Onethird of Mars surface has shallowburied H 2 O but it is currently too cold for use by life. Proposals to warm Mars using greenhouse gases require a large mass of ingredients that are rare on Mars surface. However we show here that artificial aerosols made from materials that are readily available at Marsfor example conductive nanorods that are 9 micrometers longcould warm Mars gt5  10 3 time smore effectively than the best gases. Such nanoparticles forwardscatter sunlight and efficiently block upwelling thermal infrared. Like the natural dust of Mars they are swept high into Mars atmosphere allowing delivery from the nearsurface. For a 10year particle lifetime two climate models indicate that sustained release at 30 liters per second would globally warm Mars by 30 kelvin and start to melt the ice. Therefore if nanoparticles can be made at scale on or delivered to Mars then the barrier to warming of Mars appears to be less high than previously thought. Warming Mars with artificial aerosols appears to be feasible.
['Astrophysics - Earth and Planetary Astrophysics', 'Physics - Atmospheric and Oceanic Physics', 'Physics - Geophysics']
Feasibility of keeping Mars warm with nanoparticles
{'Feasibility of keeping Mars warm with nanoparticles': 'Samaneh Ansari et al.  \n*Corresponding author: Edwin S. Kite. Email: kite@uchicago.edu', 'Authors': 'Samaneh Ansari 1 , Edwin S. Kite 2,* , Ramses Ramirez 3 , Liam J. Steele 2,4 , Hooman Mohseni 1', 'Affiliations': '- 1. Department of Electrical and Computer Engineering, Northwestern University; Evanston, IL.\n- 2. Department of the Geophysical Sciences, University of Chicago; Chicago, IL.\n- 3. Department of Physics, University of Central Florida; Orlando, FL.\n- 4. European Center for Medium-Range Weather Forecasts; Reading, UK.\n- * Corresponding author, kite@uchicago.edu', 'Abstract': "One-third of Mars' surface has shallow-buried H 2 O, but it is currently too cold for use by life. Proposals to warm Mars using greenhouse gases require a large mass of ingredients that are rare on Mars' surface. However, we show here that artificial aerosols made from materials that are readily available at Mars -for example, conductive nanorods that are ~9 μm long -could warm Mars >5×10 3 times  more  effectively  than  the  best gases.  Such  nanoparticles  forward-scatter sunlight and efficiently block upwelling thermal infrared. Similar to the natural dust of Mars, they are  swept  high  into  Mars'  atmosphere, allowing  delivery  from  the  near-surface.  For  a  particle lifetime  of  10 years, two  climate  models  indicate  that  sustained  release  at  30  liters/sec would globally warm Mars by ≳ 30 K and start to melt the ice. Therefore, if nanoparticles can be made at scale on (or delivered to) Mars, then the barrier to warming of Mars appears to not be as high as previously thought.", 'Teaser': 'Warming Mars with artificial aerosols appears to be feasible.', 'Introduction.': "Dry river valleys cross Mars's once-habitable surface (1, 2), but today the icy soil is too cold for Earth-derived life (3-5). Streams may have flowed as recently as 600 kyr ago (6), hinting at a planet on the cusp of habitability. Many methods have been proposed to warm Mars' surface by closing the spectral windows, centered around wavelengths ( λ ) 22 µm and 10 µm, through which the surface is cooled by thermal infrared radiation upwelling to space (7-9). Modern Mars has a thin (~6 mbar) CO 2 atmosphere that provides only ~5 K greenhouse warming via absorption in the 15 µm band (10), and Mars apparently lacks enough condensed or mineralized CO 2 to restore a warm climate (11). The spectral windows can be closed using artificial greenhouse gases (e.g.  \nchloroflourocarbons) (8, 12), but this would require volatilizing ~100,000 megatons of fluorine, which is sparse on the Mars surface. An alternative approach is suggested by natural Mars dust aerosol. Mars dust is almost all ultimately sourced from slow comminution (indirect estimate O (3) liters/second, ref. 13) of iron-rich minerals on Mars' surface. Due to its small size (1.5 µm effective radius), Mars dust is lofted to high altitude (altitude of peak dust mass mixing ratio 15-25 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).", 'Results.': "Consider a 9-µm long conductive nanorod (we consider aluminum and iron) with a ~60:1 aspect ratio,  not  much  smaller  than  commercially  available  glitter.  Finite-difference  time  domain calculations  (Supplementary  Methods)  show  that  such  nanorods,  randomly  oriented  due  to Brownian motion (18), would strongly scatter and absorb upwelling thermal infrared in the spectral windows,  and  forward-scatter  sunlight  down  to  the  surface,  leading  to  net  warming  (Fig.  1, Figs. S1-S4). Results are robust to changing particle material type, cross-sectional shape, and mesh resolution,  and  change  as  expected  with  particle  length  and  aspect  ratio  (Figs.  S5-S8).  The calculated thermal infrared scattering is near-isotropic (Fig. 1), which favors surface warming (19). Such nanorods would settle >10× more slowly in the Mars atmosphere than natural Mars dust (Supplementary Methods), implying that once the particles are lifted into the air they would be lofted to high altitude and have a long atmospheric lifetime.  \nThis motivates calculating surface warming (K) as a function of (artificial) aerosol column density (kg/m 2 ). The MarsWRF global climate model is suitable for such a calculation (20-22). Following many previous works (22-26), we prescribe a layer of aerosol, and calculate the resulting steadystate warming (Supplementary Methods). Our calculation does not include dynamic transport of aerosol, but includes realistic topography, seasonal forcing, and surface thermophysical properties and  albedo.  The  model  output  (Fig.  2,  Figs.  S9-S19)  shows  that  Al  nanorod  column  density 160 mg/m 2 yields surface temperatures and pressures permitting extensive summertime (i.e., the warmest  ~70  sols  period  each  year)  liquid  water  in  locations  with  shallow  ground-ice. This is >5000× more  effective,  on  a  warming-per-unit-mass-in-the-atmosphere  basis,  than  the current state of the art (8) (Supplementary Methods). Temperatures experienced by subsurface ice will be lower due to insulation by soil. Water ice at <1 m depth is almost ubiquitous poleward of ±50° latitude (blue lines in Fig. 2) (1). H 2 O ice is present further equatorward (27), but is insulated beneath >1 m soil cover and so would not melt unless the annual average surface temperature is raised close to 273K.  \nThe greenhouse effect depends on the temperature difference between the top of the optically thick IR emitting/absorbing layer and that of the planet surface; higher clouds have a bigger ΔT relative to  the  surface  (due  to  adiabatic  cooling)  and  so  give  a  stronger  greenhouse  effect.  Therefore, the results depend on both artificial-dust-layer-top height and column density (28). Supplementary  \nTable 2 and Fig. S19 show the results varying the layer-top height between ~35 km and ~28 km. The minimum column density for substantial warming (Fig. 2c) can be estimated by setting optical depth  in  the  spectral  window  ( τ win )  to  unity  and  solving  the  following  expression  for  column density M c (mg/m 2 ) (e.g. 24):  \nτ win = 3 Q eff M c / (4 r ρ )     (1)  \nHere, Q eff is  the  wavenumber-dependent extinction efficiency, ρ is  the  nanorod particle density (Al: 2.7 g/cc), and r is the effective nanorod particle radius (the radius of a sphere of equivalent volume, 0.38 µm). Here, Q eff is the ratio of the extinction cross-section in the spectral window (about one-half the maximum cross-section, i.e. 3 × 10 -11 m 2 , from Fig. 1) to the geometric crosssection of the equivalent sphere, and is ≈60. This gives a minimum column density ( M c ) of 20 mg/m 2 . At 160 mg/m 2 , the volumetric density of nanorods, 10 cm -3 , gives a Brownian coagulation timescale (for 0.1-10 µm diameter spheres, for 100% accretion efficiency) ≈ 6 years (18). This timescale estimate has substantial uncertainties; for example, actual accretion efficiency may be less,  for  example  because  monodisperse  particles  of  uniform  composition  (e.g. nanorods)  may carry similar charges and thus repel each other (29). On Mars, particles would be taken up by dry deposition and by transient CO 2 ice, and re-released to the atmosphere by dust lifting. Initial release (after manufacture) could be from a pipe extending 10-100m above the surface, as Mars turbulent updrafts strengthen with distance from the surface (30). For an effective particle lifetime of 10 years, sustaining the warming shown in Fig. 2a requires particle fountaining at an average rate of 30 liters/sec  (1  liter/sec  corresponds  to  the  flow  of  one  standard  garden  sprinkler).  