International audienc

In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward. Key Words: Exoplanets—Biosignatures—Habitability markers—Photosynthesis—Planetary surfaces—Atmospheres—Spectroscopy—Cryptic biospheres—False positives. Astrobiology 18, 663–708.

Amongst the diversity of methods used by organisms to reduce damage caused by ultraviolet (UV) radiation, the synthesis of UV-screening compounds is almost ubiquitous. UV-screening compounds provide a passive method for the reduction of UV-induced damage and they are widely distributed across the microbial, plant and animal kingdoms. They share some common chemical features. It is likely that on early earth strong selection pressures existed for the evolution of UV-screening compounds. Many of these compounds probably had other physiological roles, later being selected for the efficacy of UV screening. The diversity in physiological functions is one of the complications in studying UV-screening compounds and determining the true ecological importance of their UV-screening role. As well as providing protection against ambient UV radiation, species with effective screening may also be at an advantage during natural ozone depletion events. In this review the characteristics of a wide diversity of UV-screening compounds are discussed and evolutionary questions are explored. As research into the range of UV-screening compounds represented in the biosphere continues, so it is likely that the properties of many more compounds will be elucidated. These compounds, as well as providing us with insights into natural responses to UV radiation, may also have implications for the development of artificial UV-screening methods to reduce human exposure to UV radiation.

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We address the first several hundred million years of Earth's history. The Moonforming impact left Earth enveloped in a hot silicate atmosphere that cooled and condensed over ∼1,000 yrs. As it cooled the Earth degassed its volatiles into the atmosphere. It took another ∼2 Myrs for the magma ocean to freeze at the surface. The cooling rate was determined by atmospheric thermal blanketing. Tidal heating by the new Moon was a major energy source to the magma ocean. After the mantle solidified geothermal heat became climatologically insignificant, which allowed the steam atmosphere to condense, and left behind a ∼100 bar, ∼500 K CO 2 atmosphere. Thereafter cooling was governed by how quickly CO 2 was removed from the atmosphere. If subduction were efficient this could have K. Zahnle ( ) K. Zahnle et al.taken as little as 10 million years. In this case the faint young Sun suggests that a lifeless Earth should have been cold and its oceans white with ice. But if carbonate subduction were inefficient the CO 2 would have mostly stayed in the atmosphere, which would have kept the surface near ∼500 K for many tens of millions of years. Hydrous minerals are harder to subduct than carbonates and there is a good chance that the Hadean mantle was dry. Hadean heat flow was locally high enough to ensure that any ice cover would have been thin (<5 m) in places. Moreover hundreds or thousands of asteroid impacts would have been big enough to melt the ice triggering brief impact summers. We suggest that plate tectonics as it works now was inadequate to handle typical Hadean heat flows of 0.2-0.5 W/m 2 . In its place we hypothesize a convecting mantle capped by a ∼100 km deep basaltic mush that was relatively permeable to heat flow. Recycling and distillation of hydrous basalts produced granitic rocks very early, which is consistent with preserved >4 Ga detrital zircons. If carbonates in oceanic crust subducted as quickly as they formed, Earth could have been habitable as early as 10-20 Myrs after the Moon-forming impact.

1 2 It has long been suggested that hydrothermal systems might have provided habitats for the origin 3 and evolution of early life on Earth, and possibly other planets such as Mars. In this contribution 4 we show that most impact events that result in the formation of complex impact craters (i.e., >2-5 4 and >5-10 km diameter on Earth and Mars, respectively) are potentially capable of generating 6 a hydrothermal system. Consideration of the impact cratering record on Earth suggests that the 7 presence of an impact crater lake is critical for determining the longevity and size of the 8 hydrothermal system. We show that there are six main locations within and around impact 9 craters on Earth where impact-generated hydrothermal deposits can form: 1) crater-fill impact 10 melt rocks and melt-bearing breccias; 2) interior of central uplifts; 3) outer margin of central 11 uplifts; 4) impact ejecta deposits; 5) crater rim region; and 6) post-impact crater lake sediments. 12We suggest that these six locations are applicable to Mars as well. Evidence for impact-13 generated hydrothermal alteration ranges from discrete vugs and veins to pervasive alteration 14 depending on the setting and nature of the system. A variety of hydrothermal minerals have been 15 documented in terrestrial impact structures and these can be grouped into three broad categories: 16(1) hydrothermally-altered target-rock assemblages; (2) primary hydrothermal minerals 17 precipitated from solutions; and (3) secondary assemblages formed by the alteration of primary 18 hydrothermal minerals. Target lithology and the origin of the hydrothermal fluids strongly 19 influences the hydrothermal mineral assemblages formed in these post-impact hydrothermal 20systems. There is a growing body of evidence for impact-generated hydrothermal activity on 21 Mars; although further detailed studies using high-resolution imagery and multispectral 22 information are required. Such studies have only been done in detail for a handful of Martian 23 4 craters. The best example so far is from Toro Crater (Marzo et al., 2010). We also present new 1 evidence for impact-generated hydrothermal deposits within an unnamed ~32-km diameter crater 2 ~ 350 km away from Toro and within the larger Holden Crater. Synthesizing observations of 3 impact craters on Earth and Mars, we suggest that if there was life on Mars early in its history, 4 then hydrothermal deposits associated with impact craters may provide the best, and most 5 numerous, opportunities for finding preserved evidence for life on Mars. Moreover, 6hydrothermally altered and precipitated rocks can provide nutrients and habitats for life long 7 after hydrothermal activity has ceased. 8 5 1

Large impacts provide a mechanism for resurfacin g planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peak s transition to peak ri ngs. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Ex pedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust

Habitability is a widely used word in the geoscience, planetary science, and astrobiology literature, but what does it mean? In this review on habitability, we define it as the ability of an environment to support the activity of at least one known organism. We adopt a binary definition of "habitability" and a "habitable environment." An environment either can or cannot sustain a given organism. However, environments such as entire planets might be capable of supporting more or less species diversity or biomass compared with that of Earth. A clarity in understanding habitability can be obtained by defining instantaneous habitability as the conditions at any given time in a given environment required to sustain the activity of at least one known organism, and continuous planetary habitability as the capacity of a planetary body to sustain habitable conditions on some areas of its surface or within its interior over geological timescales. We also distinguish between surface liquid water worlds (such as Earth) that can sustain liquid water on their surfaces and interior liquid water worlds, such as icy moons and terrestrial-type rocky planets with liquid water only in their interiors. This distinction is important since, while the former can potentially sustain habitable conditions for oxygenic photosynthesis that leads to the rise of atmospheric oxygen and potentially complex multicellularity and intelligence over geological timescales, the latter are unlikely to. Habitable environments do not need to contain life. Although the decoupling of habitability and the presence of life may be rare on Earth, it may be important for understanding the habitability of other planetary bodies.