arXiv subjects

Dimitra Atri

Publications and source records attributed to Dimitra Atri.

At least 19 recordsLinked to original sources

arXiv2026

Energy-Limited Radiolytic Habitability in the Shallow Martian Subsurface: Implications for ExoMars Rosalind Franklin and Tianwen-3

The surface of Mars is sterilized by ionizing radiation and pervasive oxidants; its shallow subsurface, shielded from ultraviolet light and the most reactive oxidation, may instead preserve habitable conditions. The radiolytic habitable zone (RHZ) hypothesis holds that galactic cosmic rays can drive water and oxychlorine radiolysis there, generating H2 and oxidants that support chemolithotrophy without sunlight or geothermal heat. We develop a quantitative framework coupling Monte Carlo GCR transport, phase-resolved radiolysis chemistry, water-activity and H2-retention treatments, and microbial maintenance-power constraints for Gale Crater, Oxia Planum, southern Utopia, and Arabia/Mawrth, expressing radiolytic chemical energy as a depth-resolved redox power comparable to the power requirements of life. H2 retention is the dominant control. Sorbed or mineral-associated retention yields 4-6 x 10^-13 W kg^-1 at protected depth (>=10 cm), whereas connected-pore free-gas escape lowers redox power by two to four orders of magnitude. Gale remains inactive under both retention assumptions, consistent with published SAM evolved-gas analyses. Even in retained-H2 active terrains, supported cell density reaches only 10^3-10^5 cells cm^-3 at subseafloor maintenance powers. The framework predicts a spatially restricted, low-density RHZ, testable by stepped H2 evolved-gas analysis of protected-depth samples from ExoMars Rosalind Franklin, Tianwen-3, or Mars Sample Return.

astro-ph.EP↗

arXiv2025

A systematic study of hot O production and escape from Martian atmosphere in response to enhanced EUV Irradiance from Solar Flares

The study of the evolution of Martian atmosphere and its response to EUV irradiation is an extremely important topic in planetary science. One of the dominant effects of atmospheric losses is the photochemical escape of atomic oxygen from Mars. Increasing the magnitude of the irradiation changes the response of the atmosphere. The purpose of the current paper is to analyze the effects of enhanced EUV irradiation on the escape rates of oxygen atoms. We have used the solar flare of 2017 September 10 as the baseline flare intensity and varied the intensity of the flare from a factor of 3 up to 10 times the baseline flare. We see an increase in the escape flux by 40% for flares up to 5x the intensity of the baseline flare. However, beyond this point, the increase in escape flux tapers off, reaching only about 17% above the baseline. At 10x the baseline flare intensity, the escape flux decreases by nearly 25% compared to the escape rate of the original flare. We also found that the total escape amount of hot O peaks at 7x the original flare intensity. Additionally, we have studied the effects of the time scales over which the flare energy is delivered. We find that energy dissipative processes like radiative cooling and thermal collisions do not come into play instantaneously. The escape flux from higher intensity flares dominate initially, but as time progresses, energy dissipative processes have a significant effect on the escape rate.

astro-ph.SR↗

arXiv2023

An Explainable Deep-learning Model of Proton Auroras on Mars

Proton auroras are widely observed on the dayside of Mars, identified as a significant intensity enhancement in the hydrogen Lyman alpha (121.6 nm) emission between 110 - 150 km altitudes. Solar wind protons penetrating as energetic neutral atoms into Mars thermosphere are thought to be primarily responsible for these auroras. Recent observations of spatially localized (patchy) proton auroras suggest a possible direct deposition of protons into Mars atmosphere during unstable solar wind conditions. Improving our understanding of proton auroras is therefore important for characterizing the solar wind interaction with Mars atmosphere. Here, we develop a first purely data-driven model of proton auroras using Mars Atmosphere and Volatile EvolutioN (MAVEN) in-situ observations and limb scans of Ly-alpha emissions between 2014 - 2022. We train an artificial neural network (ANN) that reproduces individual Lyman alpha intensities and relative Lyman alpha peak intensity enhancements with a Pearson correlation of 0.94 and 0.60 respectively for the test data, along with a faithful reconstruction of the shape of the observed Lyman alpha emission altitude profiles. By performing a SHapley Additive exPlanations (SHAP) analysis, we find that solar zenith angle, solar longitude, CO2 atmosphere variability, solar wind speed and temperature are the most important features for the modeled Lyman alpha peak intensity enhancements. Additionally, we find that the modeled peak intensity enhancements are high for early local time hours, particularly near polar latitudes, as well as weaker induced magnetic fields. Through SHAP analysis, we also identify the influence of biases in the training data and interdependecies between the measurements used for the modeling, and an improvement on those aspects can significantly improve the performance and applicability of the ANN model.

