arXiv subjects

Jack T. O'Malley-James

Publications and source records attributed to Jack T. O'Malley-James.

5 recordsLinked to original sources

arXiv2019

Biofluorescent Worlds II. Biological fluorescence induced by stellar UV flares, a new temporal biosignature

Our first targets in the search for signs of life are orbiting nearby M stars, such as the planets in the Proxima Centauri, Ross-128, LHS-1140, and TRAPPIST-1 systems. Future ground-based discoveries, and those from the TESS mission, will provide additional close-by targets. However, young M stars tend to be very active, flaring frequently and causing UV fluxes on the surfaces of HZ planets to become biologically harmful. Common UV-protection methods used by life (e.g. living underground, or underwater) would make a biosphere harder to detect. However, photoprotective biofluorescence, 'up-shifting' UV to longer, safer wavelengths, could increase a biosphere's detectability. Here we model intermittent emission at specific wavelengths in the visible spectrum caused by biofluorescence as a new temporal biosignature for planets around active M stars. We use the absorption and emission characteristics of common coral fluorescent pigments and proteins to create model spectra and colours for an Earth-like planet in such a system, accounting for different surface features, atmospheric absorption, and cloud cover. We find that for a cloud-free planet biofluorescence could induce a temporary change in brightness that is significantly higher than the reflected flux alone, causing up to two orders-of-magnitude change in planet-star contrast, compared to a non-fluorescent state, if the surface is fully covered by a highly efficient fluorescent biosphere. Hence, UV-flare induced biofluorescence presents previously unexplored possibilities for a new temporal biosignature that could be detectable by instruments like those planned for the extremely large telescope and could reveal hidden biospheres.

astro-ph.EP↗

arXiv2019

Expanding the Timeline for Earth's Photosynthetic Red Edge Biosignature

When Carl Sagan observed the Earth during a Gallileo fly-by in 1993, he found a widely distributed surface pigment with a sharp reflection edge in the red part of the spectrum, which, together with the abundance of gaseous oxygen and methane in extreme thermodynamic disequilibrium, were strongly suggestive of the presence of life on Earth. This widespread pigmentation that could not be explained by geological processes alone, is caused by the cellular structure of vegetation - a mechanism for potentially limiting damage to chlorophyll and/or limiting water loss. The distinctive increase in the red portion of Earth's global reflectance spectrum is called the vegetation red edge in astrobiology literature and is one of the proposed surface biosignatures to search for on exoplanets and exomoons. Earth's surface vegetation has only been widespread for about half a billion years, providing a surface biosignature for approximately 1/9th our planet's lifetime. However, as chlorophyll is present in many forms of life on Earth, like cyanobacteria, algae, lichen, corals, as well as leafy vegetation, such a spectral red edge feature could indicate a wide range of life, expanding its use for the search for surface biosignatures beyond vegetation alone to a time long before vegetation became widespread on Earth. We show how lichens could extend the presence of Earth's red edge surface biofeature to 1.2 Gyr ago, while ocean surface algae and cyanobacteria could extend it to over 2 Gyr ago, expanding the use of a photosynthetic red edge to earlier times in Earth's history.

astro-ph.EP↗

arXiv2019

Lessons from early Earth: UV surface radiation should not limit the habitability of active M star systems

The closest potentially habitable worlds outside our Solar system orbit a different kind of star than our Sun: smaller red dwarf stars. Such stars can flare frequently, bombarding their planets with biologically damaging high-energy UV radiation, placing planetary atmospheres at risk of erosion and bringing the habitability of these worlds into question. However, the surface UV flux on these worlds is unknown. Here we show the first models of the surface UV environments of the four closest potentially habitable exoplanets: Proxima-b, TRAPPIST-1e, Ross-128b, and LHS-1140b assuming different atmospheric compositions, spanning Earth-analogue to eroded and anoxic atmospheres and compare them to levels for Earth throughout its geological evolution. Even for planet models with eroded and anoxic atmospheres, surface UV radiation remains below early Earth levels, even during flares. Given that the early Earth was inhabited, we show that UV radiation should not be a limiting factor for the habitability of planets orbiting M stars. Our closest neighbouring worlds remain intriguing targets for the search for life beyond our Solar system.

astro-ph.EP↗

arXiv2018

The Vegetation Red Edge Biosignature Through Time on Earth and Exoplanets

The high reflection of land vegetation in the near-infrared, the vegetation red edge (VRE), is often cited as a spectral biosignature for surface vegetation on exoplanets. The VRE is only a few percent change in reflectivity for a disk-integrated observation of present-day Earth. Here we show that the strength of Earth's VRE has increased over the past ~500 million years of land plant evolution and may continue to increase as solar luminosity increases and the planet warms, until either vegetation coverage is reduced, or the planet's atmosphere becomes opaque to light reflected off the surface. Early plants like mosses and liverworts, which dominated on land 500-400 million years ago, produce a weaker VRE, approximately half as strong as that of modern vegetation. We explore how the changes in land plants, as well as geological changes like ice coverage during ice-ages and interglacial periods, influence the detectability of the VRE through Earth's geological past. Our results show that the VRE has varied through the evolutionary history of land plants on Earth, and could continue to change into the future if hotter climate conditions became dominant, encouraging the spread of vegetation. Our findings suggest that older and hotter Earth-like planets are good targets for the search for a VRE signature. In addition, hot exoplanets and dry exoplanets with some water could be the best targets for a successful vegetation biosignature detection. As well as a strong red edge, lower cloud-fractions and low levels of atmospheric water vapor on such planets could make it easier to detect surface features in general.

astro-ph.EP↗

arXiv2013

Swansong Biospheres II: The final signs of life on terrestrial planets near the end of their habitable lifetimes

The biosignatures of life on Earth do not remain static, but change considerably over the planet's habitable lifetime. Earth's future biosphere, much like that of the early Earth, will consist of predominantly unicellular microorganisms due to the increased hostility of environmental conditions caused by the Sun as it enters the late stage of its main sequence evolution. Building on previous work, the productivity of the biosphere is evaluated during different stages of biosphere decline between 1 Gyr and 2.8 Gyr from present. A simple atmosphere-biosphere interaction model is used to estimate the atmospheric biomarker gas abundances at each stage and to assess the likelihood of remotely detecting the presence of life in low-productivity, microbial biospheres, putting an upper limit on the lifetime of Earth's remotely detectable biosignatures. Other potential biosignatures such as leaf reflectance and cloud cover are discussed.

astro-ph.EP↗