Super Greenhouse Gases

Super Greenhouse Gases (SGG) are hundreds or thousands of times more powerful than CO₂ in warming planets, and are regulated on Earth for that reason. On Mars, which is too cold, long lived Super Greenhouse Gases (SGG) are considered an economic and desirable way to warm the planet. Types of gases which are long lived under Martian conditions are especially valuable for this purpose. ^[1] ^[2] ^[3] ^[4]

Discussion of Greenhouse Gases

Planetary atmospheres warm planets by allowing light to hit the world, but slows the radiation of infrared (heat energy) leaving the world. (This is known as the Greenhouse effect.) Without our atmosphere, Earth would have a sub freezing temperature of -10 C. However, not all gases warm planets equally. Some such as oxygen (O₂), and nitrogen (N₂) are transparent to heat energy. More complex molecules tend to slow the radiation of heat to space. Carbon dioxide (CO₂) is a Greenhouse gas which is causing the Earth to warm, as it concentration increases in Earth's atmosphere. The strength of other green house gases are measured relative to carbon dioxide. For example: methane is 80 times more powerful than CO₂ during the 20 years it is expected to remain in the atmosphere. Water (H₂O) is a powerful greenhouse gas, but it rapidly leaves the atmosphere as rain and snow. Carbon dioxide remains in the air for a long time. (CO₂ is expected to last in the air for about 200 years, when it is typically absorbed by some sort of plant. However, the CO₂ is returned to the air when the life rots a few years later. To draw down the CO₂ permanently, it needs to be removed from the atmosphere AND the biosphere.) If we introduce life onto Mars, methane (CH₄) would be added to the atmosphere which also is a greenhouse gas.

Many gases will help retain heat, but each is best at trapping specific wavelengths of infrared radiation. To warm Mars, we would wish to pick gases which block 'windows' in the spectrum where heat can escape from Mars. (There is plenty of CO₂ on Mars, and if it warms, then water (H₂O) will also be more common in the air. So gases which block wavelengths that these two compounds don't, are of special interest.

See also Greenhouse nano-particles for another way to warm the atmosphere.

Super Greenhouse Gases (SGG)

If kilotonnes of gases are to be created then we want to create compounds which will be stable in the Martian atmosphere for many decades or centuries. Mars is bombarded by Ultraviolet light the most energetic of which has energies sufficient to break molecular bonds. Fluorine (F) has the strongest chemical bonds, so its compounds are ideally suited for SGG in Mars' atmosphere. Unfortunately, fluorine is fairly rare. Chlorine (Cl) is chemically similar and much more common and cheaper, so some fluorine atoms may be replaced with Cl, as a cheaper, less long lived substitute. However, Chlorine can destroy the ozone layer, so in the long term it should be avoided. Several industrial refrigerants such as 1,1,1-Trichloroethane, have been suggested, but with Cl as part of the structure, they will fight the goal of building up Mars' Ozone layer.)

Because these gases absorb different parts of the infrared spectrum, it is likely that the ideal solution for warming Mars would be a mix of several chemicals. (Mixtures of CF₄, C₂F₆, C₃F₈, and SF₆, are the most common gases looked at in the literature.)

Large molecules tend to have better warming potential (they can hold heat by more vibration modes between the atomic bonds), but are more likely to be broken up by Ultraviolet light, so they last for shorter periods in the atmosphere. Unless a short term 'burst' of warming is wanted, longer lived molecules will likely be preferred.

Fogg, in his book on Terraforming, points out that these gases are most useful at low concentrations. (At higher concentrations, you reach a region of diminishing returns.) This suggests that SGG would be used as a first step in terraforming, useful in the initial goal of destabilizing the South Pole CO₂ ice cap.

Climate modelling suggests that the warming would be strongest near the equator and in low lands. If we got a 20K increase in temperature planet wide, at the bottom of the Hellas basin would be about double that. (A five degree increase at the South Pole would destabilize the southern carbon dioxide ice cap. Note that the South Pole is 1 to 3 km above the average height of the planet, and is far from the equator, so warming will be lower there. I've not been able to find what the MINIMUM average temperature would destabilize the antarctic CO₂, but in several papers authors have said that an average 20K increase would be more than enough.)

The following table shows several chemicals, their greenhouse gas warming potential (relative to CO₂), and their expected lifetime in the Martian air. (Relative warming looks at how much stronger the greenhouse gas is compared to CO₂, over a 100 year period.) The lifespan is for Earth, there the lower atmosphere is protected by the Ozone layer. Mars gets 43% as much sunlight as Earth (so gases will last longer), but lacks an ozone layer (so the gases will break up more quickly). If life on Mars develops an ozone layer, the lifespan there will be higher than on Earth, for now it is likely to be lower.


Name	Formula	Relative Warming	Lifespan (Years)	Notes
Carbon Tetrafluoride	CF₄	5,700 (times more than CO₂)	50,000 y	Also known as Tetrafluoromethane, or R-14
Hexafluoroethane	C₂F₆	11,900 or 9,500?	10,000 y	Etchant in semiconductors. Also a refrigerant when mixed with trifluoromethane.
Octafluoropropane	C₃F₈	8,600 or 24,000?	2,600 y	Used in semiconductor production. Other than lower life span, an ideal gas.
Octafluorocyclobutane	C₄F₈	10,000	3,200 y	Ring of 4 carbon atoms with Fl everywhere else. Semiconductors, refrigerant.
Perflubutane	C₄F₁₀	8,600	2,600 y	Fire extinguishers, and used in ultrasound contrast agents.
Sulfur Hexafluoride	SF₆	22,200	3,200 y	Used in transformers and Magnesium production
Chlorotrifluoromethane	CF₃Cl	14,000? or 10,800?	640 y	Chlorine can destroy ozone layer which protects these gases
Fluoroform	CHF₃	12,000	260 y	Used in semiconductor industry and as a refrigerant
Nitrogen Trifluoride	NF3	17,200? or 12,300?	740 or 550 years?	Semiconductors, toxic in high concentrations (~1,000 ppm).