astro-ph.EP↗

arXiv2023

Expanding Mars Climate Modeling: Interpretable Machine Learning for Modeling MSL Relative Humidity

For the past several decades, numerous attempts have been made to model the climate of Mars with extensive studies focusing on the planet's dynamics and the understanding of its climate. While physical modeling and data assimilation approaches have made significant progress, uncertainties persist in comprehensively capturing and modeling the complexities of Martian climate. In this work, we propose a novel approach to Martian climate modeling by leveraging machine learning techniques that have shown remarkable success in Earth climate modeling. Our study presents a deep neural network designed to accurately model relative humidity in Gale Crater, as measured by NASA's Mars Science Laboratory ``Curiosity'' rover. By utilizing simulated meteorological variables produced by the Mars Planetary Climate Model, a robust Global Circulation Model, our model accurately predicts relative humidity with a mean error of 3\% and an $R^2$ score of 0.92. Furthermore, we present an approach to predict quantile ranges of relative humidity, catering to applications that require a range of values. To address the challenge of interpretability associated with machine learning models, we utilize an interpretable model architecture and conduct an in-depth analysis of its internal mechanisms and decision making processes. We find that our neural network can effectively model relative humidity at Gale crater using a few meteorological variables, with the monthly mean surface H$_2$O layer, planetary boundary layer height, convective wind speed, and solar zenith angle being the primary contributors to the model predictions. In addition to providing a fast and efficient method to modeling climate variables on Mars, this modeling approach can also be used to expand on current datasets by filling spatial and temporal gaps in observations.

astro-ph.EP↗

arXiv2022

Radiation protection and shielding materials for crewed missions on the surface of Mars

A potential crewed mission to Mars would require us to solve a number of problems, including how to protect astronauts against the devastating effects of energetic charged particles from Solar and Galactic sources. The radiation environment on Mars is of particular interest, since maintaining optimal absorbed doses by astronauts is crucial to their survival. Here, we give an overview of the conditions on Mars, as determined by theoretical models and in-situ measurements, and present the main proposed strategies to mitigate radiation exposure while on Mars. Specifically, we focus on the passive shielding technique. Several widely used materials, along with some innovative ones and combinations of those, are studied for their behavior against Solar Energetic Particle Events and Galactic Cosmic Rays in the Martian environment. For that purpose, we implement the GEANT4 package, a Monte-Carlo numerical model developed by CERN, which is specifically applied to simulate interactions of radiation with matter. A description of our model will be given, followed by outputs of the numerical model. We conclude that hydrogen-rich materials act as better attenuators, as expected, but other materials can be helpful against cosmic rays too.

astro-ph.EP↗

arXiv2022

Auroras on Mars: from Discovery to New Developments

Auroras are emissions in a planetary atmosphere caused by its interactions with the surrounding plasma environment. They have been observed in most planets and some moons of the solar system. Since their first discovery in 2005, Mars auroras have been studied extensively and is now a rapidly growing area of research. Since Mars lacks an intrinsic global magnetic field, its crustal field is distributed throughout the planet and its interactions with the surrounding plasma environment lead to a number of complex processes resulting in several types of auroras uncommon on Earth. Martian auroras have been classified as diffuse, discrete and proton aurora. With new capability of synoptic observations made possible with the Hope probe, two new types of auroras have been observed. One of them, which occurs on a much larger spatial scale, covering much of the disk, is known as discrete sinuous aurora. The other subcategory is one of proton auroras observed in patches. Further study of these phenomena will provide insights into the interactions between the atmosphere, magnetosphere and the surrounding plasma environment of Mars. We provide a brief review of the work done on the subject in the past 17 years since their discovery, and report new developments based on observations with Hope probe.

