how to heat mars
The great desire of some individuals to colonize Mars may seem crazy, especially considering the many factors that do not contribute to normal life on the red planet. One of these problems is the temperature, which on the surface of Mars fluctuates between +20 °C and -153 °C. It is also known that a third of the surface of Mars has shallow H2Oh, but because of the temperature, this resource is useless, roughly speaking. That's why scientists from all over the world have started thinking about how to heat Mars. Some ideas are quite effective on paper, but their implementation requires both infrastructure and materials that are not available on Mars. But scientists from the University of Chicago (Chicago, USA) have proposed a method that will use components native to the red planet on a nanoscale. What is the essence of the method, how does it work, and how effective is it? We will learn the answers to these questions from the scientists' report.
Research basis
Dry river valleys crisscross the once habitable surface of Mars, but today the icy soil is too cold for Earth-like life. The streams may have been active about 600,000 years ago, suggesting that Mars may have been habitable to some degree. Many methods have been proposed to warm the surface of Mars by closing the spectral windows centered around wavelengths (λ) of 22 and 10 μm, through which the surface is cooled by thermal infrared radiation rising into space. Modern Mars has a thin (~6 mbar) CO atmosphere
2
which provides only ~5 K of greenhouse effect due to absorption in the 15 μm range, and Mars apparently lacks sufficient condensed or mineralized CO
2
to restore a warm climate. Spectral windows can be closed using artificial greenhouse gases (e.g., chlorofluorocarbons), but this would require the volatilization of ~100,000 megatons of fluorine, which is rare on the Martian surface. An alternative approach is proposed using the natural Martian dust aerosol. Almost all Martian dust ultimately forms as a result of the slow grinding of iron-rich minerals on the Martian surface. Due to its small size (effective radius 1.5 μm), Martian dust rises to high altitudes, is always visible in the Martian sky, and is present at altitudes up to > 60 km. The natural Martian dust aerosol reduces daytime surface temperatures, but this is due to compositional and geometrical features that can be modified in the case of artificial dust. For example, a nanorod with a length of about half the wavelength of upwelling thermal infrared radiation should strongly interact with this radiation.
Research results
Image #1
The scientists propose to consider a 9 µm long conducting nanorod (in this paper, aluminum and iron are considered) with an aspect ratio of ~60:1. Finite difference time-domain calculations show that such nanorods, randomly oriented due to Brownian motion, would strongly scatter and absorb upward thermal infrared radiation in spectral windows and scatter sunlight downward toward the surface, resulting in net warming (plot above). The results are robust to variations in particle material type, cross-sectional shape, and mesh resolution, and vary as expected with particle length and aspect ratio. The calculated thermal infrared scattering is nearly isotropic, contributing to surface warming. Such nanorods would settle in the Martian atmosphere 10 times slower than natural Martian dust, meaning that once the particles are airborne, they will be at high altitudes and have a long atmospheric lifetime.
Image #2
This motivates the calculation of surface warming (K) as a function of the (artificial) aerosol density of the rods (kilograms per square meter). The global climate model Mars Weather Research and Forecasting (MarsWRF) is suitable for such a calculation. Following many previous works, the scientists specified an aerosol layer and calculated the resulting steady-state warming. This calculation does not include dynamic aerosol transport, but does include realistic topography, seasonal forcing, and surface thermophysical properties and albedo. The model output (schematics above) shows that the density of Al nanorods is 160 mg/m2 yields surface temperatures and pressures that allow for extensive summer (i.e. warmest period ~70 sols each year) liquid water in shallow ground ice locations. This is > 5000× more efficient, on a warming per unit mass basis in the atmosphere, than the current state of affairs. Temperatures experienced by subsurface ice will be lower due to insulation by the soil. Water ice at depths < 1 m is nearly ubiquitous poleward of ±50° latitude (blue lines in diagram). Ice H2O is present further equatorward but is sequestered under >1 m of soil cover and so will not melt unless the annual mean surface temperature rises close to 273 K (about -0.15 ℃).
The greenhouse effect depends in part on the temperature difference between the top of the optically thick infrared-emitting/absorbing layer and the planet's surface. Higher clouds have a larger ΔT relative to the surface (due to adiabatic cooling) and thus produce a stronger greenhouse effect. Therefore, the results depend on both the height of the top of the artificial dust layer and its density. The minimum density for significant warming (2C) can be estimated by setting the optical depth in the spectral window (τwin) per unit and solving the following expression for the density of rods Mc (milligrams per square meter):
τwin = 3 QeffMc / ( 4 r ρ)
Qeff — extinction efficiency depending on the wave number, ρ — density of nanorod particles (Al: 2.7 g/cm3), and r is the effective radius of the nanorod particles (the radius of a sphere of equivalent volume, 0.38 μm). Here Qeff — the ratio of the extinction cross-section in the spectral window (about half the maximum cross-section, i.e. 3 × 10-11 m2) to the geometric section of the equivalent sphere and is ≈ 60. This gives the minimum density of rods (Mc) 20 mg/m2. At 160 mg/m2 the bulk density of nanorods is 10/cm3 gives a time scale of Brownian coagulation (for spheres with diameters from 0.1 to 10 µm, for 100% accretion efficiency) of ≈ 6 years.
