Starting from planetary systems with three giant planets and an outer disc of planetesimals, we use dynamical simulations to show how dynamical instabilities can transform planetesimal discs into 102–103 au-scale isotropic clouds. The instabilities involve a phase of planet–planet scattering that concludes with the ejection of one or more planets and the inward-scattering of the surviving gas giant(s) to remove them from direct dynamical contact with the planetesimals. ‘Mini-Oort clouds’ are thus formed from scattered planetesimals whose orbits are frozen by the abrupt disappearance of the perturbing giant planet. Although the planetesimal orbits are virtually isotropic, the surviving giant planets tend to have modest inclinations (typically ∼10°) with respect to the initial orbital plane. The collisional lifetimes of mini-Oort clouds are long (10 Myr to >10 Gyr) and there is a window of ∼100 Myr or longer during which they produce spherical clouds of potentially observable dust at 70 μm. If the formation channel for hot Jupiters commonly involves planetary close encounters, we predict a correlation between this subset of extrasolar planetary systems and mini-Oort clouds.
The resiliency and adaptive ability of microbial life in real time on Earth relies heavily upon horizontal gene transfer. Based on that knowledge, how likely is earth based microbial life to colonize extraterrestrial targets such as Mars? To address this question, we consider manned and unmanned space exploration, the resident microbiota that is likely to inhabit those vehicles, the adaptive potential of that microbiota in an extraterrestrial setting especially with regards to mobile genetic elements, and the likelihood that Mars like environments could initiate and sustain colonization.
We find that terrestrial oxidation of pyrite by microbes using oxygen has contributed a substantial fraction of the total sulphur weathering flux since at least 2.5 Gyr ago, with probable evidence of such activity 2.7–2.8 Gyr ago. The late Archaean onset of terrestrial sulphur cycling is supported by marine molybdenum abundance data and coincides with a shift to more sulphidic ocean conditions5. We infer that significant microbial land colonization began by 2.7–2.8 Gyr ago. Our identification of pyrite oxidation at this time provides further support for the appearance6 of molecular oxygen several hundred million years before the Great Oxidation Event.
We present an analytic one-dimensional radiative-convective model of the thermal structure of planetary atmospheres. Our model assumes that thermal radiative transfer is gray and can be represented by the two-stream approximation. Model atmospheres are assumed to be in hydrostatic equilibrium, with a power-law scaling between the atmospheric pressure and the gray thermal optical depth. The convective portions of our models are taken to follow adiabats that account for condensation of volatiles through a scaling parameter to the dry adiabat. By combining these assumptions, we produce simple, analytic expressions that allow calculations of the atmospheric-pressure-temperature profile, as well as expressions for the profiles of thermal radiative flux and convective flux. We explore the general behaviors of our model. These investigations encompass (1) worlds where atmospheric attenuation of sunlight is weak, which we show tend to have relatively high radiative-convective boundaries; (2) worlds with some attenuation of sunlight throughout the atmosphere, which we show can produce either shallow or deep radiative-convective boundaries, depending on the strength of sunlight attenuation; and (3) strongly irradiated giant planets (including hot Jupiters), where we explore the conditions under which these worlds acquire detached convective regions in their mid-tropospheres. Finally, we validate our model and demonstrate its utility through comparisons to the average observed thermal structure of Venus, Jupiter, and Titan, and by comparing computed flux profiles to more complex models.
Oxygenic photosynthesis appears to have evolved well before O2 levels increased in the atmosphere, at around 2.4 Ga. This has led to numerous suggestions as to what may have kept O2 suppressed and then eventually allowed it to rise. These suggestions include changes in the recycling of carbon and sulfur relative to water (or hydrogen), a switch from dominantly submarine to dominantly subaerial volcanism, gradual oxidation of the continents and a concomitant decrease in reduced metamorphic gases, a decline in deposition of banded iron-formations, a decline in nickel availability, and various proposals to increase the efficiency of photosynthesis. Several of these different mechanisms could have contributed to the rise of O2, although not all of them are equally effective. To be considered successful, any proposed mechanism must make predictions that are consistent with the carbon isotope record in marine carbonates, which shows relatively little change with time, apart from transient (but occasionally spectacular) excursions. The reasons for this constancy are explored here, but are not fully resolved. In the process of making these comparisons, a self-consistent redox balance framework is developed which will hopefully prove useful to others who may work on this problem and to astronomers who may one day try to decipher spectral signatures of oxygen on Earth-like exoplanets.
We present models for the formation of terrestrial planets, and the collisional evolution of debris disks, in planetary systems that contain multiple marginally unstable gas giants. We previously showed that in such systems, the dynamics of the giant planets introduces a correlation between the presence of terrestrial planets and cold dust, i.e., debris disks, which is particularly pronounced at λ ~ 70 μm. Here we present new simulations that show that this connection is qualitatively robust to a range of parameters: the mass distribution of the giant planets, the width and mass distribution of the outer planetesimal disk, and the presence of gas in the disk when the giant planets become unstable. We discuss how variations in these parameters affect the evolution. We find that systems with equal-mass giant planets undergo the most violent instabilities, and that these destroy both terrestrial planets and the outer planetesimal disks that produce debris disks.
Earth’s climate and biosphere have always shaped one another. James F. Kasting approves of an attempt to reveal the planet’s future by reading its past.
We find evidence for oxygen production in microbial mats and localized oxygenation of surface waters. Carbon and sulphur isotopes indicate that this oxygen production occurred under a reduced atmosphere that was periodically rich in methane, consistent with the prediction of a hydrocarbon haze. We use a photochemical model to corroborate our geochemical data. Our simulations predict transitions between two stable atmospheric states, one with organic haze and the other haze-free. The transitions are presumably governed by variations in the amount of biological methane production during the Archaean eon. We find that the isotopic signatures we observe are evident in other data sets from this period and conclude that methane was an important component of the atmosphere throughout the Archaean.
Here we show that raindrop imprints in tuffs of the Ventersdorp Supergroup, South Africa, constrain surface air density 2.7 billion years ago to less than twice modern levels. We interpret the raindrop fossils using experiments in which water droplets of known size fall at terminal velocity into fresh and weathered volcanic ash, thus defining a relationship between imprint size and raindrop impact momentum. Fragmentation following raindrop flattening limits raindrop size to a maximum value independent of air density, whereas raindrop terminal velocity varies as the inverse of the square root of air density. If the Archaean raindrops reached the modern maximum measured size, air density must have been less than 2.3 kg m(-3), compared to today’s 1.2 kg m(-3), but because such drops rarely occur, air density was more probably below 1.3 kg m(-3). The upper estimate for air density renders the pressure broadening explanation possible, but it is improbable under the likely lower estimates. Our results also disallow the extreme CO(2) levels required for hot Archaean climates.