arXiv subjects

David A. Brain

Publications and source records attributed to David A. Brain.

5 recordsLinked to original sources

arXiv2026

ESCAPE: a small explorer mission to study the stellar drivers of exoplanet evolution

The long-term stability of exoplanetary atmospheres depends critically on the extreme-ultraviolet (EUV) photon and high-energy particle fluxes from the host star, which are poorly constrained. To address this key gap in our understanding of atmospheric retention, we present the Extreme-ultraviolet Stellar Characterization for Atmospheric Physics and Evolution (ESCAPE) mission, a NASA Small Explorer concept proposed in 2026. ESCAPE employs extreme- and far-ultraviolet spectroscopy (80 - 1650 Ang) to provide the first comprehensive study of the stellar EUV history and stellar coronal mass ejection (CME) environments that control atmospheric mass-loss and determine the habitability of rocky exoplanets. This paper outlines both the primary science goals of the mission, the breadth of future general observer investigations, and a detailed design study of the mission's instrumentation. The ESCAPE instrument comprises a grazing incidence telescope that feeds multiple diffraction gratings and a photon-counting detector. We describe a demonstration of the Hettrick-Bowyer telescope, etched silicon diffraction gratings, the microchannel plate detector and housing, and gold and zirconium coatings. We present a STOP analysis that verifies ESCAPE's ability to meet its structural integrity, thermal stability, and optical performance requirements throughout the mission environment.

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arXiv2026

Comparing Monte Carlo Models of Impact Alteration of Planetary Atmospheres

One process that affects atmospheric surface pressure is impact bombardment. The evolution of a planet's atmosphere under impact bombardment is an open question. We use a Monte Carlo method to evolve a range (0.006 to 92.5 bar) of initial atmospheres at Mars, Earth, and Venus under bombardment of 5x10^6 impactors using seven individual models. Since these seven models are best suited for specific impactor size regimes, we also combine these models into a composite model and compare it to other existing composites. Alterations to the existing models are required to apply to broad initial conditions. If we use each component model for every impactor, starting from present-day atmospheric pressure, we find about two or three orders of magnitude spread in the final atmospheric pressure. Given these differences, we suggest that the use of any one model to determine atmospheric change due to impact bombardment is risky. Most models and starting parameters result in net growth between +0.01 and +100 bar. Our composite model shows that the atmospheres of Venus, Earth, and Mars tend to grow under bombardment, with Earth's atmosphere growing most quickly. For an early Martian (P_0=1 bar) and an early terrestrial (with an initial pressure of P_0=0.25 bar) atmosphere, both tend to grow under bombardment. The results suggested here, where the models are universally applied, suggest that impact bombardment was likely a significant source of volatiles in the early Solar System. Additional work and careful consideration of how impact events affect the evolution of planetary atmospheres is needed.

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arXiv2026

Mars as an Exoplanet: Lessons from a Planet at the Edge of Habitability

Mars is the Solar System's canonical small, rocky planet that transitioned from early geologic activity and surface liquid water to a cold and arid planet with a thin, cold, CO$_2$-dominated atmosphere. The evolution of Mars, in the context of such planetary parameters as size, mass, atmosphere, insolation flux, magnetosphere, and impact history, harbor important diagnostics regarding the development and sustainability of habitable surface conditions. In this work, we synthesize how the study of Mars contributes to our understanding of exoplanet processes, such as volatile delivery and loss, photochemistry, climate evolution (including CO$_2$ condensation and atmospheric loss), obliquity forcing, planetary architecture, and the role of intrinsic magnetism. We also evaluate optimal methods and prospects for detecting and characterizing potential Mars analogs beyond the Solar System. We focus on relevant results from planetary missions (e.g., Mars Reconnaissance Orbiter, MAVEN, Mars Science Laboratory, Mars2020) and observational studies of exoplanet atmospheres with the James Webb Space telescope (JWST) and future facilities. Through the convergence of these parallel pathways of inquiry, we describe the primary science questions and suggested avenues for characterizing small rocky planets that lie at the edge of potentially habitable conditions.

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arXiv2026

Atmospheric Escape Rates from Mars - If it Orbited an Old M-Dwarf Star

Atmospheric escape is an important process that influences the evolution of planetary atmospheres. A variety of physical mechanisms can contribute to escape from an atmosphere, including thermal escape, ion escape, photochemical escape, and sputtering. Here we estimate escape rates via each of these processes for a hypothetical Mars-like exoplanet orbiting Barnard's star (an old, inactive M dwarf star). We place the planet at an orbital distance that receives the same total stellar flux as it does in our solar system. We use the measured stellar extreme ultraviolet (EUV) spectrum and assumptions on the star's magnetic field to determine both the high-energy radiation and the stellar wind environment around the planet. This information is used to model the response of the planet's thermosphere, exosphere and magnetosphere using a variety of models that have been validated against solar system observations. We find overall escape rates that are dominated by thermal processes and elevated by 2-5 orders of magnitude relative to present-day Mars, suggesting that a Mars-like planet orbiting Barnard's star would not retain a significant atmosphere for more than 10's of millions of years. Recently reported planets around Barnard's star should also not have retained significant atmospheres. By extension, Mars-like planets orbiting any M dwarf near the 'Habitable Zone' should not retain atmospheres for extended periods of time.

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arXiv2024

Exoplanet Magnetic Fields

Planetary magnetic fields are important indicators of planetary processes and evolution, from a planet's outer core to its surface (if it possesses one) to its atmosphere and near-space environment. Magnetic fields are most directly measured in situ, and determining whether distant planetary objects possess magnetic fields can be challenging. At present we have no unambiguous measurements of magnetic fields on exoplanets. Nevertheless, it would be surprising if at least some exoplanets did not generate a magnetic field, like many planetary bodies in the solar system. This chapter provides an overview of the current understanding of exoplanetary magnetic fields and their consequences. In the next section we review the current understanding of planetary dynamo generation as it applies to solar system objects and discuss the implications for exoplanetary magnetic field generation. Following this, we describe seven methods for determining the existence and strength of an exoplanetary magnetic field and discuss the near-term prospects for each method. We close by highlighting four main consequences of exoplanetary magnetic fields for a planet and its evolution.

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