The TRAPPIST-1 system provides an extraordinary opportunity to study multiple terrestrial extrasolar planets and their atmospheres. Here, we use the National Center for Atmospheric Research Community ...Atmosphere Model version 4 to study the possible climate and habitability of the planets in the TRAPPIST-1 system. We assume the worlds are ocean-covered, with atmospheres composed of N2, CO2, and H2O, and with orbital and geophysical properties defined from observation. Model results indicate that the inner three planets (b, c, and d) presently reside interior to the inner edge of the traditional liquid water habitable zone. Thus, if water ever existed on the inner planets, they would have undergone a runaway greenhouse and lost their water to space, leaving them dry today. Conversely, the outer three planets (f, g, and h) fall beyond the maximum CO2 greenhouse outer edge of the habitable zone. Model results indicate that the outer planets cannot be warmed, despite having as much as 30 bar CO2 atmospheres, instead entering a snowball state. The middle planet (e) represents the best chance for a presently habitable ocean-covered world in the TRAPPIST-1 system. Planet e can maintain at least some habitable surface area with 0-2 bar CO2, depending on the background N2 content. Near-present-day Earth surface temperatures can be maintained for an ocean-covered planet e with either 1 bar N2 + 0.4 bar CO2, or a 1.3 bar pure CO2 atmosphere.
Twisted graphene bilayers provide a versatile platform to engineer metamaterials with novel emergent properties by exploiting the resulting geometric moiré superlattice. Such superlattices are known ...to host bulk valley currents at tiny angles (α≈0.3°) and flat bands at magic angles (α≈1°). We show that tuning the twist angle to α*≈0.8° generates flat bands away from charge neutrality with a triangular superlattice periodicity. When doped with ±6 electrons per moiré cell, these bands are half-filled and electronic interactions produce a symmetry-broken ground state (Stoner instability) with spin-polarized regions that order ferromagnetically. Application of an interlayer electric field breaks inversion symmetry and introduces valley-dependent dispersion that quenches the magnetic order. With these results, we propose a solid-state platform that realizes electrically tunable strong correlations.
The ultrafast photoinduced ring-opening of 1,3-cyclohexadiene constitutes a textbook example of electrocyclic reactions in organic chemistry and a model for photobiological reactions in vitamin D ...synthesis. Although the relaxation from the photoexcited electronic state during the ring-opening has been investigated in numerous studies, the accompanying changes in atomic distance have not been resolved. Here we present a direct and unambiguous observation of the ring-opening reaction path on the femtosecond timescale and subångström length scale using megaelectronvolt ultrafast electron diffraction. We followed the carbon-carbon bond dissociation and the structural opening of the 1,3-cyclohexadiene ring by the direct measurement of time-dependent changes in the distribution of interatomic distances. We observed a substantial acceleration of the ring-opening motion after internal conversion to the ground state due to a steepening of the electronic potential gradient towards the product minima. The ring-opening motion transforms into rotation of the terminal ethylene groups in the photoproduct 1,3,5-hexatriene on the subpicosecond timescale.
We investigate the atmospheric dynamics of terrestrial planets in synchronous rotation within the habitable zone of low-mass stars using the Community Atmosphere Model. The surface temperature ...contrast between the day and night hemispheres decreases with an increase in incident stellar flux, which is opposite the trend seen in gas giants. We define three dynamical regimes in terms of the equatorial Rossby deformation radius and the Rhines length. The slow rotation regime has a mean zonal circulation that spans from the day to the night sides, which occurs for planets around stars with effective temperatures of 3300-4500 K (rotation period days), with both the Rossby deformation radius and the Rhines length exceeding the planetary radius. Rapid rotators have a mean zonal circulation that partially spans a hemisphere and with banded cloud formation beneath the substellar point, which occurs for planets orbiting stars with effective temperatures of less than 3000 K (rotation period days), with the Rossby deformation radius less than the planetary radius. In between is the Rhines rotation regime, which retains a thermally direct circulation from the day side to the night side but also features midlatitude turbulence-driven zonal jets. Rhines rotators occur for planets around stars in the range of 3000-3300 K (rotation period ∼5-20 days), where the Rhines length is greater than the planetary radius but the Rossby deformation radius is less than the planetary radius. The dynamical state can be observationally inferred from a comparison of the morphologies of the thermal emission phase curves of synchronously rotating planets.