Multi-year lifetimes  are  consistent  with one-pass  fall-out  of  ~0.1 µm-diameter  particles  (31),  and  particle lifetime  might  be  greatly  extended  if  the  particles  are  engineered  to  self-loft  (32-34),  further reducing the sustaining mass flux; however, effective particle lifetime remains a major uncertainty in our model.  \nAs  a  check  on  the  3-D  results,  we  ran  a  1-D  model  using  annual-average  Mars  insolation (Supplementary Methods). This predicts 245 K global temperature for ~160 mg/m 2 Al nanorods (Fig. 2c, Figs. S16-S17). For further increases in nanorod loading, the 1-D model predicts lower global  temperatures  than  does  the  3-D  model.  This  may  be  due  to  differences  in  the  vertical temperature structure of the two models (Supplementary Information).  \nEven in the warmed climate, the south pole is cold enough for seasonal CO 2 condensation.  \nWithin  months  of  warming  Mars,  the  atmospheric  pressure  increases  by  ~20%  as  CO 2 ice sublimes, a positive warming feedback. On a warmed Mars, atmospheric pressure will further increase by a factor of 2-20 as adsorbed CO 2 desorbs (35), and polar CO 2 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). However, raising Mars' temperature, by itself, 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 O 2 , as did Earth's air prior to the arrival  of  cyanobacteria.  Remediating  perchlorate-rich  soil  might  require  bioremediation  by perchlorate-reducing bacteria, which yield molecular oxygen as a natural byproduct (38).", 'Discussion.': "The results from this relatively simple workflow are subject to several uncertainties that motivate more sophisticated modeling. As one of several examples, modeling of coupled dust flow and ice nucleation on Mars is currently at an early stage (39). Modeling the effect of nanorods as ice nuclei - which could either be a positive or a negative feedback, depending on the size and altitude of the resulting water ice cloud particles and their precipitation efficiency - is additional motivation to study this coupling. A thin coating on the nanorods could alter their hydrophobicity level, and potentially the ice nucleation, and might also protect against oxidation. The optimal location(s) for particle fountaining require further research. Release into the ascending limb of the Hadley cell should  allow  dispersal  in  both  hemispheres.  The  radiative  effect  of  water  vapor  feedback  is unambiguously positive. Tests varying nanorod size, composition, and shape suggest that further improvements  to  warming  effectiveness  are  possible  (Figs.  S7-S8).  For  example,  extinction efficiency decreases approximately linearly with rod radius, but mass decreases quadratically with rod radius.  \nWith the caveats above in mind, Fig. 2c allows a first estimate of how much surface material would be  needed  to  supply  the  fountains.  For  surface-material  density  2  g/cc,  and Al 2 O 3 content  of ~10 wt% (e.g., 40), raising the surface temperature to that shown in Fig. 2a over 10 years would require processing 2 × 10 7 m 3 /yr surface material to obtain 7 × 10 5 m 3 /yr of metal, corresponding to a prismatic mine of half-width 350m and side-wall slope 20°, lengthening by 250m per year. This  is  much  easier  than the  current  state-of-the-art  (8),  because  fluorine  is  only  infrequently detected by rovers (41), and is present in most Mars meteorites only at 15-90 ppmw concentrations (42).  However,  even  this  reduced  material-processing  demand  still  corresponds  to 1 ×  10 -3 of Earth's  metal  production,  and  this  defines  a  major  manufacturing  problem  that  remains  to  be solved. Processing of surface material into nanoparticles might use lenses or mirrors to concentrate sunlight for vacuum evaporation, followed by colloidal growth. Synthetic biology (e.g. magnetite nanorods) is a possible alternative (43). Metal 3D printing of parts (e.g. Relativity's Stargate; 44) and/or  assembly  on  Mars  might  reduce  launch  costs.  Due  to  their  <2  nm  width,  carbon nanomaterials might warm Mars more effectively than the nanoparticles shown in Figs. 1-2. For example, graphene's density is 0.77 mg/m 2 ,  and graphene nanodisks that are ~10 2 nm diameter have strong mid-infrared resonances (45).  \nAlthough nanoparticles could warm Mars (Fig. 2c, Fig. 3), both the benefits and potential costs of this course of action are currently 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, that might increase the Solar System's capacity for human flourishing. On the costs 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. Examples include Mars-pressure wind-tunnel experiments for nanorod and dust re-uptake  and  release  rate  from  realistic  rough  surfaces  (including  icy  surfaces),  and  \nmesoscale/large-eddy-simulation modeling of nanorod dispersal and lofting. In addition to the nanorod warming option and the no-action option, cost-benefit calculations should also consider local-warming methods, such as silica aerogel tiling (9).  \nMore work is needed on the very-long-term sustainability of a warmed Mars. Atmospheric escape to  space  would  take  at  least  300  Myr  to  deplete  the  atmosphere  at  the  present-day  rate  (46). However, 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 (47).  \nThe efficiency of nanoparticle warming suggests that any entity with the goal of strong planetscale warming would use this approach. This suggests polarization as a technosignature for cold terrestrial worlds with geodynamos.  \nNanoparticle warming, by itself, is not sufficient to make the planet's surface habitable again. Nevertheless, our study suggests that nanoparticle warming  may  be  of  interest  to the nanophotonics and planetary science communities, among others, and that further investigation might be fruitful.", 'Materials and Methods.': 'Calculation  of  optical  properties  of  nanorods. We  carried  out  finite-difference  time  domain simulations  (FDTD:  3D  Electromagnetic  Simulator).  Optical  properties  are  obtained  at 75 wavelengths,  approximately  log-uniformly  spaced  from  0.24-55 µm  (Table  S1). Additional details are given in the Supplementary Materials.  \nCalculation of the surface-warming effect of the nanorods using 1D climate model. Our singlecolumn radiative-convective climate model (RCM) subdivides atmospheres into multiple vertical log-layers (we implement 201 layers here) that extend from the ground to the top of the atmosphere (1 × 10 -5 bar here) (e.g. 25).  The RCM has 55 infrared and 38 solar spectral intervals and applies a  standard  moist  convective  adjustment.  Additional  details  are  given  in  the  Supplementary Materials.  \nCalculation of the surface-warming effect of the nanorods using 3D climate model. We use the MarsWRF code (20-22), with a horizontal resolution of 5.625° × 3.75°, corresponding to a grid of 64 points in longitude × 48 points in latitude. A 40-layer vertical grid is used. A prescribed natural dust aerosol distribution is imposed, giving an average dust optical depth of 0.20 (peaking at 0.43 during southern summer). Additional details are given in the Supplementary Materials.  \nCompeting interests: Authors declare that they have no competing interests.  \nData and materials availability. All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data, for example full 3D climate  model  output  files,  are  stored  at  Zenodo  (doi:10.5281/zenodo.8352416). FDTD:  \n3D Electromagnetic Simulator is commercial code (Lumerical). The MarsWRF source code can be made available by Aeolis Research pending scientific review and a completed Rules of the Road  agreement.  Requests  for  the  MarsWRF  source  code  should  be  submitted  to mir@aeolisresearch.com.', 'References.': "- 1. Mangold, N., Gupta, S.; Gasnault, O.; Dromart, G.; Tarnas, J. D.; Sholes, S. F.; Horgan, B.; Quantin-Nataf, C.; Brown, A. J.; Le Mouélic, S.; Yingst, R. A.; Bell, J. F.; Beyssac, O.; Bosak, T.; Calef, F.; Ehlmann, B. L.; Farley, K. A.; Grotzinger, J. P.; Hickman-Lewis, K.; Holm-Alwmark, S.; Kah, L. C.; Martinez-Frias, J.; McLennan, S. M.; Maurice, S.; Nuñez, J. I.; Ollila, A. M.; Pilleri, P.; Rice, J. W.; Rice, M.; Simon, J. I.; Shuster, D. L.; Stack, K. M.; Sun, V. Z.; Treiman, A. H.; Weiss, B. P.; Wiens, R. C.; Williams, A. J.; Williams, N. R.; & Williford, K. H., Perseverance rover reveals an ancient delta-lake system and flood deposits at Jezero crater, Mars. Science 374 , 6568, 711-717 (2021).\n- 2. Grotzinger, J. P.,   Sumner, D. Y.,   Kah, L. C.,   Stack, K.,   Gupta, S.,   Edgar, L.,   Rubin, D., Lewis, K.,   Schieber, J.,   Mangold, N.,   Milliken, R.,   Conrad, P. G.,   DesMarais, D.,   Farmer, J.,   Siebach, K.,   Calef, F., III,   Hurowitz, J.,   McLennan, S. M.,   Ming, D.,   Vaniman, D., Crisp, J.,   Vasavada, A.,   Edgett, K. S.,   Malin, M.,   Blake, D.,   Gellert, R.,   Mahaffy, P., Wiens, R. C.,   Maurice, S.,   Grant, J. A.,   Wilson, S.,   Anderson, R. C.,   Beegle, L.,   Arvidson, R.,   Hallet, B.,   Sletten, R. S.,   Rice, M.,   Bell, J., III,   Griffes, J.,   Ehlmann, B.,   Anderson, R. B.,   Bristow, T. F.,   Dietrich, W. E.,   Dromart, G.,   Eigenbrode, J.,   Fraeman, A.,   Hardgrove, C.,   Herkenhoff, K.,   Jandura, L.,   Kocurek, G.,   Lee, S.,   Leshin, L. A.,   Leveille, R.,   Limonadi, D.,   Maki, J.,   McCloskey, S.,   Meyer, M.,   Minitti, M.,   Newsom, H.,   Oehler, D.,   Okon, A., Palucis, M.,   Parker, T.,   Rowland, S.,   Schmidt, M.,   Squyres, S.,   Steele, A.,   Stolper, E., Summons, R.,   Treiman, A.,   Williams, R.,   Yingst, A., and MSL Science Team, A habitable fluvio-lacustrine  environment  at  Yellowknife  Bay,  Gale  Crater,  Mars. Science 343 ,  no.  6169 (2014): 1242777.\n- 3. Feldman, W.C., Prettyman, T.H., Maurice, S., Plaut, J.J., Bish, D.L., Vaniman, D.T., Mellon, M.T., Metzger, A.E., Squyres, S.W., Karunatillake, S. and Boynton, W.V.,  Global distribution of near-surface hydrogen on Mars. J. Geophys. Res. - Planets 109 , E9 (2004).\n- 4. Dundas, C.M., Bramson, A.M., Ojha, L., Wray, J.J., Mellon, M.T., Byrne, S., McEwen, A.S., Putzig, N.E., Viola, D., Sutton, S. and Clark, E., Exposed subsurface ice sheets in the Martian midlatitudes. Science 359 , 6372 (2018): 199-201.\n- 5. McKay, C.P. (2014), Requirements and limits for life in the context of exoplanets. Proc. Natl. Acad. Sci. USA 111 (35), 12628-12633.\n- 6.  Dickson,  J.L.,  Palumbo,  A.M.,  Head,  J.W.,  Kerber,  L.,  Fassett,  C.I.  and  Kreslavsky,  M.A.. Gullies  on  Mars  could  have  formed  by  melting  of  water  ice  during  periods  of  high obliquity. Science 380 , 6652 (2023),  pp.1363-1367.  \n- 7. McKay, C.P., Toon, O.B. and Kasting, J.F., Making Mars habitable. Nature 352 , 6335 (1991), 489-496.\n- 8. Marinova, M. M., C. P. McKay, and H. Hashimoto. Radiative-convective model of warming Mars with artificial greenhouse gases. J. Geophys. Res. - Planets 110 , no. E3 (2005).\n- 9. Wordsworth, R., L. Kerber, and C. Cockell. Enabling Martian habitability with silica aerogel via the solid-state greenhouse effect. Nature Astronomy 3 , 10 (2019): 898-903.\n- 10. Haberle, R. M. Estimating the power of Mars' greenhouse effect. Icarus 223 , 1 (2013): 619620.\n- 11. Jakosky, B. M., and C. S. Edwards. Inventory of CO 2 available for terraforming Mars. Nature Astronomy 2 (8) (2018): 634-639.\n- 12. Gerstell, M. F., J. S. Francisco, Y. L. Yung, C. Boxe, and E. T. Aaltonee. Keeping Mars warm with new super greenhouse gases. Proc. Natl. Acad. Sci. USA 98 , 5 (2001): 2154-2157.\n- 13. Kite, E. S., and D. P. Mayer. Mars sedimentary rock erosion rates constrained using crater counts, with applications to organic-matter preservation and to the global dust cycle. Icarus 286 (2017): 212-222.\n- 14. McCleese, D.J., Heavens, N.G., Schofield, J.T., Abdou, W.A., Bandfield, J.L., Calcutt, S.B., Irwin, P.G.J., Kass, D.M., Kleinböhl, A., Lewis, S.R. and Paige, D.A., Structure and dynamics of the Martian lower and middle atmosphere as observed by the Mars Climate Sounder: Seasonal variations in zonal mean temperature, dust, and water ice aerosols. J. Geophys. Res. - Planets 115 , no. E12 (2010).\n- 15. Kahre, M. A., J. R. Murphy, C. E. Newman, R. J. Wilson, B. A. Cantor, M. T. Lemmon, and M. J. Wolff. The Mars dust cycle. in Haberle, R. (Ed.), 2017, The Atmosphere and Climate of Mars (2017): 295, Cambridge University Press.\n- 16. Streeter, P.M., S.R. Lewis, M.R. Patel, J.A. Holmes, and D.M. Kass. Surface warming during the 2018/Mars year 34 global dust storm. Geophys. Res. Lett. 47 , no. 9 (2020): e2019GL083936.\n- 17. Van Vleck, J. H., F. Theory of radar reflection from wires or thin metallic strips . Journal of Applied Physics 18 , no. 3 (1947): 274-294.\n- 18. Seinfeld, J., & S. Pandis (2016), Atmospheric chemistry and physics, 3rd edn., Wiley.\n- 19. Forget, F., and R. T. Pierrehumbert. Warming early Mars with carbon dioxide clouds that scatter infrared radiation. Science 278 , no. 5341 (1997): 1273-1276.  \n- 20. Richardson, M. I., A. D. Toigo, and C. E. Newman. PlanetWRF: A general purpose, local to global  numerical  model  for  planetary  atmospheric  and  climate  dynamics. J.  Geophys.  Res.  Planets 112 (2007), E09001.\n- 21.  Toigo,  A.  D.,  C.  Lee,  C.  E.  Newman,  and  M.  I.  Richardson.  The  impact  of  resolution  on the dynamics of the martian global atmosphere: Varying resolution studies with the MarsWRF GCM. Icarus 221 , no. 1 (2012): 276-288.\n- 22.  Kite,  E.  S.,  L.  J.  Steele,  M.  A.  Mischna,  and  M.  I.  Richardson.  Warm  early  Mars  surface enabled by high-altitude water ice clouds. Proceedings of the National Academy of Sciences 118 , no. 18 (2021): e2101959118.\n- 23. Conrath, B.J., Thermal structure of the Martian atmosphere during the dissipation of the dust storm of 1971. Icarus 24 (1) (1975), 36-46.\n- 24. Ramirez, R. M., and J. F. Kasting. Could cirrus clouds have warmed early Mars?, Icarus 281 (2017): 248-261.\n- 25. Ramirez, R.M., 2017. A warmer and wetter solution for early Mars and the challenges with transient warming. Icarus 297 , 71-82.\n- 26. Turbet, M., C. Gillmann, F. Forget, B. Baudin, A. Palumbo, J. Head, and Ö. Karatekin, 2020, The environmental effects of very large bolide impacts on early Mars explored with a hierarchy of numerical models, Icarus 335 , 113419.\n- 27. Dundas, C.M., Mellon, M.T., Conway, S.J., Daubar, I.J., Williams, K.E., Ojha, L., Wray, J.J., Bramson, A.M., Byrne, S., McEwen, A.S. and Posiolova, L.V., Widespread exposures of extensive clean  shallow  ice  in  the  midlatitudes  of  Mars. J. Geophys.  Res.  -  Planets 126 ,  no.  3  (2021): e2020JE006617.\n- 28. Pierrehumbert, R.T. (2010). Principles of planetary climate. Cambridge University Press.\n- 29. Fedorova, A. A., F. Montmessin, A. V. Rodin, O. I. Korablev, A. Määttänen, L. Maltagliati, and J-L. Bertaux. Evidence for a bimodal size distribution for the suspended aerosol particles on Mars. Icarus 231 (2014): 239-260.\n- 30. Spiga, A., F. Forget, S. R. Lewis, and D. P. Hinson. Structure and dynamics of the convective boundary  layer  on  Mars  as  inferred  from  large-eddy  simulations  and  remote-sensing measurements. Quarterly Journal of the Royal Meteorological Society 136 , 647 (2010): 414-428.\n- 31. Murphy, J. R., Toon, O. B., Haberle, R. M., & Pollack, J. B. (1990). Numerical simulations of the decay of Martian global dust storms. Journal of Geophysical Research: Solid Earth , 95 (B9), 14629-14648.  \n- 32. Keith, D. W. Photophoretic levitation of engineered aerosols for geoengineering. Proc. Natl. Acad. Sci. USA 107 (2010): 16428-16431.\n- 33. Daerden, F., J. A. Whiteway, L. Neary, L. Komguem, M. T. Lemmon, N. G. Heavens, B. A. Cantor, E. Hébrard, and M. D. Smith. A solar escalator on Mars: Self-lifting of dust layers by radiative heating. Geophys. Res. Lett. 42 , no. 18 (2015): 7319-7326.\n- 34. Azadi, M., G. A. Popov, Z. Lu, A. G. Eskenazi, A. Ji Won Bang, M. F. Campbell, H. Hu, and I. Bargatin. Controlled levitation of nanostructured thin films for sun-powered near-space flight. Science Advances 7 , no. 7 (2021): eabe1127.\n- 35. Buhler, P. B., and S. Piqueux. Obliquity-Driven CO 2 Exchange Between Mars' Atmosphere, Regolith, and Polar Cap. J. Geophys. Res. - Planets 126 , no. 5 (2021): e2020JE006759.\n- 36. Phillips, R.J., Davis, B.J., Tanaka, K.L., Byrne, S., Mellon, M.T., Putzig, N.E., Haberle, R.M., Kahre, M.A.,  Campbell,  B.A.,  Carter,  L.M.  and  Smith,  I.B.,  2011.  Massive  CO 2 ice  deposits sequestered in the south polar layered deposits of Mars. Science 332 (6031), 838-841.\n- 37. Stern, J. C., Sutter, B., Jackson, W. A., Navarro-González, R., McKay, C. P., Ming, D. W., ... & Mahaffy, P. R. (2017). The nitrate/(per) chlorate relationship on Mars. Geophysical Research Letters 44 (6), 2643-2651.\n- 38. Davila, A. F., D. Willson, J. D. Coates, and C. P. McKay. Perchlorate on Mars: a chemical hazard and a resource for humans. Intl. Journal of Astrobiology 12 (2013): 321-325.\n- 39. Bertrand, T. R. J. Wilson, M. A. Kahre, R. Urata, and A. Kling. Simulation of the 2018 global dust storm on Mars using the NASA Ames Mars GCM: a multitracer approach . J. Geophys. Res. - Planets 125 , no. 7 (2020): e2019JE006122.\n- 40. O'Connell-Cooper, C. D., Spray, J. G., Thompson, L. M., Gellert, R., Berger, J. A., Boyd, N. I.,  ...  &  VanBommel,  S.  J.  (2017).  APXS-derived  chemistry  of  the  Bagnold  dune  sands: Comparisons  with  Gale  Crater  soils  and  the  global  Martian  average. Journal  of  Geophysical Research: Planets 122 (12), 2623-2643.\n- 41. Forni, O., Gaft, M., Toplis, M. J., Clegg, S. M., Maurice, S., Wiens, R. C., ... & Dyar, M. D. (2015). First detection of fluorine on Mars: Implications for Gale Crater's geochemistry. Geophysical Research Letters 42 (4), 1020-1028.\n- 42. Rampe, E.B., Cartwright, J.A., McCubbin, F.M., Osterloo, M.M. (2018). The Role of Halogens During Fluid and Magmatic Processes on Mars. In: Harlov, D., Aranovich, L. (eds) The Role of Halogens  in  Terrestrial  and  Extraterrestrial  Geochemical  Processes.  Springer  Geochemistry. Springer, Cham. https://doi.org/10.1007/978-3-319-61667-4\\_16\n- 43.  Lang,  C.,  D.  Schüler,  and  D.  Faivre.  Synthesis  of  magnetite  nanoparticles  for  bio-  and nanotechnology. Macromolecular bioscience 7 , 2 (2007): 144-151.  \n- 44. Salmi, B. 3D-printing a rocket. IEEE Spectrum 56, no. 11 (2019): 22-29.\n- 45.  Koppens,  F.  H.,  Chang,  D.  E.,  &  García  de  Abajo,  F.  J.  (2011).  Graphene  plasmonics:  a platform for strong light-matter interactions. Nano letters 11 (8), 3370-3377.\n- 46.  Jakosky,  B.M.,  Brain,  D.,  Chaffin,  M.,  Curry,  S.,  Deighan,  J.,  Grebowsky,  J.,  Halekas,  J., Leblanc, F., Lillis, R., Luhmann, J.G. and Andersson, L.,  Loss of the Martian atmosphere to space: Present-day loss rates determined from MAVEN observations and integrated loss through time. Icarus 315 (2018): 146-157.\n- 47. Grimm, R. E. Low-frequency electromagnetic exploration for groundwater on Mars. Journal of Geophysical Research: Planets 107 , no. E2 (2002): 1-1.\n- 48.  Wolff,  M.  J.,  and  R.  T.  Clancy.  Constraints  on  the  size  of  Martian  aerosols  from Thermal Emission Spectrometer observations. J. Geophys. Res. - Planets 108 , no. E9 (2003).\n- 49. Iwabuchi, H., and P. Yang. Temperature dependence of ice optical constants: Implications for simulating the single-scattering properties of cold ice clouds. Journal of Quantitative Spectroscopy and Radiative Transfer 112 , no. 15 (2011): 2520-2525.\n- 50. Querry, M. R. Optical constants. US Army Armament, Munitions & Chemical Command, Chemical Research & Development Center (1985).\n- 51. Rakić, A. D. Algorithm for the determination of intrinsic optical constants of metal films: application to aluminum. Applied optics 34 , no. 22 (1995): 4755-4767.\n- 52. Cahill, J., D. T. Blewett, N. V. Nguyen, A. Boosalis, S. J. Lawrence, and B. W. Denevi. Optical constants of iron and nickel metal and an assessment of their relative influences on silicate mixture spectra from the FUV to the NIR. Icarus 317 (2019): 229-241.\n- 53. Neubrech, F., and A. Pucci. Plasmonic enhancement of vibrational excitations in the infrared. IEEE Journal of selected topics in quantum electronics 19 , no. 3 (2012): 4600809.\n- 54. Novotny, L. Effective wavelength scaling for optical antennas. Phys. Rev. Lett. 98 ,  no.  26 (2007): 266802.\n- 55. Kasting, J., Whitmire, D., and Reynolds, R., Habitable Zones Around Main Sequence Stars, Icarus 101 (1993), 108-128.\n- 56. Manabe, S. and Wetherald, R.T. (1967), Thermal equilibrium of the atmosphere with a given distribution of relative humidity. J. Atmospheric Sciences 24 (3), 241-258.\n- 57. Thekaekara, M.P. (1973), Solar energy outside the earth's atmosphere. Solar energy 14 (2), 109-127.  \n- 58. Ramirez, R. M., Kopparapu, R., Zugger, M. E., Robinson, T. D., Freedman, R., & Kasting, J. F. (2014). Warming early Mars with CO 2 and H 2 . Nature Geoscience , 7 (1), 59-63.\n- 59. Mischna, M. A., Lee, C., & Richardson, M. (2012). Development of a fast, accurate radiative transfer model for the Martian atmosphere, past and present. Journal of Geophysical Research: Planets , 117 (E10).\n- 60.  Millour,  E.,  F.  Forget,  F.  González-Galindo,  A.  Spiga,  S.  Lebonnois,  S.  R.  Lewis,  L. Montabone., The Mars climate database (version 4.3). No. 2009-01-2395. SAE Technical Paper, 2009.\n- 61. Guzewich, S. D., Talaat, E. R., Toigo, A. D., Waugh, D. W., & McConnochie, T. H. (2013). High-altitude dust layers on Mars: Observations with the Thermal Emission Spectrometer. Journal of Geophysical Research: Planets , 118 (6), 1177-1194.\n- 62. Guzewich, S. D., Toigo, A. D., Richardson, M. I., Newman, C. E., Talaat, E. R., Waugh, D. W.,  &  McConnochie, T. H. (2013).  The  impact  of  a  realistic  vertical  dust  distribution  on  the simulation of the Martian General Circulation. Journal of Geophysical Research: Planets , 118 (5), 980-993.\n- 63. Guzewich, S. D., C. E. Newman, M. D. Smith, J. E. Moores, C. L. Smith, C. Moore, M. I. Richardson, D. Kass, A. Kleinbohl, M. Mischna, F. J. Martin- Torres, M.-P. Zorzano-Mier, and M.  Battalio  (2017),  'The  Vertical  Dust  Profile  Over  Gale  Crater,  Mars', J.  Geophys.  Res. (Planets) , 122(12), pp. 2779-2792.\n- 64. Hong, S.-Y., and H.-L. Pan, 'Nonlocal Boundary Layer Vertical Diffusion in a Medium-Range Forecast Model', Mon. Wea. Rev., 124(10), pp. 2322, doi: 10.1175/15200493(1996)124<2322:NBLVDI>2.0.CO;2, 1996.", 'Acknowledgements.': "We thank M. Turbet, C. Willard, M.A. Mischna, A. Geller, M.I. Richardson, C. Lee, F. Sharipov, A. Noblet, and the PlanetWRF development team. This research was supported in part through the computational  resources  and  staff  contributions  provided  for  the  Quest  high  performance computing facility  at  Northwestern  University  which  is  jointly  supported  by  the  Office  of  the Provost, the Office for Research, and Northwestern University Information Technology, and with resources provided by the University of Chicago's Research Computing Center.  \nFunding: The  authors  acknowledge  that  they  received  no  external  funding  in  support  of  this research.  \nAuthor contributions. ESK conceived research. SA, ESK, RR, LJS, and HM designed research. SA, ESK, and RR carried out research. ESK drafted the manuscript. SA, ESK, RR, LJS, and HM edited the manuscript and contributed to the final manuscript.  \nFig. 1. Orientation-averaged optical properties of a 9 μm-long Al nanorod with cross-section 0.16 µm × 0.16 µm, calculated using a 3D Finite-Difference Time-Domain (FDTD) approach. Top  panel:  Planck  functions  (normalized  flux  density,  W/m 2 /µm)  for  215 K  (Mars  thermal emission now, red) and 6000 K (insolation, blue). For context, the CO 2 band is overlain (gray shading) at 12-16  µm. Middle panel: Solid black line corresponds to total extinction, dotted black line  to  scattering.  Lower  panel  shows  scattering  asymmetry.  Also  shown  are  wavelength dependence of total extinction and asymmetry factor for natural dust assuming a log-gaussian particle size distribution centered on 2.5 µm (48) (red dotted lines).  \n<!-- image -->  \nFig. 2. 3D model output. Warm-season temperatures (K) (color shading) on (A) Mars with addition of ~160 mg/m 2 of Al nanorods, (B) control case. This corresponds to the average surface temperature  during  the  warmest  36°  of solar  longitude  (~70  days)  of  the  year. White  contour  corresponds  to  610  Pa (~6 mbar)  mean  pressure  level.  Black contours correspond to topographic elevation in m (dashed: -5 km and -2 km, solid:  0 km,  +2km,  and  +5  km).  Blue lines: approximate latitudinal (equatorward)  extent  of  ice  at  <1  m depths.  Results  do  not  include  CO 2 outgassing from within polar ice, which would cause further warming. (C) Dependence of planet-averaged surface warming on Al-nanorod column mass. The blue envelope corresponds to the  modeled  seasonal  range  in  global mean T surf . Gray  corresponds  to  1-D results  (see  text  for  details).  Blue  line corresponds to 3-D results. Blue triangle corresponds  to  panel  (a),  and  white triangle  marks  onset  of  warm-season temperatures above the freezing point of water  at  50°S.  Symbols  on  y-axis  are temperatures  for  the  no-nanorod  case, with  the  red  asterisk  corresponding  to observed Mars value. Additional details including comparison to Fe nanorods are shown in Figs. S10-S19.  \n<!-- image -->  \n<!-- image -->  \ncolumn density of Al nanorods (mg/m?)  \n<!-- image -->  \nFig. 3. Graphical summary. This figure shows a graphical summary of the proposed warming method. Figure credit: Aaron M. Geller, Northwestern, CIERA + IT-RCDS.  \n<!-- image -->", 'This PDF file includes:': 'Supplementary Text Figs. S1 to S19 Tables S1 to S3', '1. Calculation of optical properties of nanorods.': "Simple calculations (e.g., ref. 17) suggest that 9 µm long conductive nanorods with a 60:1 aspect ratio would have a strong and broad extinction in the ~22 µm spectral window. To test this, we carried  out  finite-difference  time  domain  simulations  (FDTD:  3D  Electromagnetic  Simulator). First,  we  verified  that  the  FDTD  simulations  reproduce  Mie theory  for  water  ice  spheres  (49) (Fig. S1). The nanorod FDTD simulations use a pulse of light whose interaction with the simulated nanoparticle is Fourier-decomposed to obtain the λ -dependence of the absorption and scattering cross sections as well as the scattering asymmetry. The representation of the angular distribution of scattered light by a single parameter, the scattering asymmetry, is standard in climate modeling (e.g.,  28).  75 wavelengths, approximately log-uniformly spaced from 0.24-55 µm, are obtained (Table S1). We used refractive indices for Fe from Ref. 50, and for Al from Ref. 51. Ref. 52 suggest  that  Fe  refractive  indices  obtained  prior  to  their  own  work  might  be  affected  by  Feoxidation, but as Mars' atmosphere is ~0.1% O 2 some degree of oxidation is inevitable, so this is acceptable. Implementation involved combining three different simulations for different λ ranges, for  computational  feasibility.  75 simulations  were  carried  out  for  the  9  µm  long  nanorod, corresponding to the product of 5 orientations in θ, 5 orientations in φ (0°, 30°, 45°, 60°, and 90°, for both angles), and three λ ranges. Here, θ corresponds to rotations in the E-H plane and the other rotation is termed φ (k-E plane, symmetric and similar to k-H plane) (Fig. S2). Far-field methods were employed to obtain the scattering phase function. We anticipate that actual nanorods will have  circular  cross-sections.  Because  of  computational  limitations  associated  with  the  FDTD approach, we model a nanorod with square cross-section, but find that switching between circular and square cross-sections makes negligible difference to the calculated optical properties (Fig. S3).  \nAs  expected,  the  simulated  nanorods  showed  strong,  broad  absorptions  near  22  µm  (Fig. 1), supporting  their  suitability  for  Mars  warming.  This  is  consistent  with  previous  work  on Ag nanorods (53). The wavelength of the absorption peak is slightly longer than double the rod length, due to plasma effects (54). In order to average over orientations (Fig. S4), rod orientation was uniformly sampled on the sphere relative to the incident electric field vector (~1000 samples between 0 ° and 90 ° for both angles) and optical properties interpolated using a spline in the grid of  25  computed  orientations  for each λ .  The  orientations  of  nanorods  in  the  atmosphere  are assumed  to  be  random  and uniform,  hence  averaging  over  a  spherically  equidistant  grid  of orientations would result in proper orientation-averaged characteristics. We found (minor) infrared back-scattering in these simulations.  A sensitivity test using the 3D model for Fe nanorods showed that this choice makes little difference (~0.3 K) to the calculated temperatures.  \nTo check the interpolation accuracy, we did check runs on multiple intermediate points (Fig. S5). When the rod is viewed nearly end-on, the interpolation results grow less accurate. However, endon geometries are infrequently encountered, and at the higher angles, the scattering and absorption cross-sections are substantially smaller, so the effect of these points on the overall orientation average is  insignificant.  To  check  the  simulation  accuracy,  we  did  a  check  run  increasing  the  \nresolution across the width of the nanorod from 8 simulation mesh points (20 nm spacing) to 10 simulation mesh points (16 nm spacing), finding negligible differences (Fig. S6).  \nWe also did calculations for 7.5 µm-long Fe nanorods with cross-section 0.08 µm × 0.08 µm, finding peak extinction around 20 µm (Fig. S7), and less climate warming per nanorod, but more climate warming per kg of nanorods. An optimal warming approach might use a mix of rods of different lengths. Moreover, we find that the total extinction (absorption plus scattering) for Al nanorods is the same as for Fe nanorods (Fig. S8), and the corresponding warming is also about the same (Table S2), even though the density of Al is three times less than that of Fe. Fe, Al, and Mg are all present at >4 wt% level in Mars soil. We focus here on Al and Fe because optical properties are available over the full wavelength range of interest (other metals are worthy of future investigation).  This  suggests  that  further  research  might  yield  further  improvements  in  the effectiveness of warming.", '2. Calculation of the surface-warming effect of the nanorods using 1D climate model.': "Our  single-column  radiative-convective  climate  model  (RCM)  subdivides  atmospheres  into multiple vertical log-layers (we implement 201 layers here) that extend from the ground to the top of the atmosphere (1 × 10 -5 bar here) (e.g. 25).  The RCM has 55 infrared and 38 solar spectral intervals  (55).  The  model  applies  a  standard  moist  convective  adjustment  (e.g.  56).  