Note: different references have very different values for the relative warming. First value is from IPCC report (100 year warming potential), second value is the one most commonly found. Trying to resolve why some of these values are so different.

The very long lifespan of carbon tetrafluoride (CF₄) and its high relative warming makes it attractive.

The paper "Keeping Mars Warm with New Super Greenhouse Gasses":

https://www.pnas.org/content/98/5/2154

.. looked at a number of chemicals, and studied the following 5 gases as the most likely useful:

SF₄(CF₃)₂
CR₃CF₂CF₃
CF₃SCF₂CF₃
CF₅CF₃
SF₆

These were picked since they covered a lower set of frequencies than the carbon - fluoride compounds listed above. However the large complex molecules broke down more quickly, and were intended to be used after Mars had an ozone layer to protect them.

After studying these gases they suggested that sulphur hexafluoride (SF₆) was the best to use of the ones studied.

This paper: Radiative-convective model of warming Mars with artificial greenhouse gases ^[5]

.. suggested that C₃F₈ had an absorption spectrum which was well suited to the lower pressure atmosphere of Mars. (They said on Mars, it had a higher relative warming potential, higher than SF₆.) Given that this would be an ideal gas, other gases were compared to it under Martian conditions and it was found that:

-- CF₄ was 17% as good as C₃F₈,

-- C₂F₆ was 49% as good, and

-- SF₆ was 48% as good as C₃F₈.

An ideal mixture of these 3 gases with C₃F₈ was 16% more effective at warming the planet than C3F8 alone.

They concluded that their energy balance calculations suggest that the addition of ∼0.2 Pa of the best greenhouse gases mixture, or ∼0.4 Pa of C₃F₈ alone, would shift the equilibrium by 20 degrees K. This would be enough that CO₂ would no longer be stable at the Martian poles and a runaway greenhouse effect would result. (Another study I read suggested ~1 Pa would be needed for this large of an increase in temperature, but they used a different mix of gases.)

Compounds After the Breakdown of SGG

Carbon tetrafluoride (CF₄) is an ideal greenhouse gas, but eventually it will be broken down by Ultraviolet (UV) light by losing a Fluorine atom. The fluorine is HIGHLY reactive, likely combining with CO₂ to form COF (which still missing a valence electron, so it will perhaps collect an H, or an OH eventually). The CF₃ will eventually form another molecule such as CF₃OH or CHF₃.

These 'frangment' molecules will also be greenhouse gases, but little is know about them as far as their lifetime in the Martian atmosphere, or their greenhouse potential. This should be a subject of future study.

Effects of terraforming on Super Greenhouse Gases (SGG)

The hope of adding a significant amount of these compounds to the air, is to warm Mars for terraforming purposes. As the planet warms, the atmosphere will thicken (warming it further) and water (also a greenhouse gas) will become stable on the Martian surface. If plants, or cyanobacteria are introduced, Mars will slowly start building up oxygen in the atmosphere.

Oxygen (O₂) will be broken up by UV light, and form Ozone (O₃) which strongly absorbs hard Ultraviolet light. This will reduce the UV flux on the planet's surface making it friendlier to plant life, AND it will slow the break up of super greenhouse gases. Thus instead of lasting thousands of years before breakup, SGG should last much longer. This is a positive feedback loop (as terraforming progresses, terraforming becomes easier).

Cost of Creating These Gases

As Martian industry expands, these gases will be created for various purposes. For example, SF6 will be created for use in electrical transformers. On Earth, great care (and expense) must be taken to prevent it from leaking into the air. On Mars such effort is not needed since we WANT the planet to warm. Thus small amounts of these chemicals will inevitably build up, for 'free', in the Martian atmosphere as Mars' industry develops.

That said, here are the 2021 prices for some of the compounds listed above:

CF4.. $7,000 / tonne. This chemical is very easy to produce, (just burn carbon in fluorine). The high price likely reflects no widespread industrial use.
C2F6.. $150 / tonne
C3F8.. $500 to $700 / tonne
SF6.. $6,000 / tonne
CHF3.. $45,000 / tonne

(Note, these prices are from quick google searches. Someone more knowledgable please correct these values if they are off.)

If industries developed to mass produce the gases by the kilotonne, these prices would drop.

The 'Keeping Mars Warm' paper discussed above suggests that no more than 170 kilotonne per Earth year would be needed to make up for the loss of these gases by UV photolysis. If we assume the average cost of these gases is $1,000 / tonne, and we want to produce 500 kilotonnes / year to build up the concentration, then the price to engineer Mars' atmosphere would be $500 million per year. This price would drop to a maintenance level of about 1/3 that, once we reach the target temperature.

References

"Terraforming: Engineering Planetary Environments", by Martin J. Fogg, ISBN 1-56091-609-5.

Wanke, H. & Dreibus, G. (1988) Phil. Trans. R. Soc. London A 235, 545–557. // Paper suggests fluorine may be more common on Mars than on Earth.

// Paper discussing which SGG would be idea for terraforming Mars.

https://www.pnas.org/content/98/5/2154

M. Gerstell, J. Francisco, Y. Yung, C. Boxe, and E. Aaltonee. Keeping mars warm with new super greenhouse gases. Proceedings of the National Academy of Sciences, 98(5):2154–2157

Y. L. Yung and W. B. DeMore. Photochemistry of planetary atmospheres. Oxford University Press, 1999. ISBN 9780195105018. URL http://books.google.com.au/books?id=Q4pHLv9TvksC.