arXiv subjects

Dan Marsh

Publications and source records attributed to Dan Marsh.

4 recordsLinked to original sources

arXiv2023

Degenerate interpretations of O$_3$ spectral features in exoplanet atmosphere observations due to stellar UV uncertainties: a 3D case study with TRAPPIST-1e

TRAPPIST-1e is a potentially habitable terrestrial exoplanet orbiting an ultra-cool M Dwarf star and is a key target for observations with the James Webb Space Telescope (JWST). One-dimensional photochemical modelling of terrestrial planetary atmospheres has shown the importance of the incoming stellar UV flux in modulating the concentration of chemical species, such as O$_3$ and H$_2$O. In addition, three-dimensional (3D) modelling has demonstrated anisotropy in chemical abundances due to transport in tidally locked exoplanet simulations. We use the Whole Atmosphere Community Climate Model Version 6 (WACCM6), a 3D Earth System Model, to investigate how uncertainties in the incident UV flux, combined with transport, affect observational predictions for TRAPPIST-1e (assuming an initial Earth-like atmospheric composition). We use two semi-empirical stellar spectra for TRAPPIST-1 from the literature. The UV flux ratio between them can be as large as a factor of 5000 in some wavelength bins. Consequently, the photochemically-produced total O$_3$ columns differ by a factor of 26. Spectral features of O$_3$ in both transmission and emission spectra vary between these simulations (e.g. differences of 19 km in transmission spectra effective altitude for O$_3$ at 0.6 $\mu$m). This leads to potential ambiguities when interpreting observations, including overlap with scenarios that assume alternative O$_2$ concentrations. Hence, to achieve robust interpretations of terrestrial exoplanetary spectra, characterisation of the UV spectra of their host stars is critical. In the absence of such stellar measurements, atmospheric context can still be gained from other spectral features (e.g. H$_2$O), or by comparing direct imaging and transmission spectra in conjunction.

astro-ph.EP↗

arXiv2023

Higher Water Loss on Earth-like Exoplanets in Eccentric Orbits

The climate of a terrestrial exoplanet is controlled by the type of host star, the orbital configuration and the characteristics of the atmosphere and the surface. Many rocky exoplanets have higher eccentricities than those in the Solar System, and about 18% of planets with masses $< 10 \mathrm{M}_{\oplus}$ have $e>0.1$. Underexplored are the implications of such high eccentricities on the atmosphere, climate, and potential habitability on such planets. We use WACCM6, a state-of-the-art fully-coupled Earth-system model, to simulate the climates of two Earth-like planets; one in a circular orbit ($e=0$), and one in an eccentric orbit ($e=0.4$) with the same mean insolation. We quantify the effects of eccentricity on the atmospheric water abundance and loss given the importance of liquid water for habitability. The asymmetric temperature response in the eccentric orbit results in a water vapour mixing ratio in the stratosphere ($> 20$ ppmv) that is approximately five times greater than that for circular orbit ($\sim 4$ ppmv). This leads to at most $\sim 3$ times increases in both the atmospheric hydrogen loss rate and the ocean loss rate compared with the circular case. Using the Planetary Spectrum Generator, we simulate the idealised transmission spectra for both cases. We find that the water absorption features are stronger at all wavelengths for the $e=0.4$ spectrum than for the circular case. Hence, highly-eccentric Earth-like exoplanets may be prime targets for future transmission spectroscopy observations to confirm, or otherwise, the presence of atmospheric water vapour.

astro-ph.EP↗

arXiv2022

Variability due to climate and chemistry in observations of oxygenated Earth-analogue exoplanets

The Great Oxidation Event was a period during which Earth's atmospheric oxygen (O$_2$) concentrations increased from $\sim 10^{-5}$ times its present atmospheric level (PAL) to near modern levels, marking the start of the Proterozoic geological eon 2.4 billion years ago. Using WACCM6, an Earth System Model, we simulate the atmosphere of Earth-analogue exoplanets with O$_2$ mixing ratios between 0.1% and 150% PAL. Using these simulations, we calculate the reflection/emission spectra over multiple orbits using the Planetary Spectrum Generator. We highlight how observer angle, albedo, chemistry, and clouds affect the simulated observations. We show that inter-annual climate variations, as well short-term variations due to clouds, can be observed in our simulated atmospheres with a telescope concept such as LUVOIR or HabEx. Annual variability and seasonal variability can change the planet's reflected flux (including the reflected flux of key spectral features such as O$_2$ and H$_2$O) by up to factors of 5 and 20, respectively, for the same orbital phase. This variability is best observed with a high-throughput coronagraph. For example, HabEx (4 m) with a starshade performs up to a factor of two times better than a LUVOIR B (6 m) style telescope. The variability and signal-to-noise ratio of some spectral features depends non-linearly on atmospheric O$_2$ concentration. This is caused by temperature and chemical column depth variations, as well as generally increased liquid and ice cloud content for atmospheres with O$_2$ concentrations of $<$1% PAL.

astro-ph.EP↗

arXiv2021

A revised lower estimate of ozone columns during Earth's oxygenated history

The history of molecular oxygen (O$_2$) in Earth's atmosphere is still debated; however, geological evidence supports at least two major episodes where O$_2$ increased by an order of magnitude or more: the Great Oxidation Event (GOE) and the Neoproterozoic Oxidation Event. O$_2$ concentrations have likely fluctuated (between $10^{-3}$ and $1.5$ times the present atmospheric level) since the GOE $\sim 2.4$ Gyr ago, resulting in a time-varying ozone (O$_3$) layer. Using a three-dimensional chemistry-climate model, we simulate changes in O$_3$ in Earth's atmosphere since the GOE and consider the implications for surface habitability, and glaciation during the Mesoproterozoic. We find lower O$_3$ columns (reduced by up to $4.68$ times for a given O$_2$ level) compared to previous work; hence, higher fluxes of biologically harmful UV radiation would have reached the surface. Reduced O$_3$ leads to enhanced tropospheric production of the hydroxyl radical (OH) which then substantially reduces the lifetime of methane (CH$_4$). We show that a CH$_4$ supported greenhouse effect during the Mesoproterozoic is highly unlikely. The reduced O$_3$ columns we simulate have important implications for astrobiological and terrestrial habitability, demonstrating the relevance of three-dimensional chemistry-climate simulations when assessing paleoclimates and the habitability of faraway worlds.

astro-ph.EP↗