On water‐dominated planets, warming from increased solar insolation is strongly amplified by the water vapor greenhouse feedback. As the Sun brightens due to stellar evolution, Earth will become ...uninhabitable due to rising temperatures. Here we use a modified version of the Community Earth System Model from the National Center for Atmospheric Research to study Earth under intense solar radiation. For small (≤10%) increases in the solar constant (S0), Earth warms nearly linearly with climate sensitivities of ~1 K/(W m−2) and global mean surface temperatures below 310 K. However, an abrupt shift in climate is found as the solar constant is increased to +12.5% S0. Here climate sensitivity peaks at ~6.5 K/(W m−2), while global mean surface temperatures rise above 330 K. This climatic transition is associated with a fundamental change to the radiative‐convective state of the atmosphere. Hot, moist climates feature both strong solar absorption and inefficient radiative cooling in the low atmosphere, thus yielding net radiative heating of the near‐surface layers. This heating forms an inversion that effectively shuts off convection in the boundary layer. Beyond the transition, Earth continues to warm but with climate sensitivities again near unity. Conditions conducive to significant water loss to space are not found until +19% S0. Earth remains stable against a thermal runaway up to at least +21% S0, but at that point, global mean surface temperatures exceed 360 K, and water loss to space becomes rapid. Water loss of the oceans from a moist greenhouse may preclude a thermal runaway.
Key Points
Increasing solar luminosity will render the Earth uninhabitable within ~2 Gyr
A radiative‐convective transition marks the onset of a moist greenhouse
A runaway greenhouse may never occur, precluded by rapid water loss
The conversion of photon energy into other energetic forms in molecules is accompanied by charge moving on ultrafast timescales. We directly observe the charge motion at a specific site in an ...electronically excited molecule using time-resolved x-ray photoelectron spectroscopy (TR-XPS). We extend the concept of static chemical shift from conventional XPS by the excited-state chemical shift (ESCS), which is connected to the charge in the framework of a potential model. This allows us to invert TR-XPS spectra to the dynamic charge at a specific atom. We demonstrate the power of TR-XPS by using sulphur 2p-core-electron-emission probing to study the UV-excited dynamics of 2-thiouracil. The method allows us to discover that a major part of the population relaxes to the molecular ground state within 220-250 fs. In addition, a 250-fs oscillation, visible in the kinetic energy of the TR-XPS, reveals a coherent exchange of population among electronic states.
Instrumental behavior can be controlled by goal-directed action-outcome and habitual stimulus-response processes that are supported by anatomically distinct brain systems. Based on previous findings ...showing that stress modulates the interaction of "cognitive" and "habit" memory systems, we asked in the presented study whether stress may coordinate goal-directed and habit processes in instrumental learning. For this purpose, participants were exposed to stress (socially evaluated cold pressor test) or a control condition before they were trained to perform two instrumental actions that were associated with two distinct food outcomes. After training, one of these food outcomes was selectively devalued as subjects were saturated with that food. Next, subjects were presented the two instrumental actions in extinction. Stress before training in the instrumental task rendered participants' behavior insensitive to the change in the value of the food outcomes, that is stress led to habit performance. Moreover, stress reduced subjects' explicit knowledge of the action-outcome contingencies. These results demonstrate for the first time that stress promotes habits at the expense of goal-directed performance in humans.