astro-ph.EP↗

arXiv2022

Crewed Missions to Mars: Modeling the Impact of Astrophysical Charged Particles on Astronauts and Their Health

The impact of exposure to astrophysical ionizing radiation on astronaut health is one of the main concerns in planning crewed missions to Mars. Astronauts will be exposed to energetic charged particles from Galactic and Solar origin for a prolonged period with little protection from a thin spacecraft shield in transit and from the rarefied Martian atmosphere when on the surface. Adverse impacts on astronaut health include, for example, Acute Radiation Syndrome, damage to the nervous system, and increased cancer risk. We rely on medical studies to assess the impact of enhanced radiation dose levels on various physiological systems and the overall health of astronauts. Using a combination of radiation measurements and numerical modeling with the GEANT4 package, we calculate the distribution of radiation dose in various human body organs for various expected scenarios simulated with a model human phantom. We suggest mitigation strategies, such as improved ways of shielding and dietary supplements, and make recommendations for the safety of astronauts in future crewed missions to Mars.

physics.space-ph↗

arXiv2022

Estimating the potential of ionizing radiation-induced radiolysis for microbial metabolism on terrestrial planets and satellites with rarefied atmospheres

Ionizing radiation is known to have a destructive effect on biology by causing damage to the DNA, cells, and production of Reactive Oxygen Species (ROS), among other things. While direct exposure to high radiation dose is indeed not favorable for biological activity, ionizing radiation can, and in some cases is known to produce a number of biologically useful products. One such mechanism is the production of biologically useful products via charged particle-induced radiolysis. Some of the byproducts are impossible to produce with lower-energy radiation (such as sunlight), opening up new avenues for life to utilize them. The main objective of the manuscript is to explore the concept of a Radiolytic Habitable Zone (RHZ), where the chemistry of GCR-induced radiolysis can be potentially utilized for metabolic activity. We first calculate the energy deposition and the electron production rate using the GEANT4 numerical model, then estimate the current production and possible chemical pathways which could be useful for supporting biological activity on Mars, Europa and Enceladus. The concept of RHZ provides a novel framework for understanding the potential for life in high-radiation environments. By combining energy deposition calculations with the energy requirements of microbial cells, we have defined the RHZ for Mars, Europa, and Enceladus. These zones represent the regions where radiolysis-driven energy production is sufficient to sustain microbial metabolism. We find that bacterial cell density is highest in Enceladus, followed by Mars and Europa. We discuss the implications of these mechanisms for the habitability of such objects in the Solar system and beyond.

astro-ph.EP↗

arXiv2022

Modeling the effectiveness of radiation shielding materials for astronaut protection on Mars

The surface of Mars is bombarded by energetic charged particles of solar and cosmic origin with little shielding compared to Earth. As space agencies are planning for crewed missions to the red planet, a major concern is the impact of ionizing radiation on astronaut health. Keeping exposure below acceptable radiation dose levels is crucial for the health of the crew. In this study, our goal is to understand the radiation environment of Mars and describe the main strategies to be adopted to protect astronauts from the harmful impacts of cosmic radiation. Specifically, we investigate the shielding properties of various materials in the Martian radiation field using the Geant4 numerical model, after validating its accuracy with in-situ instrument measurements by MSL RAD. Our results indicate that composite materials such as types of plastic, rubber or synthetic fibers, have a similar response against cosmic rays and are the best shields. Martian regolith has an intermediate behavior and therefore could be used as an additional practical option. We show that the most widely used aluminum could be helpful when combined with other low atomic number materials.

astro-ph.EP↗

arXiv2022

Diurnal variation of the surface temperature of Mars with the Emirates Mars Mission: A comparison with Curiosity and Perseverance rover measurements

