Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p13c1332t&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P13C-1332
Mathematics
Logic
5416 Glaciation, 5418 Heat Flow, 5422 Ices, 5455 Origin And Evolution, 6296 Extra-Solar Planets
Scientific paper
Existence of liquid water on the planetary surface is essential for life. However, terrestrial planets with abundant water have multiple climate modes, including an ice-free, a partially ice-covered, and a globally ice- covered state, even when the incident flux from the central star and the abundance of greenhouse gasses in the atmosphere are the same. This multiplicity of climate mode is derived from large difference in the albedo of ice and water. Recent geological studies have revealed that the Earth experienced global glaciations ("snowball Earth" events) in its history. In the snowball glaciations, liquid water is thought to have existed under the ice shell because of geothermal heat flow from the Earth"fs interior. By analogy with the snowball glaciations, I discuss the conditions for an extrasolar terrestrial planet which is covered with ice but has an internal ocean for the timescale of planetary evolution owing to geothermal heat flow from the planetary interior. I show that liquid water can exist if the planetary mass and the water abundance are comparable to the Earth, although a planet with a mass of <0.4 Me (Me is the Earth's mass) would not be able to maintain the internal ocean. Liquid water would be absolutely stable for a planet with a mass of >4 Me (i.e., super- Earth), irrespective of planetary orbit and luminosity of the central star. It is therefore implied that super-Earth inevitably have liquid water either on its surface (for the ice-freee or partially ice-covered modes) or beneath the ice (for the globally ice-covered mode). Searches for terrestrial planets in extrasolar planetary systems should consider such a "snowball planet", which is a possible type of water-rich terrestrial planet other than an Earth-like "ocean planet". Because a snowball planet is much brighter than (more than twice) an ocean planet with the same size, it would be a good target for the astronomical observation in the future.
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