I want to return to Mars this morning because an emerging idea on how to terraform it is in the news. The idea is to block infrared radiation from escaping into space by releasing engineered dust particles about half as long as the wavelength of this radiation, which is centered around wavelengths of 22 and 10 μm, into the atmosphere. Block those escape routes and the possibility of warming Mars in a far more efficient way than has previously been suggested emerges. The paper on this work even suggests a SETI implication (!), but more about that in a moment.

Grad student Samaneh Ansari (Northwestern University) is lead author of the paper, working with among others Ramses Ramirez (University of Central Florida), whose investigations into planetary habitability and the nature of the habitable zone have appeared frequently in these pages (see, for example, Revising the Classical ‘Habitable Zone’). The engineered ‘nanorods’ at the heart of the concept could raise the surface temperature enough to allow survivability of microbial life, which would at least be a beginning to the long process of making the Red Planet habitable.

As opposed to using artificial greenhouse gases, a method that would involve vast amounts of fluorine scarce on the Martian surface, the nanorod approach takes advantage of the properties of the planet’s dust, which is lofted to high altitudes as an aerosol. The authors calculate, using the Mars Weather Research and Forecasting global climate model, that releasing 9-μm-long conductive nanorods made of aluminum “not much smaller than commercially available glitter” would provide the needed infrared blocking that natural dust cannot, and once at high altitude settle more slowly to the surface.

What stands out in the authors’ modeling is that their method is over 5,000 times more efficient than other methods of terraforming, and relies on materials already available on Mars. Natural dust particles, you would think, should warm the planet if released in greater quantities, but the result of doing so is actually to cool the surface even more. Let me quote the paper on this counter-intuitive (to me at least) result:

Because of its small size (1.5-μm effective radius), Mars dust is lofted to high altitude (altitude of peak dust mass mixing ratio, 15 to 25 km), is always visible in the Mars sky, and is present up to >60 km altitude (14–15). Natural Mars dust aerosol lowers daytime surface temperature [e.g., (16)], but this is due to compositional and geometric specifics that can be modified in the case of engineered dust. For example, a nanorod about half as long as the wavelength of upwelling thermal infrared radiation should interact strongly with that radiation (17).

Edwin Kite (University of Chicago) is a co-author on the work:

“You’d still need millions of tons to warm the planet, but that’s five thousand times less than you would need with previous proposals to globally warm Mars. This significantly increases the feasibility of the project… This suggests that the barrier to warming Mars to allow liquid water is not as high as previously thought.”

Image: This is Figure 3 from the paper. Caption: The proposed nanoparticle warming method. Figure credit: Aaron M. Geller, Northwestern, Center for Interdisciplinary Exploration and Research in Astrophysics + IT-RCDS.

Strikingly, the effects begin to emerge quite quickly. Within months of the beginning of the process, atmospheric pressure rises by 20 percent as CO2 ice sublimes, creating a positive warming feedback. Note this from the paper:

On a warmed Mars, atmospheric pressure will further increase by a factor of 2 to 20 as adsorbed CO2 desorbs (35), and polar CO2 ice (36) is volatilized on a timescale that could be as long as centuries. This will further increase the area that is suitable for liquid water (6).

That said, we’re still not in range for creating a surface habitable by humans. We have to deal with barriers to oxygenic photosynthesis, including the makeup of the Martian sands, which are laden with potentially toxic levels of nitrates, and an atmosphere with little oxygen. Toxic perchlorates in the soil would require ‘bioremediation’ involving perchlorate-reducing bacteria, which yield molecular oxygen as a byproduct. We’re a long way from creating an atmosphere humans can breathe, but we’re in range of the intermediate goal of warming the surface, possibly enough to sustain food crops.

Addendum: I made a mistake above, soon caught by Alex Tolley. Let me insert his comment here to straighten out my mistake:

“… which are laden with potentially toxic levels of nitrates,”

I think you misinterpreted the sentence from the paper:

“…is not sufficient to make the planet’s surface habitable for oxygenic photosynthetic life: barriers remain (7). For example, Mars’ sands have ~300 ppmw nitrates (37), and Mars’ air contains very little O2, as did Earth’s air prior to the arrival of cyanobacteria. Remediating perchlorate-rich soil…”

300 ppm nitrates is very low and will not support much plant or bacterial life. [You want ~ 10,000 ppm ] That is why N and P are added to simulated Mars regolith when testing plant growth for farming or terraforming. IIRC, there have been suggestions of importing nitrogen from Titan to meet its needs on Mars.

Thanks for catching this, Alex!

Although nanoparticles could warm Mars… both the benefits and potential costs of this course of action are now uncertain. For example, in the unlikely event that Mars’ soil contains irremediable compounds toxic to all Earth-derived life (this can be tested with Mars Sample Return), then the benefit of warming Mars is nil. On the other hand, if a photosynthetic biosphere can be established on the surface of Mars, perhaps with the aid of synthetic biology, then that might increase the Solar System’s capacity for human flourishing. On the cost side, if Mars has extant life, then study of that life could have great benefits that warrant robust protections for its habitat. More immediately, further research into nanoparticle design and manufacture coupled with modeling of their interaction with the climate could reduce the expense of this method.

That’s a robust way forward, one the authors suggest could involve wind tunnel experiments at Mars pressure to analyze how both dust and nanomaterials are released from modeled Mars surfaces, from dusty flat terrain to the ice of the poles. Large eddy simulations (LES) are ways to model larger flows such as winds and weather patterns. Deploying these should be useful in learning how the proposed nanorods will disperse in the atmosphere, while local warming methods also demand consideration.

A question I had never thought to ask about terraforming was how long the effects can be expected to last, and indeed the authors point out how little is known about long-term sustainability. A 2018 paper on current loss rates in the Martian atmosphere suggests that it would take at least 300 million years to fully deplete the atmosphere. The big unknown here is the Martian ice, and what may lie beneath it:

…if the ground ice observed at meters to tens of meters depth is underlain by empty pore space, then excessive warming over centuries could allow water to drain away, requiring careful management of long-term warming. Subsurface exploration by electromagnetic methods could address this uncertainty regarding how much water remains on Mars deep underground.

Image: Will we ever get to this? The ‘nanorod’ approach oculd be the beginning. Credit: Daein Ballard, Wikimedia Commons CC BY-SA 3.0.

The SETI implication? Nanoparticle warming is efficient, so much so that we might expect other civilizations to use the technique. A potential technosignature emerges in the polarization of light, because a terrestrial world with a magnetic field will show the interaction of polarized light with the planet’s atmosphere, the latter conceivably laden with the nanoparticles at work in terraforming. Polarization will occur when light interacts with nanoparticles, aerosols, or dust in the atmosphere or the magnetic field. This would be an elusive signature to spot, but not outside the range of possibility.

In the absence of an active geodynamo to drive a magnetic field, Mars would not be a candidate for this kind of remote observation. But an exoplanet of terrestrial class with a magnetic field should, by these calculations, be a candidate for this kind of study.

The paper is Ansari et al., “Feasibility of keeping Mars warm with nanoparticles,” Science Advances Vol. 10, No. 32 (7 August 2024). Abstract / Preprint. Thanks to Centauri Dreams reader Ivan Vuletich for the pointer to this paper.