For the first time, the Emirates Mars Infrared Spectrometer (EMIRS) instrument on board the Emirates Mars Mission (EMM) "Hope", is providing us with the temperature measurements of Mars at all local times covering most of the planet. As a result, it is now possible to compare surface temperature measurements made from orbit with those from the surface by rovers during the same time period. We use data of diurnal temperature variation from the Rover Environmental Monitoring Station (REMS) suite on board the Mars Science Laboratory (MSL) "Curiosity" rover, and the Mars Environmental Dynamics Analyzer (MEDA) suite on board the Mars 2020 "Perseverance" rover, between June and August 2021 and compare them with EMIRS observations and estimates of the Mars Climate Database (MCD) model. We show that although the overall trend of temperature variation is in excellent agreement across missions, EMIRS measurements are systematically lower at night compared to Mars 2020. The lower spatial resolution of EMIRS compared to the rovers and consequently lower average thermal inertia of the observed regions in this particular case primarily contributed to this discrepancy, among other factors. We discuss the implications of these results in improving our understanding of the Martian climate which would lead to better modeling of local weather prediction, useful for future robotic and crewed missions.

astro-ph.EP↗

arXiv2022

Assessment of Microbial Habitability Across Solar System Targets

With a fleet of exploratory space missions on the horizon, the study of target specific biospheres is crucial for accurately determining the probability of the existence of microbial life on various planetary bodies and prioritising targets accordingly. Although previous studies have compared the potential habitability of objects in our solar system by bulk characteristics, it is less common that precise qualitative methods are developed for ranking candidates hospitable to microbial life on a local environment basis. In this review we create a planetary environmental database and use it to motivate a list of primary habitability candidates and essential criteria for microbial survival. We then propose a new method, the Microbial Habitability Index (MHI) which uses a metric of microbial survival factor values in target environments compared with appropriate Earth analogues to assess their potential for life. We arrive at a selection of eight primary candidates and from this set conclude that Europa, Mars, and Enceladus have the highest potential for facilitating microbial survival.

astro-ph.EP↗

arXiv2021

Habitability Models for Astrobiology

Habitability has been generally defined as the capability of an environment to support life. Ecologists have been using Habitat Suitability Models (HSMs) for more than four decades to study the habitability of Earth from local to global scales. Astrobiologists have been proposing different habitability models for some time, with little integration and consistency among them, being different in function to those used by ecologists. Habitability models are not only used to determine if environments are habitable or not, but they also are used to characterize what key factors are responsible for the gradual transition from low to high habitability states. Here we review and compare some of the different models used by ecologists and astrobiologists and suggest how they could be integrated into new habitability standards. Such standards will help to improve the comparison and characterization of potentially habitable environments, prioritize target selections, and study correlations between habitability and biosignatures. Habitability models are the foundation of planetary habitability science and the synergy between ecologists and astrobiologists is necessary to expand our understanding of the habitability of Earth, the Solar System, and extrasolar planets.

astro-ph.EP↗

arXiv2020

Modeling Solar Proton Event-induced Martian Surface Radiation Dose

Solar Proton Events (SPEs) can cause abrupt and significant enhancements to the Martian surface radiation dose. Observations of the impact of SPEs on the Martian surface are available from satellites and surface detectors, but the data set is very limited in time, and the energy range is limited in scope, which makes it insufficient to estimate the impact of major events on the Martian surface. On the other hand, long-term data of SPEs impacting the Earth spanning a large energy range is widely available, and can be used to estimate the impact of major events on Mars on long timescales. Herein, we take major SPEs observed during the past several decades on Earth (1956 - 2014), along with PAMELA observations (2006 - 2014) and use the GEANT4 Monte Carlo code to calculate the Martian surface radiation dose. We study the contribution of proton fluence and spectral shape of events on the surface radiation dose and estimated the impact of possible major SPEs on the Martian surface in the future. These results have major implications for the planned human exploration of Mars. Overall we find that the radiation dose from extreme events can have a significant impact on astronaut health, and in rare, worst case scenarios, the estimated dose can even reach lethal levels.

astro-ph.EP↗

arXiv2020

From Starspots to Stellar Coronal Mass Ejections -- Revisiting Empirical Stellar Relations

Upcoming missions, including the James Webb Space Telescope, will soon characterize the atmospheres of terrestrial-type exoplanets in habitable zones around cool K- and M-type stars searching for atmospheric biosignatures. Recent observations suggest that the ionizing radiation and particle environment from active cool planet hosts may be detrimental for exoplanetary habitability. Since no direct information on the radiation field is available, empirical relations between signatures of stellar activity, including the sizes and magnetic fields of starspots, are often used. Here, we revisit the empirical relation between the starspot size and the effective stellar temperature and evaluate its impact on estimates of stellar flare energies, coronal mass ejections, and fluxes of the associated stellar energetic particle events.