Should tropospheric radiative lapse rates exceed their moist adiabatic values, the model relaxes to a moist H 2 O adiabat at high temperatures or to a moist CO 2 adiabat when temperatures are low enough for CO 2 to condense. The RCM implements a standard solar spectrum (57). For present Mars (solar flux = 585 W/m 2 ), we assume a typical stratospheric temperature of 155 K and a surface albedo of 0.22 (e.g. 24). The atmospheric pressure is a Mars-like 650 Pa with an assumed acceleration due to gravity of 3.73 m/s 2 . Although we prescribe a tropospheric relative humidity of 50%, our results are insensitive to this parameter. The baseline mean surface temperature of the resultant pure CO 2 atmosphere (without nanorods) is 218 K. For comparison, Mars' observed global mean surface temperature is 202 K (10). The 1D model runs warmer in the 6 mbar no-nanorods case mainly because it lacks day-night and equator-pole temperature contrast, and because it lacks clouds, dust and topography (10). With an assumed surface albedo of 0.22 and solar flux of 585 W/m 2 ,  the predicted equilibrium temperature ( T eq ) would be ~212 K (assuming a perfect blackbody). This 212 K T eq value is the minimum surface temperature for a 1D model with no greenhouse effect. However,  our  1D  model  predicts  6 K  greenhouse  effect  (consistent  with  data  and  with  other models), raising the mean surface temperature to ~218 K (e.g. 58). In summary, the warm 1D model output gives a useful benchmark comparison on the importance of 3D factors like dust, clouds and topography on modeling predictions in comparison with that from the nanorods.  \nOne of the reasons for the differences in temperature response between the models is the baseline surface temperature, which is higher in the 1D model (for reasons explained above). The main cause, however, is differences in the vertical temperature profiles across models (e.g., Fig. S18). For instance, lapse rates differ slightly (the top of the cloud deck is at a slightly higher pressure level, or closer to the ground, in the 3D model) and the 3D model has a near-surface temperature inversion which is absent in the 1D code (Fig. S18). The 3D model also has a larger effective  \nconvective region. Tests confirm that the 1D and 3D results agree more closely when the 3D profile is  imposed  on  the  1D  model.  For  example,  at τ Fe =  0.25,  imposing  the  same  mean  surface temperature in the 1D model as is obtained for the 3D model (~225.5 K) yields a top-of atmosphere (TOA) net outgoing longwave radiation/net incident stellar flux ratio of ~0.69 (a value equal to 1 corresponds to a TOA radiative equilibrium). In other words, the 1D model 'wants' to warm. However, when the 3D model vertical temperature profile was applied instead (at the same surface temperature), this ratio had increased to 0.86 (removing ~60% of the difference from 1). In other words,  most  of  the  difference  between  the  3D  model  and  the  1D  model  can  be  explained  by differences in the vertical thermal profile. This suggests that even across 3D models, differences in  convective  schemes  (and  temperature  profiles)  could  produce  some  spread  in  the  results, motivating investigation of engineered warming of Mars using different 3D schemes. Despite these numerical differences, the atmospheric response to nanorod addition in both the 1D and 3D models is  qualitatively very similar. In both cases, nanoparticle warming provides a greenhouse effect >5,000× greater than the current state of the art.  \nFollowing Refs. 24-25, we calculate the wavenumber-dependent optical depths ( τ ) for the nanorods from the following expression:  \nτ = 3 Q eff NRC Δz / (4 r ρ) (2)  \nHere, NRC is the nanorod content (g/m 3 ) and Δz is the path length. This equation differs from (1) only in that the optical depth is integrated over all nanorod heights and across all wavenumbers, not just in the spectral window. The nanorods are well-mixed throughout the atmosphere (up to 35 km) above the imposed 500 Pa nanorod-layer base and so NRC was assumed to scale linearly with the local pressure. We computed the radius of the sphere with the equivalent nanorod volume, which yields an effective nanorod particle radius of ~0.38 µm for the 9 µm nanorods.  \nWithin the 1D model framework, we implement a simple procedure to calculate nanorod warming. For each assumed nanorod optical depth, we find the surface temperature that yields stratospheric energy balance (i.e. the net outgoing and net incoming fluxes must balance each other) (Figs. S16S17).", '3. Calculation of the surface-warming effect of the nanorods using 3D climate model.': 'The FDTD output is interpolated to 2000 log-uniformly spaced λ \'s. We set optical properties at λ >55 µm equal to those at 55 µm (Fig. S9). This approximation is acceptable, because extinction (W/m 2 /µm) is minor at such long λ .  MarsWRF uses a two-stream radiation code (e.g. 22, 59). Radiative transfer calculations include both gas (for this work, CO 2 ) and aerosol (for this work, natural dust, nanorods, and CO 2 ice)  radiative  effects.  This  implementation  of  MarsWRF uses Planck-weighted averaging to bin down high-spectral-resolution optical properties. The blackbody temperature used for the Planck-weighted averaging is 6000 K for solar bands (6 bins from 0.244.5 µm, with bin edges at 0.24, 0.40, 0.8, 1.31, 1.86, 2.48, 3.24, and 4.5 µm), and 215 K for the thermal infrared bands (6 bins from 4.5-1000 µm, with bin edges at 4.5, 8.0, 12.0, 14.1, 16.0, 24.0,  \n60, and 1000 µm) (Fig. S9). Thermal equilibrium between gas and dust is assumed. We neglect radiation  pressure,  magnetic  effects,  quantum  size  effects,  and  temperature  dependence  of the optical constants.  \nIn the fixed-cloud runs, present-day Mars values are used for orbital parameters, surface albedo, and surface thermal inertia. Mars\' southern summer solstice occurs near perihelion, so (by Kepler\'s second law) northern summer is long and relatively cool, and southern summer is short and warm relative to annual average (Figs. S10-S12). Water vapor abundance in the atmosphere is fixed to zero, which is conservative in that water vapor radiative feedback would increase warming. A prescribed  natural  dust  aerosol  distribution  is  imposed  corresponding  to  the  Mars  Climate Database MGS-dust-scenario (60), giving an average dust optical depth of 0.20 (peaking at 0.43 during southern summer). Natural dust aerosol warms the atmosphere but lowers daytime surface temperature. The artificial-aerosol layer is parameterized by a layer-base pressure (in Pa), a layer vertical thickness (in units of model levels), and the nanorod orientation-averaged optical depth at a reference λ (0.67 µm). Because the model levels are specified in η coordinates (where η = P/P surf , where  the  model-top  pressure  has  been  subtracted  from  both  pressures),  the  aerosol  layer  is physically more compact over the poles because low temperatures compact the air column. We refer to the altitude away from the poles below which 95% of the particles are contained as the "aerosol layer top height". The nanorod mixing ratio is assumed to be uniform within the nanorod layer  (Fig.  S13).  We prescribe  a  layer-base  level  of  500  Pa  and  vary  the  layer  thickness.  This approach gives a maximum in artificial-aerosol opacity above the surface, which is also observed for natural Mars dust aerosol for some seasons (61-63). However, natural dust on Mars persists all the  way  to  the  surface  (63)  which  has  average  pressure  ~600  Pa.  This  has  no  effect  on  our conclusions, as results are only weakly sensitive to the vertical distribution of simulated aerosol for example, in Table S3, the "Thinner cloud (200 Pa base, 8 levels)" run differs from the reference case with 500 Pa base by only 0.3 K in average temperature. Buffering by latent heat implies that melting either ice cap to form seas would take at least centuries.  \nSettling for a 0.1 µm-radius dust particle for 1 scale height at Mars surface pressure takes 3 years (Murphy 1990). Extrapolation to Al nanorods with radius 0.04-0.08 µm suggests settling times that are similar or greater. Increase of Mars\' atmospheric pCO 2 under warming will decrease the Knudsen number and hinder dust settling. One-pass settling rates are much slower than that of natural Mars dust, and effective particle lifetime will be longer due to re-entrainment.  \nThe runs presented here use a horizontal resolution of 5.625° × 3.75°, corresponding to a grid of 64 points in longitude × 48 points in latitude. A 40-layer vertical grid is used, using a modifiedη (terrain-following) coordinate ( η = P/P surf , where the model-top pressure has been subtracted from both pressures). The dynamical timestep varies between runs but is never longer than 1-min. The planetary  boundary  layer  scheme  is  based  on  that  in  Ref.  64  (Medium-Range  Forecast  model scheme), with Mars implementation similar to that in Ref. 20, and the surface layer uses a MoninObukhov scheme. Runs are initialized from a cold state and continue until the simulated annual seasonal cycle is highly repeatable from year to year.  We found that runs require only a single year of spin-up adjustment, as modern Mars, lacking seas, has low effective thermal inertia. Output is  reported from the third year of each model run. Mean wind speed ~30 m above the surface  \nincreases from 8 m/s in the no-nanorods case to 13 m/s in the Fig. 2a (with-nanorods) case, which would stir up more dust (and nanorods). Daytime near-surface turbulence on Mars is sufficient to loft  dust (and therefore nanorods) released at 10-100m altitude at all latitudes for P > 6 mbar. Sensitivity tests adjusting model resolution, and varying nanorod distribution and other parameters are listed in Table S3.  \nFor the no-nanorods case (Fig. 2b), the global and annual average shortwave radiation reaching the surface is 133.1 W/m 2 (119.7 W/m 2 direct beam and the remainder diffuse/scattered), of which 104.2 W/m 2 is  absorbed with the rest being reflected. The spatially- and time-averaged surface albedo,  including  ice,  is  0.237.  The  global/annual  average  longwave  radiation  at  the  surface (greenhouse effect) is 23.9 W/m 2 . The global/annual average longwave emission from the surface is 125.4 W/m 2 . The mismatch of -2.7 W/m 2 is caused by limited output sampling, fluxes into the surface (e.g. CO 2 condensation, conduction), and model imprecision.  For the with-nanorods case shown  in  Fig.  2a,  the  global  and  annual  average  shortwave  radiation  reaching  the  surface  is 89.2 W/m 2 (41.4 W/m 2 direct beam), of which 70.6 W/m 2 is absorbed with the rest being reflected. The spatially-  and  time-averaged  surface  albedo,  including  ice,  is  0.208.  A  greater  fraction  of sunlight is absorbed due to reduction in reflective seasonal CO 2 ice. The global/annual average longwave radiation at the surface (greenhouse effect) is 186.2 W/m 2 . The global/annual average longwave emission from the surface is 254.3 W/m 2 .  The  mismatch of -2.5 W/m 2 (i.e.,  surface temperature under-stated relative to radiative fluxes) is caused by limited output sampling, fluxes into the surface (e.g. CO 2 condensation, conduction), and model imprecision.  \nThe runs shown in Fig. 2c correspond to optical depths at λ = 0.67 µm of {0, 0.125, 0.25, 0.5, 0.75, 1, 1.5, 2}. Details for selected values are given in Fig. S14. The conversion factor is 545 mg/m 2 for Fe and 212 mg/m 2 for Al for one optical depth. Optical depths in the spectral windows are much greater than at 0.67 µm (Fig. 1). Output was sampled at 260 equally spaced intervals per Mars year. This provides (deliberately aliased) sampling of the day-night cycle during each season at each longitude and latitude. A sensitivity test using 1300 output steps showed negligible (≤0.1 K) differences in both annual-average and seasonal warming (Table S3). Mars\' atmosphere is thin, with low thermal inertia and limited ability to transport heat laterally, and the tests with artificial imposed patchy or unsteady nanorod distribution show that the corresponding warming is sharply confined in space and time. This suggests that it might be possible to enhance warming at preferred latitudes and seasons. The atmosphere and surface temperature quickly responds to the radiative forcing at the release site/time, and after injection ceases, spreading of the particles away from the release site ensures that the warming at the release site soon decreases.', '4. Possible hazards.': 'Natural Mars air is unsafe for humans to breathe because it has almost no oxygen (insufficient for deflagration) and also has a high natural concentration of PM 2.5 (Mars mineral aerosol dust). The nanorod density is ~10 µg/m 3 , which would not substantially alter this situation. A more immediate concern is asbestosis, as humans would bring both natural dust and nanoparticles into settlements via airlocks. One way to mitigate this hazard would be to make nanorods that dissolve or fragment in liquid water.', '5. Comparison to previous work.': 'To our knowledge, the most effective (on a per-unit-mass-in-the-atmosphere basis) Mars warming agent that has been previously been proposed is the "optimal [gas] mix" of ref. 8, which is mostly C 3 F 8 (molecular mass 188 Da). This gives 37.5 K warming for 1 Pa, which corresponds to 170 kg/m 2 × (1 Pa / 650 Pa) × (~188 Da / 44 Da) ≈ 1.1 kg/m 2 . This warming is about the same as the nanorod case shown in Fig. 2a, which corresponds to ~160 mg/m 2 of Al nanorods. Therefore this nanorod loading (using non-optimized nanoparticles) is >5000× more effective than the optimal gas mix.  \nFig. S1. (a,b,c) Verification of calculations by comparison to Mie-theory results for a water-ice sphere (49), showing single particle albedo, scattering asymmetry, and extinction efficiency.  (d,e) Shows the comparison of analytical Mie scattering results for an ice sphere with simulation outputs.  \n<!-- image -->  \nFig.  S2. Illustration  showing  the  definition  of  the  angles θ and φ in  the  FDTD  calculation. "k" corresponds to the direction of the propagation of the incident electromagnetic wave. The gray bar corresponds to the nanorod.  \n<!-- image -->  \n<!-- image -->  \nSingle particle albedo  \n<!-- image -->  \nFig. S3. Switching between a square and circular cross-section for the nanorod has only minor effects  on  the  calculated  optical  properties.  Fe  nanorod,  9  µm-long,  ~60:1  aspect  ratio, orientation: θ = 45°, φ = 45°.  \n<!-- image -->  \n<!-- image -->  \nFig. S4. FDTD output figure, showing orientation dependence of the optical properties of a 9 µm-long, ~60:1 aspect ratio Fe nanorod. (a) Cross-sections as a function of wavelength. (b) Cross-sections and scattering asymmetry as a function of orientation for wavelengths 21.9 µm (solid lines, bold labels) and 0.70 µm (dashed lines, italic labels).  \n<!-- image -->  \nResults of interpolation and simulation at 0-60,0=15  \n<!-- image -->  \n<!-- image -->  \n<!-- image -->  \nFig. S5. Results of interpolation check. 9 µm-long, ~60:1 aspect ratio Fe nanorod. Units of angles are degrees.  \n<!-- image -->  \nFig. S6. Sensitivity test showing change in optical properties when mesh resolution is increased (decreasing mesh spacing). 9 µm-long, ~60:1 aspect ratio Fe nanorod. Orientation: θ = 45°, φ = 45°.  \n<!-- image -->  \nFig. S7. Showing sensitivity of calculated optical properties to changing nanorod composition (Al  vs.  Fe),  for  a  9 µm-long,  ~60:1  aspect  ratio  nanorod.  The  total  extinction  cross  section (scattering plus absorption) and the location of the resonances remains about the same; a small increase in Al nanorod length would be sufficient to closely match the Fe cross-sections. As Al is 3× less dense than Fe, this suggests Al is more effective on a warming-per-unit-mass basis. Conductive-nanorod size and shape, and not composition, are the main controls on simulated nanorod optical properties. Aluminum material properties are obtained from ref. 51. Orientation: θ = 45°, φ = 45°.  \n<!-- image -->  \nFig. S8. Showing the calculated optical properties varying particle length and width for the same particle composition: specifically, 7.5 µm-long Fe nanorods with cross-section 0.08 µm × 0.08 µm, compared to 9 µm-long Fe nanorods with cross-section 0.16 µm × 0.16 µm.  