Main article: Anti Greenhouse Technology
Unlike conventional Greenhouse Gases, used to increase global temperature of an Outer Planet, the anti-greenhouse gasses are used to cool an Inner Planet. Please note that there are not many known anti-greenhouse gasses and their effect on the environment is not well studied. The way these gasses work is completely different from the way greenhouse gasses work.
Any gas has a certain greenhouse effect, only that the effect is far greater for some gasses compared to others. The greenhouse effect of an atmosphere is the cumulative effect of all gasses in the atmosphere:
G = G1*m1 + G2*m2 + G3*m3 + ... + Gn*mn
where G is the cumulative greenhouse effect, G1...Gm is the greenhouse effect of each gas in the atmosphere and m1...mn is the mass of each gas in the atmosphere.
From this formula, one can see that adding a new gas will only increase the greenhouse effect. Therefore, no gas, added in small or large amounts, will be able to cool a planet.
On any planet, there is an equilibrium state between the energy it receives from its parent star and the energy it dissipates into space. Some planets have also an internal source of heat, but for a terraformed rocky world, that is negligible. The only difference is the form this energy moves. A star sends 3 types of radiation: infrared, visible and ultraviolet. The difference is that both the atmosphere and surface transforms part of visible and ultraviolet light into infrared. Greenhouse gasses do two things: they reflect infrared light from the star back to the cosmos and infrared waves from the planet back to the surface. The stronger the greenhouse effect is, the more infrared remains trapped in the atmosphere and less is radiated to the cosmos. Greenhouse gasses decrease the percent of heat radiated to the cosmos. The planet warms up, until a new equilibrium is achieved, but the percent of radiated heat from the heat of the atmosphere remains constant.
Anti-greenhouse technologies work in two different ways: they reflect infrared from the star (preventing for a short period of time the planet to heat-up) or they reflect part of the incoming visible or ultraviolet light, which would be transformed into infrared on the surface or the atmosphere. Some gasses work on their own, while others create hazes, helping water to condensate.
Greenhouse and anti greenhouse effect
Greenhouse gasses allow light to pass through them, but reflect infrared (heat). This way, heat radiated from the surface is reflected back towards the planet. In addition, infrared radiation from the sun is also reflected, but towards the cosmos. Ironically, Venus reflects almost all light it receives (visible or infrared), but the tiny amount of radiation that penetrates the atmosphere is transformed into heat and is trapped.
Anti greenhouse gasses should reflect heat from the sun, but allow heat from the planet to radiate into space. This can be possible in a few ways.
A few examples
Dimethylsulfide, also known as DMS, is a sweet-smelling gas, produced by plankton, that has an influence in creating clouds above the ocean . This gas is short lived. In theory, DMS can be used to create clouds in the morning, while during night, the gas would have disintegrated, allowing heat to escape into space.
Sulfur dioxide is another anti-greenhouse gas. It has a half life of a few years. It can be dumped into the upper atmosphere above equator. The gas reflects part of solar light and heat. However, if sulfur dioxide remains for long and covers the whole planet, it will also have a greenhouse effect, because it also reflects heat that radiates from the planet. Sulfur dioxide depletes ozone and can cause acid rains.
Helium conducts heat much faster then nitrogen or oxygen and has a very low greenhouse effect. Building an atmosphere based on helium as an inert gas could be a solution to terraform a planet heated like Venus. However, helium is a light gas and it escapes into space. Inside the atmosphere, it separates into an upper layer. Living organisms need nitrogen for proteins, but still, an atmosphere that contains 50% helium, 30% nitrogen and 20% oxygen would be acceptable. Helium can be used indirectly for Micro Helium Balloons.
Theoretical gasses (used in science fiction, real gasses with similar effects are very expensive, unstable or toxic):
- A gas that reflects infrared when exposed to visible or ultraviolet light (so that it will reflect solar radiation during the day and cool the planet during night).
- A gas that allows only a band of infrared light to pass (the wavelength radiated from the planet), but reflects much of the near infrared (wavelength of the star).
- A gas that reflects visible light but is permeable for far infrared (it prevents visible light from reaching the planet).
- A gas that changes heat reflectivity according to average temperature (becomes infrared opaque during the heat of the day, but then changes into transparent during the cold nights). By using this gas, temperature will grow fast during daytime, until a maximum is achieved. Beyond that, temperature will grow slowly. During night, heat is trapped until temperature drops below a certain level, when the gas becomes permeable for heat and temperature drops fast.
Anti greenhouse gasses have some major problems. They are short lived and need to be replenished daily or at every year, resulting in high maintenance costs. Some of them can deplete ozone layers or cause acid rains. Uncontrolled, the gasses or hazes they create can generate the opposite, a greenhouse effect. Also, there is a risk that, these gasses can send a planet into an ice age. Ice reflects much more light and when a planet becomes covered with ice, it will stay cooler for long.