Sensitivity of biomarkers to changes in chemical emissions in the Earth’s Proterozoic atmosphere     

J.L. Grenfell  S. Gebauer  M. Godolt  A.B.C. Patzer  H. Rauer 《Icarus》2011,211(1):81-88

The search for life beyond the Solar System is a major activity in exoplanet science. However, even if an Earth-like planet were to be found, it is unlikely to be at a similar stage of evolution as the modern Earth. It is therefore of interest to investigate the sensitivity of biomarker signals for life as we know it for an Earth-like planet but at earlier stages of evolution. Here, we assess biomarkers, i.e. species almost exclusively associated with life, in present-day and in 10% present atmospheric level oxygen atmospheres corresponding to the Earth’s Proterozoic period. We investigate the impact of proposed enhanced microbial emissions of the biomarker nitrous oxide, which photolyses to form nitrogen oxides which can destroy the biomarker ozone. A major result of our work is regardless of the microbial activity producing nitrous oxide in the early anoxic ocean, a certain minimum ozone column can be expected to persist in Proterozoic-type atmospheres due to a stabilising feedback loop between ozone, nitrous oxide and the ultraviolet radiation field. Atmospheric nitrous oxide columns were enhanced by a factor of 51 for the Proterozoic “Canfield ocean” scenario with 100 times increased nitrous oxide surface emissions. In such a scenario nitrous oxide displays prominent spectral features, so may be more important as a biomarker than previously considered in such cases. The run with “Canfield ocean” nitrous oxide emissions enhanced by a factor of 100 also featured additional surface warming of 3.5 K. Our results suggest that the Proterozoic ozone layer mostly survives the changes in composition which implies that it is indeed a good atmospheric biomarker.  相似文献   

Cometary nuclear magnitudes from sky survey observations     

Michael Weiler  Heike Rauer  Christiaan Sterken 《Icarus》2011,212(1):351-366

The determination of the nuclear magnitudes of comets, and with it nuclear size frequency distributions, is strongly complicated by cometary activity. By now, only nuclear size frequency distributions for Jupiter Family comets are available, and they are still subject of uncertainties. For comets of other dynamical classes, nuclear magnitudes are known for only a few comets. The size frequency distributions are thus not well constrained.In this work we study whether nuclear magnitudes of comets can be constrained from sky survey observations as published by the Minor Planet Center. Observations from sky survey programs in which the comet was classified as a point-like source are analyzed in this respect.From the available published observations from 1998 to 2008, we derive nuclear magnitudes, as well as nuclear radii, for 84 comets. Among these are comets of the Jupiter Family, dynamically old and new isotropic comets, Halley-type comets and Centaurs. For Jupiter Family comets and for isotropic comets, the size frequency distributions are presented.Uncertainties of derived nuclear magnitudes arise from photometry and from potentially undetected activity. However, a comparison with objects with well known nuclear parameters shows that, despite substantial observational uncertainties, nuclear magnitudes are constrained to ±0.6 mag, thereby providing first indications for nuclear sizes. This is particularly relevant for isotropic comets with so far ill-constrained size distributions. Exponents of the differential size frequency distributions of for Jupiter Family comets and for isotropic comets are presented. The values derived here form a basis for future, dedicated observational studies which provide higher measurement accuracy.  相似文献   

三峡工程影响的初步评价     

Heike HARTMANN  Stefan BECKER 《湖泊科学》2003,15(Z1):249-254

全世界都在评论三峡工程的修建,这些评论经常是不太客观的.本文从正负两方面对三峡工程进行了讨论,重点在防洪工程和区域经济方面.作者认为修建三峡工程作为发电的功能远大于防洪的功能.尽管上游来的洪峰经过三峡水库调蓄后能够被削弱.但中游的洪水风险仍然很大.一般而言,中游地区的洪水是大降水事件的产物.三峡工程不可能完全控制中游的洪水.三峡水库发电对中国中部及周边地区经济的可持续发展将产生积极作用,也将能确保东部沿海地区尤其是上海市电能的供给.  相似文献