Greenhouse nano-particles

A conducting rod, about 9 micrometers long (about half the wavelength of IR light) would forward scatter visible light coming down from the sun, but act as a greenhouse gas to infrared light (IR) light coming back from the surface. Such particles would be lifted into the Martian air, and remain there as dust.

Greenhouse nano-particles are estimated to be 5,000 times more effective than Super Greenhouse Gases, and use materials common on Mars (iron or aluminum), where as super Greenhouse gases require fluorine which is rare on Mars.

It is estimated that such nano-particles would have a 10 year half-life in Mars' atmosphere.

Advantages of this method of terraforming

A key tipping point of terraforming is warming the planet enough that CO2 does not condense at the poles at local winter. (Especially the South Pole which has longer winters than the north due to Mars' elliptical orbit.) A sudden, and intense warming from Greenhouse nano-particles could allow us to reach this tipping point much faster, and cheaper.

These rods have high surface area to weight ratios compared to normal Martian dust and are calculated to remain in the air ~10 times longer than normal Martian dust. When they eventually land on the surface, it is likely that they could be lofted again by a strong wind, just as regular dust is.

160 milligrams of this nano-dust above each square meter of Mars' surface would warm Mars enough to allow the (slow) melting of permafrost in summer. This is obviously a massive project, but it requires less than 1,000 times less industrial effort than a pure super-greenhouse gas strategy. To warm Mars as suggested in this study would require a massive mining operation approximately 1/1,000 of Earth's current mining volume.

Using rods of different lengths will strongly absorb different IR wavelengths. This could be deliberately engineered, or natural variation in the nano-particle manufacturing may be enough to absorb a wide variety of IR wavelengths.

Tho this study looked at iron and aluminum nano-particles, it is possible that carbon nano-tubes would work as well or better.

Disadvantages of this terraforming technique

The major concern is the lifespan of these particles. If water molecules are attracted to them, they may soon leave the Martian air as snow. It is possible that coatings on the particles can make them not attract water, but this would add to the cost of creating them.

These particles are cheaper than super greenhouse gases, but the gases have a much longer lifetime in the Martian air. For example, carbon tetra-fluoride (CF4) is a powerful greenhouse gas which is expected to last 50,000 years.

As terraforming is more successful, more water will enter the atmosphere, and snow and rain will sweep dust (including these rods) from the air. This suggests that other forms of warming the planet will become more important in later phases of terraforming.