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

• Abstract
• Introduction
• Infrared Quantum-Mechanical Calculations
• Required Greenhouse Columns
• Photolysis
• Discussion
• References
• Figure
• Tables

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 C_nF_2n+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 m² 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.

[ 1 ]

Here,  is the Stefan-Boltzmann constant, _g is the temperature at the planet's surface, F_s is the downward solar flux, and  is the gray optical thickness. To obtain  directly from , 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 _g is a simple function of . Taking the present gray opacity on Mars to be nearly zero, Eq. 1 implies that a gray opacity near  = 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 cm¹ and above 1,400 cm¹ 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 cm¹, 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% (= e³).

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 cm¹ 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.