Mathematics – Logic
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt........23w&link_type=abstract
Thesis (PHD). THE PENNSYLVANIA STATE UNIVERSITY , Source DAI-B 59/06, p. 2806, Dec 1998, 140 pages.
Mathematics
Logic
5
Scientific paper
The recent discoveries of extrasolar planets have generated widespread anticipation of detecting a life-supporting environment, such as an Earth-like planet or moon, around a nearby solar-type star. Future observations will enable life on such worlds to be detected remotely through the spectral identification of CH4 and O3 in their atmospheres. This thesis addresses the climatic and dynamic factors affecting whether an Earth-like biosphere might exist around another star and, hence, the likelihood that extraterrestrial life will be discovered in the foreseeable future. To remain habitable for billions of years, a planetary body must be large enough to form and retain an atmosphere. Earth's Moon (~0.01M⊕) does not satisfy this basic criterion. Objects with atmospheres must orbit their stars within the habitable zone (HZ) for liquid water to exist on their surfaces. Otherwise habitable worlds can have their climates destabilized by the slow brightening of their-stars as the age, or by chaotic variability of their orbits and obliquities over time. Earth's 23.5o-obliquity is presently stable, but the spin-stability of extrasolar Earths will depend on the masses and proximity of satellites and neighboring planets. Climates of planets with high obliquities are investigated using an energy-balance climate model. At high obliquity, Earth's climatic zonation is reversed so that the lower latitudes are permanently frozen and the poles are subjected to extraordinary swings in seasonal temperature. Planets within the outer HZs around their stars are less affected by obliquity because they develop dense-CO2 atmospheres as a result of the carbonate-silicate geochemical cycle. Efficient heat transport within such atmospheres reduce latitudinal temperature gradients and limit the amplitudes of seasonal temperature extremes. Geologic evidence for low-latitude glaciation during the Precambrian era suggests that the obliquity of early-Earth may have been much higher than it is today. Earth's obliquity could have been reduced to its present value as a consequence of obliquity-oblateness feedback. In this process, obliquity-driven changes to continental ice volume and oblateness may have caused a secular downward drift in obliquity of ~30o between 600 Ma and 500 Ma. Such an event may account for the present non-zero inclination of the lunar orbit.
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