Terrestrial planets under extreme radiative forcings: applications to habitable zones, early Mars, and a high-co2 Earth

Open Access
Ramirez, Ramses Mario
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
February 20, 2014
Committee Members:
  • James Kasting, Dissertation Advisor
  • Christopher Howard House, Committee Member
  • Eugene Edmund Clothiaux, Committee Member
  • Steinn Sigurdsson, Committee Member
  • habitable zones
  • habitability
  • Mars
  • collision-induced absorption
  • hydrogen
  • runaway greenhouse
  • carbon dioxide
The CO2-H2O habitable zone is defined as the region around a star where liquid water is stable on a planetary surface. The inner edge is defined by the initiation of a wet stratosphere followed by escape of water to space, whereas the location of the outer edge is determined by the maximum greenhouse effect of CO2. Previous calculations have shown that a conservative estimate of the habitable zone is relatively wide at ~0.95-1.67 AU (Kasting et al., 1993). A wide habitable zone facilitates the search for extraterrestrial life because the probability of finding potentially habitable planets increases, relaxing design specifications for observing telescopes. In our own solar system, the solar flux received by early Mars 3.8 Ga is only slightly less than that predicted at the outer edge, suggesting that the Red Planet may have once been habitable if it contained additional greenhouse gases in its atmosphere. In the case of Earth, our close proximity to the inner edge implies susceptibility to various climatic catastrophes at elevated temperatures. The calculations of Kasting et al. have become obsolete as line-by-line absorption databases are continually updated with newly discovered lines, leading to increased absorption for key greenhouse gases such as CO2, H2O, CH4, and H2. This suggests that current climate models may just be able to produce warm conditions for early Mars, although it also means that life on Earth may be more vulnerable than previously thought. The aim of this Thesis is to update the 1-D climate model of Kasting et al., derive updated habitable zone boundaries and use them to a) refine the telescopic search for life in exoplanetary systems, b) assess whether early Mars may have once manifested a warm and wet climate, and c) evaluate the implications for the habitability of the Earth as a result of continued increases in CO2. With our new climate model we find: a) that these new absorption databases move the inner edge past the orbit of Earth, demonstrating the inadequacy of 1-D models, b) 5 – 20% H2, along with 1.3 – 4 bar CO2, generates above freezing mean temperatures for early Mars, and c) a runaway greenhouse cannot be triggered from increased fossil fuel emissions. All of these problems should be revisited using 3-D models.