Habitable Zone Limits for Dry Planets

Abstract

Most discussion of habitable planets has focused on Earth-like planets with globally abundant liquid water. For an "aqua planet" like Earth, the surface freezes if far from its sun, and the water vapor greenhouse effect runs away if too close. Here we show that "land planets" (desert worlds with limited surface water) have wider habitable zones than aqua planets. For planets at the inner edge of the habitable zone, a land planet has two advantages over an aqua planet: (i) the tropics can emit longwave radiation at rates above the traditional runaway limit because the air is unsaturated and (ii) the dry air creates a dry stratosphere that limits hydrogen escape. At the outer limits of the habitable zone, the land planet better resists global freezing because there is less water for clouds, snow, and ice. Here we describe a series of numerical experiments using a simple three-dimensional global climate model for Earth-sized planets. Other things (CO2, rotation rate, surface pressure) unchanged, we found that liquid water remains stable at the poles of a low-obliquity land planet until net insolation exceeds 415 W/m2 (170% that of modern Earth), compared to 330 W/m2 (135%) for the aqua planet. At the outer limits, we found that a low-obliquity land planet freezes at 77%, while the aqua planet freezes at 90%. High-obliquity land and aqua planets freeze at 58% and 72%, respectively, with the poles offering the last refuge. We show that it is possible that, as the Sun brightens, an aqua planet like Earth can lose most of its hydrogen and become a land planet without first passing through a sterilizing runaway greenhouse. It is possible that Venus was a habitable land planet as recently as 1 billion years ago.

A poll

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Gosh

Rebekah Brooks, Rupert Murdoch's former British CEO, says she is "assisting the police with their inquiries" after being arrested in the British phone hacking and police bribery scandal.

Brooks, 43, was arrested at a London police station at noon Sunday by appointment. She is being questioned on suspicion of conspiring to intercept communications — phone hacking — and on suspicion of corruption, which relates to bribing police for information.

Odd detail noticed in ancient comic

Specifically, in Archie Goodwin and Walt Simonson's '70s revival of Manhunter as a backup feature in Detective Comics, a man targeted for death is responsible for stirring up rebellion in a nation only identified as being Nepal's neighbor. How oddly imprecise.

Is there a reason they wouldn't just say "Tibet", "China" or "India" in a 1973 comic?

Of course, this being DC, there's at least one fictional city-state near Nepal but why would a sympathetic character stir up trouble in Nanda Parbat?