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A study indicating that if Mars could somehow acquire an Earth-like atmospheric …


Biology Articles » Astrobiology » Keeping Mars warm with new super greenhouse gases » Introduction

Introduction
- Keeping Mars warm with new super greenhouse gases

Greenhouse gases may be of some use in making Mars habitable, or at least in sustaining its habitability if otherwise achieved. Previously suggested gases include the chlorofluorocarbons (CFCs; refs. 1 and 2), SF6 (2), and CnF2n+2 (2, 3). Recent detailed calculations for C2F6 and the CFCs (3) have led to optimism about terraforming. Probably some means other than greenhouse gases must be used for the initial warming, even though a few heavy gases may absorb strongly (4) at current Martian pressure (600 Pa). They are unlikely to cover the low-frequency half of the emission spectrum. We introduce some new candidate gases, and we frame our inquiry by asking what mixture of trace gases could sustain an Earth-like temperature if Mars were endowed somehow with an Earth-like atmospheric composition and surface pressure. We suggest that a 70-K greenhouse effect might be maintainable with as little as 5 × 1022 m-2 column amount of a mixture of "designer" greenhouse molecules. This molecular column corresponds to about 240 parts per billion by volume in Earth's atmosphere. Our argument begins with a simple energy balance (5) arising from a two-stream approximation.

&sfgr;&thgr;<SUP>4</SUP><SUB><UP>g</UP></SUB>=F<SUB><UP>S</UP></SUB>(1&plus;3&tgr;&cjs1134;4) [ 1 ]
Here, sigma is the Stefan-Boltzmann constant, thetag is the temperature at the planet's surface, Fs is the downward solar flux, and tau is the gray optical thickness. To obtain tau directly from theta, we must assume no absorption of incoming solar radiation, but our assumption about a window implies the presence of much water, hence of some absorption in the interval 1 to 3 µm, and also the presence of water clouds. The cooling or warming effect of clouds depends in a complex way on their distribution in altitude and latitude (6), so we prefer not to address it.

If we ignore absorption of incoming solar radiation, a fractional increase in thetag is a simple function of tau. Taking the present gray opacity on Mars to be nearly zero, Eq. 1 implies that a gray opacity near tau = 3 should increase the surface temperature to 280 K from the current 210 K. If Mars can acquire an Earth-like atmosphere somehow, the transmission of outgoing radiation at frequencies below 700 cm-1 and above 1,400 cm-1 will be just a few percentage points as on Earth. We therefore direct our attention to filling the window.

In the interval between 700 and 1,400 cm-1, we approximate the spectral transmission of the Earth's atmosphere at a crude resolution (7). We then double the spectral optical thicknesses, because the lower gravity of Mars requires 2.6 times Earth's column airmass to achieve 100 kPa pressure at the surface. Next, we seek to add a mixture of greenhouse gases that will lower the overall transmission through the window to 5% (= e-3).

We considered 21 fluorine compounds, some of which were not previously synthesized, observed, or documented. Fluorine in the bulk composition of Mars has been estimated at 32 ppm by mass vs. 19.4 ppm for the Earth (8). Abundances of the other elements are more than sufficient for our purposes. We performed ab initio calculations to find vibrational fundamental frequencies and band intensities. After estimating bandwidth and selecting five of the candidate molecules, we stepped along the window spectrum at intervals of 1 cm-1 to calculate spectral transmission and derive gray opacity. Then we iterated with the aim of minimizing the required number of molecules of manufactured gas.

Finally, we will demonstrate that Earth-like ozone and oxygen, if present, would shield all of the selected molecules effectively against rapid photolysis, such that their continuous replenishment by synthesis on Mars then might be feasible.


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