Williams College

About: Williams College is a education organization based out in Williamstown, Massachusetts, United States. It is known for research contribution in the topics: Population & Politics. The organization has 2257 authors who have published 5015 publications receiving 213160 citations. The organization is also known as: Williams.

Changes in Pluto’s Atmosphere: 1988-2006

Abstract: The 2006 June 12 occultation of the star P384.2 (2UCAC 26039859) by Pluto was observed from five sites in southeastern Australia with high-speed imaging photometers that produced time-series CCD images. Light curves were constructed from the image time series and fit by least-squares methods with model light curves. A new modeling procedure is presented that allows a simultaneous fit of the atmospheric parameters for Pluto and the astrometric parameters for the occultation to all of the light curves. Under the assumption of a clear atmosphere and using this modeling procedure to establish the upper atmosphere boundary condition, immersion and emersion temperature profiles were derived by inversion of the Siding Spring light curve, which had our best signal-to-noise ratio. Above ~1230 km radius, atmospheric temperatures are ~100 K and decrease slightly with altitude—the same as observed in 1988 and 2002. Below 1210 km, the temperature abruptly decreases with altitude (gradients ~2.2 K km-1), which would reach the expected N2 surface-ice temperature of ~40 K in the 1158-1184 km radius range. This structure is similar to that observed in 2002, but a much stronger thermal gradient (or stronger extinction) is implied by the 1988 light curve (which shows a "kink" or "knee" at 1210 km). The temperature profiles derived from inversion of the present data show good agreement with a physical model for Pluto's atmosphere selected from those presented by Strobel et al. (1996). Constraints derived from the temperature profiles (and considering the possibility of a deep troposphere) yield a value of 1152 ± 32 km for Pluto's surface radius. This value is compared with surface-radius values derived from the series of mutual occultations and eclipses that occurred in 1985-1989, and the limitations of both types of measurements for determining Pluto's surface radius are discussed. The radius of Pluto's atmospheric shadow at the half-intensity point is 1207.9 ± 8.5 km, the same as obtained in 2002 within measurement error. Values of the shadow radius cast by Pluto's atmosphere in 1988, 2002, and 2006 favor frost migration models in which Pluto's surface has low thermal inertia. Those models imply a substantial atmosphere when New Horizons flies by Pluto in 2015. Comparison of the shape of the stellar occultation light curves in 1988, 2002, and 2006 suggests that atmospheric extinction, which was strong in 1988 (15 months before perihelion), has been dissipating.

Coccolith Sr/Ca records of productivity during the Paleocene-Eocene thermal maximum from the Weddell Sea

Abstract: [1] A major perturbation of the global carbon cycle ∼55 million years ago, believed to result from release of 1000–2000 Gt of C from methane hydrates, correlates with an intense but transient greenhouse warming event known as the Paleocene Eocene Thermal Maximum (PETM). The rapid (105 years) recovery of global temperatures reflects important negative feedbacks in the climate system and carbon cycle. Enhanced marine productivity may be one important feedback, but indicators for productivity changes have yielded conflicting results. Here we use a new independent indicator, Sr/Ca in coccolith carbonate, which covaries with the productivity of coccolithophorid algae, to investigate the biotic response in the most complete PETM deep sea record which was recovered at ODP Site 690 in the Weddell Sea. In the dominant coccolithophorid genus Toweius a large (40%) Sr/Ca increase immediately after the gas hydrate release signals a dramatic productivity increase. Productivity levels remained high for 60,000 years but decreased to pre-event levels by 120,000 years after the gas hydrate release. Productivity levels during the PETM are higher than observed at any other time in our ∼400,000 year record. Other coccolithophorid genera Chiasmolithus and Discoaster show a brief modest (25% Sr/Ca increase) increase in productivity that lags behind the methane event by 50,000 years and is within the range of productivity variation elsewhere in the record. The timing of the Toweius productivity increase agrees well with Os isotope records of globally increased weathering intensity, which may have provided higher nutrient fluxes to stimulate algal productivity. If this type of productivity response occurred globally, it would also be consistent with the timing of C drawdown that may have returned temperatures to near pre-event levels.

Macro Financial Determinants of the Great Financial Crisis: Implications for Financial Regulation

Abstract: By analysing the macro financial determinants of the Great Financial Crisis of 2007-2009 on 83 countries, we find that the probability of suffering the crisis in 2008 was larger for countries having higher levels of credit deposit ratio whereas it was lower for countries having higher levels of: i) net interest margin, ii) concentration in the banking sector, iii) restrictions to bank activities, iv) private monitoring. Our findings contribute to the ongoing discussion that can help policymakers calibrate new regulation, by achieving a reasonable trade-off between financial stability and economic growth.