19 research outputs found
Faint young Sun paradox remains
The Sun was fainter when the Earth was young, but the climate was generally
at least as warm as today; this is known as the `faint young Sun paradox'.
Rosing et al. [1] claim that the paradox can be resolved by making the early
Earth's clouds and surface less reflective. We show that, even with the
strongest plausible assumptions, reducing cloud and surface albedos falls short
by a factor of two of resolving the paradox. A temperate Archean climate cannot
be reconciled with the low level of CO2 suggested by Rosing et al. [1]; a
stronger greenhouse effect is needed.Comment: 3 pages, no figures. In press in Nature. v2 corrects typo in author
list in original submissio
Atmospheric Evolution
Earth's atmosphere has evolved as volatile species cycle between the
atmosphere, ocean, biomass and the solid Earth. The geochemical, biological and
astrophysical processes that control atmospheric evolution are reviewed from an
"Earth Systems" perspective, with a view not only to understanding the history
of Earth, but also to generalizing to other solar system planets and
exoplanets.Comment: 34 pages, 3 figures, 2 tables. Accepted as a chapter in
"Encyclopaedia of Geochemistry", Editor Bill White, Springer-Nature, 201
The Detectability of Earth's Biosignatures Across Time
Over the past two decades, enormous advances in the detection of exoplanets
have taken place. Currently, we have discovered hundreds of earth-sized
planets, several of them within the habitable zone of their star. In the coming
years, the efforts will concentrate in the characterization of these planets
and their atmospheres to try to detect the presence of biosignatures. However,
even if we discovered a second Earth, it is very unlikely that it would present
a stage of evolution similar to the present-day Earth. Our planet has been far
from static since its formation about 4.5 Ga ago; on the contrary, during this
time, it has undergone multiple changes in it's atmospheric composition, it's
temperature structure, it's continental distribution, and even changes in the
forms of life that inhabit it. All these changes have affected the global
properties of Earth as seen from an astronomical distance. Thus, it is of
interest not only to characterize the observables of the Earth as it is today,
but also at different epochs. Here we review the detectability of the Earth's
globally-averaged properties over time. This includes atmospheric composition
and biosignatures, and surface properties that can be interpreted as sings of
habitability (bioclues). The resulting picture is that truly unambiguous
biosignatures are only detectable for about 1/4 of the Earth's history. The
rest of the time we rely on detectable bioclues that can only establish an
statistical likelihood for the presence of life on a given planet.Comment: To appear in "Handbook of Exoplanets", eds. Deeg, H.J. & Belmonte,
J.A, Springer (2018). arXiv admin note: text overlap with
arXiv:astro-ph/0609398 by other author
Earth: Atmospheric Evolution of a Habitable Planet
Our present-day atmosphere is often used as an analog for potentially
habitable exoplanets, but Earth's atmosphere has changed dramatically
throughout its 4.5 billion year history. For example, molecular oxygen is
abundant in the atmosphere today but was absent on the early Earth. Meanwhile,
the physical and chemical evolution of Earth's atmosphere has also resulted in
major swings in surface temperature, at times resulting in extreme glaciation
or warm greenhouse climates. Despite this dynamic and occasionally dramatic
history, the Earth has been persistently habitable--and, in fact,
inhabited--for roughly 4 billion years. Understanding Earth's momentous changes
and its enduring habitability is essential as a guide to the diversity of
habitable planetary environments that may exist beyond our solar system and for
ultimately recognizing spectroscopic fingerprints of life elsewhere in the
Universe. Here, we review long-term trends in the composition of Earth's
atmosphere as it relates to both planetary habitability and inhabitation. We
focus on gases that may serve as habitability markers (CO2, N2) or
biosignatures (CH4, O2), especially as related to the redox evolution of the
atmosphere and the coupled evolution of Earth's climate system. We emphasize
that in the search for Earth-like planets we must be mindful that the example
provided by the modern atmosphere merely represents a single snapshot of
Earth's long-term evolution. In exploring the many former states of our own
planet, we emphasize Earth's atmospheric evolution during the Archean,
Proterozoic, and Phanerozoic eons, but we conclude with a brief discussion of
potential atmospheric trajectories into the distant future, many millions to
billions of years from now. All of these 'Alternative Earth' scenarios provide
insight to the potential diversity of Earth-like, habitable, and inhabited
worlds.Comment: 34 pages, 4 figures, 4 tables. Review chapter to appear in Handbook
of Exoplanet
Methane bursts as a trigger for intermittent lake-forming climates on post-Noachian Mars
Lakes existed on Mars later than 3.6 billion years ago, according to sedimentary evidence for deltaic deposition. The observed fluviolacustrine deposits suggest that individual lake-forming climates persisted for at least several thousand years (assuming dilute flow). But the lake watersheds’ little-weathered soils indicate a largely dry climate history, with intermittent runoff events. Here we show that these observational constraints, although inconsistent with many previously proposed triggers for lake-forming climates, are consistent with a methane burst scenario. In this scenario, chaotic transitions in mean obliquity drive latitudinal shifts in temperature and ice loading that destabilize methane clathrate. Using numerical simulations, we find that outgassed methane can build up to atmospheric levels sufficient for lake-forming climates, if methane clathrate initially occupies more than 4% of the total volume in which it is thermodynamically stable. Such occupancy fractions are consistent with methane production by water–rock reactions due to hydrothermal circulation on early Mars. We further estimate that photochemical destruction of atmospheric methane curtails the duration of individual lake-forming climates to less than a million years, consistent with observations. We conclude that methane bursts represent a potential pathway for intermittent excursions to a warm, wet climate state on early Mars
Nitrogen-enhanced greenhouse warming on early Earth
Early in Earth's history, the Sun provided less energy to the Earth than it does today. However, the Earth was not permanently glaciated, an apparent contradiction known as the faint young Sun paradox. By implication, the Earth must have been warmed by a stronger greenhouse effect or a lower planetary albedo. Here we use a radiative?convective climate model to show that more N2 in the atmosphere would have increased the warming effect of existing greenhouse gases by broadening their absorption lines. With the atmospheric CO2 and CH4 levels estimated for 2.5 billion years ago, a doubling of the present atmospheric nitrogen (PAN) level would cause a warming of 4.4 degrees C. Our new budget of Earth's geological nitrogen reservoirs indicates that there is a sufficient quantity of nitrogen in the crust (0.5 PAN) and mantle (greater than 1.4 PAN) to have supported this, and that this nitrogen was previously in the atmosphere. In the mantle, N correlates with 40Ar, the daughter product of 40K, indicating that the source of mantle N is subducted crustal rocks in which NH4+ has been substituted for K+. We thus conclude that a higher nitrogen level probably helped warm the early Earth, and suggest that the effects of N2 should be considered in assessing the habitable zone for terrestrial planets