Small exoplanets of nearby M-dwarf stars present the possibility of finding and characterizing habitable worlds within the next decade. TRAPPIST-1, an ultracool M-dwarf star, was recently found to ...have seven Earth-sized planets of predominantly rocky composition. The planets e, f, and g could have a liquid water ocean on their surface given appropriate atmospheres of N2 and CO2. In particular, climate models have shown that the planets e and f can sustain a global liquid water ocean, for ≥0.2 bar CO2 plus 1 bar N2, or ≥2 bar CO2, respectively. These atmospheres are irradiated by ultraviolet emission from the star's moderately active chromosphere, and the consequence of this irradiation is unknown. Here we show that chemical reactions driven by the irradiation produce and maintain more than 0.2 bar O2 and 0.05 bar CO if the CO2 is ≥0.1 bar. The abundance of O2 and CO can rise to more than 1 bar under certain boundary conditions. Because of this O2-CO runaway, habitable environments on the TRAPPIST-1 planets entail an O2- and CO-rich atmosphere with coexisting O3. The only process that would prevent runaway is direct recombination of O2 and CO in the ocean, a reaction that is facilitated biologically. Our results indicate that O2, O3, and CO should be considered together with CO2 as the primary molecules in the search for atmospheric signatures from temperate and rocky planets of TRAPPIST-1 and other M-dwarf stars.
Abstract Stress leads to an enhanced activity of the hypothalamus–pituitary adrenal (HPA) axis resulting in an increased release of glucocorticoids from the adrenal cortex. These hormones influence ...target systems in the periphery as well as in the brain. The present review paper describes the impact of the human stress hormone cortisol on episodic long-term memory. Starting out with our early observation that stress as well as cortisol treatment impaired declarative memory, experiments by the author are described, which result in an enhanced understanding of how cortisol influences memory. The main conclusions are that stress or cortisol treatment temporarily blocks memory retrieval. The effect is stronger for emotional arousing material independent of its valence. In addition cortisol only influences memory when a certain amount of testing induced arousal occurs. A functional magnetic resonance imaging (fMRI) study suggests that the neuronal correlate of the cortisol induced retrieval blockade is a reduced activity of the hippocampus. In contrast to the effects on retrieval cortisol enhances memory consolidation. Again this effect is often stronger for emotionally arousing material and sometimes occurs at the cost of memory for neutral material. A fMRI study revealed that higher cortisol levels were associated with a stronger amygdala response to emotional stimuli. Thus stimulatory effects of cortisol on this structure might underlie the cortisol induced enhancement of emotional memory consolidation. The findings presented are in line with models derived from experiments in rodents and are of relevance for our understanding of stress associated psychiatric disorders.
The past 100 years of empirical research in ecology have generated tremendous knowledge about the component interactions that structure ecological communities. Yet, we still lack the ability to ...reassemble these puzzle pieces to predict community responses to perturbations, a challenge that grows increasingly urgent given rapid global change. We summarize key advances in community ecology that have set the stage for modeling ecological systems and briefly review the evolution of ecological modeling efforts to identify critical hurdles to progress. We find that while Robert May demonstrated that quantitative models could theoretically predict community interactions nearly 50 years ago, in practice, we still lack the ability to predict ecological outcomes with reasonable accuracy for three reasons: (1) quantitative models require precise data for parameterization (often unavailable) and have restrictive assumptions that are rarely met; (2) estimating interaction strengths for all network components is extremely challenging; and (3) determining which species are essential to include in models is difficult (model structure uncertainty). We propose that fuzzy interaction webs (FIW), borrowed from the social sciences, hold the potential to overcome these modeling shortfalls by integrating quantitative and qualitative data (e.g., categorical data, natural history information, expert opinion) for generating reasonably accurate qualitative predictions sufficient for addressing many ecological questions. We outline recent advances developed for addressing model structure uncertainty, and we present a case study to illustrate how FIWs can be applied for estimating community interaction strengths and predicting complex ecological outcomes in a multitrophic (plants, herbivores, predators), multi‐interaction‐type (competition, predation, facilitation, omnivory) grassland ecosystem. We argue that incorporating FIWs into ecological modeling could significantly advance empirical and theoretical ecology.
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