Orientation-averaged  optical  properties  calculated  using  a  3D  FiniteDifference Time-Domain (FDTD) approach. Upper panel: Solid black line corresponds to total extinction, dotted black line to scattering, both for 7.5 µm-long Fe nanorods with cross-section 0.08 µm × 0.08 µm. Lower panel shows scattering asymmetry. Also shown in both panels are spectra for natural dust assuming a log-gaussian particle size distribution centered on 2.5 µm (48)  (red  dotted  lines).  In  both  panels,  gray  lines  correspond  to  the  results  for  a  9  μm-long Fe nanorod with cross-section 0.16 µm × 0.16 µm. As expected given its spectrum, the 7.5 µmlong nanorod gives ~2× less warming per nanorod in climate simulations. However, as it uses 5× less Fe, this design is more effective on a warming-per-unit-mass basis.  \n<!-- image -->  \nFig. S9. Showing how the spectra are represented within the 3-D climate model. 9 µm-long, ~60:1 aspect ratio nanorod. 4.5 µm marks the separation between solar bins (Planck-weighted using the orange 6000 K blackbody curve, normalized W/m 2 /µm flux density), and thermal IR bins (Planckweighted using the pink 215 K blackbody curve, normalized W/m 2 /µm flux density). The relative heights of the orange and pink Planck functions are to guide the eye only and have no physical significance.  \n<!-- image -->  \nFig. S10. Annual average temperatures (K) for (top) control simulation without nanorods (i.e., Fig. 2b), and (bottom) warmed case with ~160 mg/m 2 Al nanorods (i.e., Fig. 2a). Labeled black contours correspond to topographic elevation in m. Green contour corresponds to 610 Pa (~6 mbar) mean pressure level. Blue lines: approximate latitudinal (equatorward) extent of ice at <1 m depths.  \n<!-- image -->  \n<!-- image -->  \nFig.  S11. Seasonal  variation  of  diurnal-average  surface  temperature  (K)  for  (left)  control simulation without nanorods (i.e., Fig. 2b), and (right) warmed case with ~160 mg/m 2 nanorods (i.e., Fig. 2a), showing longitudinal average surface temperature. Each of the 10 increments on the x-axis, which are equally spaced in time, corresponds to 1/10 of a Mars year (69 Earth days). To make this figure, a 9-point smoother has been used (3 points in time, and 3 points in latitude) in  order  to  damp  oscillations  associated  with  aliasing  in  the  sampling  of  the  day-night temperature cycle in the output. The black lines show the limit of substantial polar seasonal CO 2 ice.  \n<!-- image -->  \nFig. S12. Sensitivity tests showing how warm-season temperatures vary for different parameter choices. Baseline is Fe nanorod results shown in Fig. S19. Labeled black contours correspond to topographic elevation in m. Green contour corresponds to 610 Pa (~6 mbar) mean pressure level. Blue lines: approximate latitudinal (equatorward) extent of ice at <1 m depths. (a) More nanorods ( τ Fe = 1.5): the average surface pressure exceeds 610 Pa (green contour) at all locations below +2 km elevation. (b) Fewer nanorods ( τ Fe = 0.375).  \n<!-- image -->  \nFig. S13. Cross section of steady (imposed) nanorod layer ( τ Fe = 0.75, Fig. S19). The purple shading corresponds to the relative volume density of nanorods. The mixing ratio of nanorods within the purple layer is uniform, but there is more gas closer to the planet\'s surface so there are also more nanorods lower down. The tenuous part of the nanorod layer extending to high altitude  is  disproportionately  important  because  of  its  strong  contribution  to  the  greenhouse effect.  The  Y-axis  uses  terrain-following  η-coordinates  (η = P/P surf ,  where  the  model-top pressure has been subtracted from both pressures), and the cloud lines are tilted down and to the left because the southern hemisphere surface is topographically higher and thus closer to the fixed-pressure  base  of  the  nanorod  layer.  The  yellow  lines  are  contours  of  annual  average atmospheric temperature. The green lines are contours of altitude in meters, which bow upwards at the poles where low temperatures compact the gas column.  \n<!-- image -->  \nFig. S14. Summary of the latitudinal and seasonal dependence of surface temperature for (red lines) the 3D model with-nanorods τ Fe = 0.75 case (Fig. S19a), and (blue lines) the 3D model without  nanorods  (Fig.  S19b).  Solid  line  corresponds  to  the  average  temperature  during  the warmest  season  (~70 day  period)  during  the  year,  dotted  line  corresponds  to  the  average temperature during the coldest season (~70 day period) of the year, and dashed line corresponds to the annual average. The flattening-out of the lines around 145 K corresponds to buffering at the frost point of CO 2 .  \n<!-- image -->  \nFig. S15. As Fig 2c, but with a linear x-axis instead of a logarithmic x-axis. Dependence of planet-averaged surface warming on nanorod column mass. Blue triangle corresponds to Fig. 2a, the intersection of the blue line with the zero-nanorods axis corresponds to Fig. 2b, and white triangle marks onset of warm-season temperatures above the freezing point of water at 50°S. Blue corresponds to 3-D results. The blue envelope corresponds to the modeled seasonal range in  global  mean T surf .  Gray  corresponds  to  1-D  results.  The  red  asterisk  corresponds  to  the observed modern Mars value.  \n<!-- image -->  \nFig. S16. Summary of 1-D climate model output as a function of nanorod column density. Left column corresponds to Fe, right column corresponds to Al.  \n<!-- image -->  \nFig. S17. Outgoing Longwave Radiation (OLR) from the 1D model. The integrated top-ofatmosphere OLR values are ~114.6,  96.6, and 120 W/m 2 for the no-nanorod, Al and Fe nanorod cases, respectively. (a) The no-nanorods case. Spectral windows are visible as peaks on either side of the CO 2 band which is at 600-800 per cm. (b) The nanorods case with τ Fe = 0.75. Absence of prominent peaks corresponds to closure of the spectral windows. (c) The nanorods case with τ Al = 0.75.  \n<!-- image -->  \nFig. S18. 1D model vs. 3D (MarsWRF) model temperature profile comparison for τ Fe = 0.75. The 3D result is the global and annual average. At this optical depth, surface temperature for both models is ~ 250 K.  \n<!-- image -->  \nFig.  S19. As Fig. 2, but for 400 mg/m 2 loading  of  Fe  nanorods  instead  of  160 mg/m 2 loading  of  Al  nanorods. Warmseason temperatures (K) (color shading) on (a) Mars with addition of ~400 mg/m 2 of nanorods,  (b)  control  case.  This corresponds to the average surface temperature  during  the  warmest  36°  of solar  longitude  (~70  days)  of  the  year. White  contour  corresponds  to  610  Pa (~6 mbar)  mean  pressure  level.  Black contours correspond to topographic elevation in m (dashed: -5 km and -2 km, solid:  0 km,  +2km,  and  +5  km).  Blue lines: approximate latitudinal (equatorward)  extent  of  ice  at  <1  m depths.  Results  do  not  include  CO 2 outgassing from within polar ice, which would cause further warming. (c) Dependence of planet-averaged surface  warming  on  nanorod  column mass.  Blue  line  corresponds  to  3-D results, varying layer-top height between ~35 km (solid line) and ~28 km (dashed line).  The  blue  envelope  corresponds  to the  modeled  seasonal  range  in  global mean T surf . Gray  corresponds  to  1-D results (see text for details). Blue triangle corresponds  to  panel  (a),  and  white triangle  marks  onset  of  warm-season temperatures above the freezing point of water  at  50°S.  Symbols  on  y-axis  are temperatures  for  the  no-nanorod  case, with  the  red  asterisk  corresponding  to observed  Mars  value.  More  detail  is shown in Figs. S10-S17.  \n<!-- image -->  \n<!-- image -->  \n<!-- image -->  \nSupplementary Table 1: Orientation-averaged optical properties of nanorods as used in the  \nclimate simulations, corresponding to a 9 µm-long nanorod with cross-section (0.16×0.16) µm.  \nSupplementary Table 2. Summary of climate model output. The nanorod column mass differs between the Al and Fe cases by a factor of 2.57 (ratio of material densities, corrected for the 13% greater extinction cross-section of Fe rods at 0.67 µm relative to Al rods).  \nSupplementary Table 3. Summary output for additional sensitivity tests using 3D model. All were carried out using Fe nanorods.'}