astro-ph.SR↗

arXiv2020

Gamma Ray Bursts: Not so Much Deadlier than We Thought

We analyze the additional effect on planetary atmospheres of recently detected gamma-ray burst afterglow photons in the range up to 1 TeV. For an Earth-like atmosphere we find that there is a small additional depletion in ozone versus that modeled for only prompt emission. We also find a small enhancement of muon flux at the planet surface. Overall, we conclude that the additional afterglow emission, even with TeV photons, does not result in a significantly larger impact over that found in past studies.

astro-ph.EP↗

arXiv2020

Stellar Flares versus Luminosity: XUV-induced Atmospheric Escape and Planetary Habitability

Space weather plays an important role in the evolution of planetary atmospheres. Observations have shown that stellar flares emit energy in a wide energy range (10^30-10^38 ergs), a fraction of which lies in X-rays and extreme ultraviolet (XUV). These flares heat the upper atmosphere of a planet, leading to increased escape rates, and can result in atmospheric erosion over a period of time. Observations also suggest that primordial terrestrial planets can accrete voluminous H/He envelopes. Stellar radiation can erode these protoatmospheres over time, and the extent of this erosion has implications for the planet's habitability. We use the energy-limited equation to calculate hydrodynamic escape rates from these protoatmospheres irradiated by XUV stellar flares and luminosity. We use the Flare-Frequency Distribution of 492 FGKM stars observed with TESS to estimate atmospheric loss in Habitable Zone planets. We find that for most stars, luminosity-induced escape is the main loss mechanism, with a minor contribution from flares. However, flares dominate the loss mechanism of $\sim$20\% M4-M10 stars. M0-M4 stars are most likely to completely erode both their proto- and secondary atmospheres, and M4-M10 are least likely to erode secondary atmospheres. We discuss the implications of these results on planetary habitability.

astro-ph.EP↗

arXiv2020

Habitability Models for Planetary Sciences

Habitability has been generally defined as the capability of an environment to support life. Ecologists have been using Habitat Suitability Models (HSMs) for more than four decades to study the habitability of Earth from local to global scales. Astrobiologists have been proposing different habitability models for some time, with little integration and consistency between them and different in function to those used by ecologists. In this white paper, we suggest a mass-energy habitability model as an example of how to adapt and expand the models used by ecologists to the astrobiology field. We propose to implement these models into a NASA Habitability Standard (NHS) to standardize the habitability objectives of planetary missions. These standards will help to compare and characterize potentially habitable environments, prioritize target selections, and study correlations between habitability and biosignatures. Habitability models are the foundation of planetary habitability science. The synergy between the methods used by ecologists and astrobiologists will help to integrate and expand our understanding of the habitability of Earth, the Solar System, and exoplanets.

astro-ph.IM↗

arXiv2020

Investigating the biological potential of galactic cosmic ray-induced radiation-driven chemical disequilibrium in the Martian subsurface environment

There is growing evidence suggesting the presence of aqueous environment on ancient Mars, raising the question of the possibility of life in such an environment. Subsequently, with the erosion of the Martian atmosphere resulting in drastic changes in its climate, surface water disappeared, shrinking habitable spaces on the planet, with only a limited amount of water remaining near the surface in form of brines and water-ice deposits. Life, if it ever existed, would have had to adapt to harsh modern conditions, which includes low temperatures and surface pressure, and high radiation dose. Presently, there is no evidence of any biological activity on the planet's surface, however, the subsurface environment, which is yet to be explored, is less harsh, has traces of water in form of water-ice and brines, and undergoes radiation-driven redox chemistry. I hypothesize that Galactic Cosmic Ray (GCR)-induced radiation-driven chemical disequilibrium can be used for metabolic energy by extant life, and host organisms using mechanisms seen in similar chemical and radiation environments on Earth. I propose a GCR-induced radiolytic zone, and discuss the prospects of finding such life with Rosalind Franklin rover of the ExoMars mission.

astro-ph.EP↗