Atmospheric energy for subsurface life on Mars?

Footnotes

• ↵ † To whom reprint requests should be addressed. E-mail: bweiss{at}gps.caltech.edu.

• ↵ § Less abundant gases would provide less energy partly because of the lower number of molecules that could be consumed. There would also be a larger energy cost associated with extracting and concentrating them out of the surrounding atmosphere, a process that reduces the entropy of the gas.

• Article published online before print: Proc. Natl. Acad. Sci. USA, 10.1073/pnas.030538097.

• Article and publication date are at www.pnas.org/cgi/doi/10.1073/pnas.030538097

• ↵ †† Note, however, that Antarctic soils typically contain 10⁴ times more organic carbon in nonliving form than in living bacteria (4).

• ↵ ‖ This would overestimate the flux if the diffusing gases experienced adsorption on regolith particles possibly followed by loss due to heterogeneous reactions with the regolith. However, these processes are unlikely to be important because, as demonstrated in the last paragraph of Section II, the Nair et al. model strongly suggests that little of the available CO and H₂ is being consumed by the surface (abiotically or otherwise), which would not be the case if the regolith were reactive with these gases. Furthermore, laboratory measurements on analogs of the Martian regolith (21) suggest that CO adsorption would not be significant.

• ↵ ** The soil porosity and temperature probably do not vary substantially in the top 1 km (22). The behavior of pore size and tortuosity are less well understood, although they are obviously correlated with porosity. If pore size and tortuosity were fairly constant over the top kilometer, then over that range the flux that would be delivered to a biotic layer would be approximately proportional to depth of the layer. In reality, however, even if the soil parameters were constant, this linear scaling should only be appropriate for depths substantially less than 1 km because the diffusing CO and H₂ have finite lifetimes due to photochemical reconversion back to CO₂ and H₂O. Because the timescale for diffusion to a depth l is approximately l ²/D, CO and H₂ should only be able to penetrate 64 and 890 m, respectively, before they become depleted by reconversion. Below these depths, the energy sources postulated here are still available, but at a small fraction (≈10%) of what would be estimated from the above linear scaling. This is because the HO_x radicals that catalyze the reconversions in the atmosphere should strongly adsorb to the Martian surface, causing them to diffuse into the regolith very slowly (23). Because HO_x are short-lived, this means that they will not penetrate very deeply and so will not be encountered during the random-walking of deeper CO and H₂ molecules.

• ↵ ‡‡ This assumes that the energy flux is large enough to be biologically useful. Bacteria are usually restricted from becoming larger than a few micrometers in diameter, in part because their surface area to volume ratio declines with radius r, limiting their energy intake (8). The maintenance energy M is probably proportional to mass (24) and hence to r ³, and so the surface area to maintenance energy ratio is A/M∝1/r. For survival, the energy flux must fulfill ΦA ≥ M. Thus, r ≤ kΦ, for some constant k. This means that low-density energy fluxes are capable of supporting only small organisms. There may also be a lower limit (about r = 50 nm) on the size of organisms set by the sizes of the molecules of which they are built (8). If this lower size limit is real, a viable energy source must not be so diffuse that it would require organisms to violate it. For organisms like A. eutrophus (26, 27), which has a fairly high maintenance energy compared to many chemolithotrophs, we find that k = 5 × 10⁷ cm³⋅s⋅kcal⁻¹. Thus, for the energy flux when z _b = 10 m, we see that r ≤ 45 nm, which is near the above lower limit. The maximum permissible radius is actually somewhat larger than suggested by this analysis because the radii of cylindrical bacteria are governed by the same flux-radius equation but can be arbitrarily long and thus have significantly more internal space. For the reasons stated above, one might expect that, in energy-starved environments, organisms would be smaller than those living in more energy-rich regions. Given that the absolute lower limit for self-sustaining life is still a matter of some debate, it remains conceivable that some of the diminutive (50 nm in radius) structures interpreted as nanobacteria in the Martian meteorite ALH84001 are such energy-starved resting stages (28).

• ↵ ¶ This CO surface abundance is that from the photochemical model with modified chemical rate constants [case f of Nair et al. (16)].

• Abbreviation:

  R1,

  reaction 1.

• Copyright © 2000, The National Academy of Sciences