This time scale estimate has significant uncertainties. For example, the actual accretion efficiency may be lower, for example because monodisperse particles of uniform composition (e.g. nanorods) may carry identical charges and thus repel each other. On Mars, the particles would be absorbed by dry deposition and transient CO ice.2 and re-emitted into the atmosphere by dust lift. The initial release (after fabrication) could be from a stack extending 10–100 m above the surface, as turbulent updrafts on Mars increase with distance from the surface. For an effective particle lifetime of 10 years, maintaining the warming shown in 2Arequires a particle fountaining rate of 30 liters/s on average. The multi-year lifetime is consistent with a single-pass fallout of ~0.1 μm diameter particles, and the particle lifetime could be significantly increased if the particles were designed to rise on their own, further reducing the supporting mass flux. However, the effective particle lifetime remains a major uncertainty in the model under study.
To test the three-dimensional (3D) results, the scientists ran a 1D model using the average annual insolation of Mars. It predicts a global temperature of 245 K for Al nanorods of ~160 mg/m2 (2C). To further increase the nanorod loading, the 1D model predicts lower global temperatures than the 3D model. This may be due to differences in the vertical temperature structure of the two models. Even in a warming climate, the South Pole is cold enough for seasonal CO condensation2.
Image #3
Over the course of several months of warming on Mars, atmospheric pressure increases by about 20% as CO2 ice sublimates, which is a positive feedback of heating. As Mars warms, atmospheric pressure will increase by another 2–20 times, since adsorbed CO2 is desorbed, and polar CO2 ice evaporates over a period of time that could last for centuries. This would further increase the area suitable for liquid water. However, increasing the temperature of Mars is not enough by itself to make the planet's surface suitable for life with oxygen-based photosynthesis. For example, the sands of Mars contain approximately 300 parts per million by weight (ppmw of parts per million by weight) nitrates, and the air of Mars contains very little O2like the Earth's air before cyanobacteria arrived. Remediation of perchlorate-rich soil may require bioremediation using perchlorate-reducing bacteria that release molecular oxygen as a natural byproduct.
For a more detailed look at the nuances of the study, I recommend taking a look at scientists' report And additional materials to him.
Epilogue
The results of this relatively simple workflow are subject to several uncertainties that motivate more complex modeling. As one of several examples, modeling of the coupled dust flux and ice nucleation on Mars is currently in its early stages. Modeling the effect of nanorods as ice nuclei—which could be either a positive or negative feedback, depending on the size and height of the resulting water ice particles and their settling efficiency—is an additional motivation to explore this relationship. A thin coating of the nanorods could alter their level of hydrophobicity and possibly ice nucleation, and could also protect against oxidation. The optimal location for particle fountaining requires further study. Release into the ascending limb of the Hadley cell should ensure dispersion in both hemispheres. The radiative forcing of the water vapor feedback is clearly positive. Tests with different nanorod sizes, compositions, and shapes suggest that further improvements in warming efficiency are possible. For example, the attenuation efficiency decreases approximately linearly with the rod radius, but the mass decreases quadratically with the rod radius.
Given the above caveats, 2C allows a first estimate of how much surface material will be required to provide fountains. With a surface material density of 2 g/cm3 and Al content2O3 ~10 wt% increase in surface temperature to the temperature shown in 2A in 10 years would require a treatment of 2 × 107 m3/year of surface material to obtain 7 × 105 m3/year of metal, which corresponds to a prismatic shaft with a half-width of 350 m and a side wall slope of 20°, extending by 250 m per year.
While nanoparticles could warm Mars, both the benefits and potential costs of this course of action are currently uncertain. For example, in the unlikely event that the soil of Mars contains irreparable compounds that are toxic to all life on Earth (which could be tested by Mars Sample Return), the benefit of warming Mars would be zero. On the other hand, if a photosynthetic biosphere could be created on the surface of Mars, perhaps through synthetic biology, this could increase the chance of successful colonization of the red planet.
More work is needed on the very long-term sustainability of a warm Mars. At current rates, it would take at least 300 million years for the atmosphere to be depleted into space. However, if the subsurface ice observed at depths of meters to tens of meters is trapped in empty pore space, then excessive warming over centuries could result in water leakage, requiring careful management of long-term warming. Electromagnetic subsurface probing could resolve this uncertainty about how much water remains deep underground on Mars.
The effectiveness of nanoparticle heating suggests that any entity seeking strong planet-scale warming would use this approach. This suggests polarization as a technosignature for cold terrestrial worlds with geodynamos. Nanoparticle heating alone is not sufficient to make a planet’s surface habitable again. However, this study suggests that nanoparticle heating may be of interest to the nanophotonics and planetary science communities, among others, and that further exploration could be extremely fruitful.
The full colonization of Mars is still a long, long way off. However, scientists from a variety of fields are making gigantic efforts to bring this new stage in human development closer. The unification of the accumulated knowledge of all these areas of science is the cornerstone of the success of such a dangerous mission.
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