement at equatorial latitudes. The eccentricity and the inclination of Mars' orbit produces a significant shift of the ablation peak altitude at high latitudes as Mars moves toward, or away, from the northern/southern solstices

Explanation for the increase in high altitude water on Mars observed by NOMAD during the 2018 global dust storm

The Nadir and Occultation for MArs Discovery (NOMAD) instrument on board ExoMars Trace Gas Orbiter (TGO) measured a large increase in water vapor at altitudes in the range of 40‐100 km during the 2018 global dust storm on Mars. Using a three‐dimensional general circulation model, we examine the mechanism responsible for the enhancement of water vapor in the upper atmosphere. Experiments with different prescribed vertical profiles of dust show that when more dust is present higher in the atmosphere the temperature increases and the amount of water ascending over the tropics is not limited by saturation until reaching heights of 70‐100 km. The warmer temperatures allow more water to ascend to the mesosphere. Photochemical simulations show a strong increase in high‐altitude atomic hydrogen following the high‐altitude water vapor increase by a few days

MAVEN IUVS observations of the aftermath of the Comet Siding Spring meteor shower on Mars

We report the detection of intense emission from magnesium and iron in Mars' atmosphere caused by a meteor shower following Comet Siding Spring's close encounter with Mars. The observations were made with the Imaging Ultraviolet Spectrograph, a remote sensing instrument on the Mars Atmosphere and Volatile EvolutioN spacecraft orbiting Mars. Ionized magnesium caused the brightest emission from the planet's atmosphere for many hours, resulting from resonant scattering of solar ultraviolet light. Modeling suggests a substantial fluence of low-density dust particles 1-100μm in size, with the large amount and small size contrary to predictions. The event created a temporary planet-wide ionospheric layer below Mars' main dayside ionosphere. The dramatic meteor shower response at Mars is starkly different from the case at Earth, where a steady state metal layer is always observable but perturbations caused by even the strongest meteor showers are challenging to detect

Global Variations in Water Vapor and Saturation State Throughout the Mars Year 34 Dusty Season

To understand the evolving martian water cycle, a global perspective of the combined vertical and horizontal distribution of water is needed in relation to supersaturation and water loss and how it varies spatially and temporally. The global vertical water vapor distribution is investigated through an analysis that unifies water, temperature and dust retrievals from several instruments on multiple spacecraft throughout Mars Year (MY) 34 with a global circulation model. During the dusty season of MY 34, northern polar latitudes are largely absent of water vapor below 20 km with variations above this altitude due to transport from mid‐latitudes during a global dust storm, the downwelling branch of circulation during perihelion season and the intense MY 34 southern summer regional dust storm. Evidence is found of supersaturated water vapor breaking into the northern winter polar vortex. Supersaturation above around 60 km is found for most of the time period, with lower altitudes showing more diurnal variation in the saturation state of the atmosphere. Discrete layers of supersaturated water are found across all latitudes. The global dust storm and southern summer regional dust storm forced water vapor at all latitudes in a supersaturated state to 60–90 km where it is more likely to escape from the atmosphere. The reanalysis data set provides a constrained global perspective of the water cycle in which to investigate the horizontal and vertical transport of water throughout the atmosphere, of critical importance to understand how water is exchanged between different reservoirs and escapes the atmosphere

A highly mutagenised barley (cv. Golden Promise) TILLING population coupled with strategies for screening-by-sequencing

Background:We developed and characterised a highly mutagenised TILLING population of the barley (Hordeum vulgare) cultivar Golden Promise. Golden Promise is the 'reference' genotype for barley transformation and a primary objective of using this cultivar was to be able to genetically complement observed mutations directly in order to prove gene function. Importantly, a reference genome assembly of Golden Promise has also recently been developed. As our primary interest was to identify mutations in genes involved in meiosis and recombination, to characterise the population we focused on a set of 46 genes from the literature that are possible meiosis gene candidates. Results:Sequencing 20 plants from the population using whole exome capture revealed that the mutation density in this population is high (one mutation every 154 kb), and consequently even in this small number of plants we identified several interesting mutations. We also recorded some issues with seed availability and germination. We subsequently designed and applied a simple two-dimensional pooling strategy to identify mutations in varying numbers of specific target genes by Illumina short read pooled-amplicon sequencing and subsequent deconvolution. In parallel we assembled a collection of semi-sterile mutants from the population and used a custom exome capture array targeting the 46 candidate meiotic genes to identify potentially causal mutations. Conclusions:We developed a highly mutagenised barley TILLING population in the transformation competent cultivar Golden Promise. We used novel and cost-efficient screening approaches to successfully identify a broad range of potentially deleterious variants that were subsequently validated by Sanger sequencing. These resources combined with a high-quality genome reference sequence opens new possibilities for efficient functional gene validation.Miriam Schreiber, Abdellah Barakate, Nicola Uzrek, Malcolm Macaulay, Adeline Sourdille, Jenny Morris, Pete E. Hedley, Luke Ramsay and Robbie Waug