Mars surface color

Rock-strewn surface imaged by Mars Pathfinder

The apparent color of the Martian surface enabled humans to distinguish it from other planets early in human history and motivated them to weave fables of war in association with Mars. One of its earliest names, Har decher, literally meant "Red One" in Egyptian.^[1] Its color may have also contributed to a malignant association in Indian astrology, as it was given the names Angaraka and Lohitanga, both reflecting the distinctively red color of Mars as seen by the naked eye.^[1] Modern robotic explorers have shown that not only the surfaces, but also the skies above may appear red under sunlit conditions on Mars.

Reason for the color and its extensiveness

Modern observations indicate that Mars's redness is skin deep. The Martian surface looks reddish primarily because of an ubiquitous dust layer (particles are typically between 3 µm to 45 µm across ^[2][3]) that is typically on the order of millimeters thick. Even where the thickest deposits of this reddish dust occur, such as the Tharsis area, the dust layer is probably not more than 2 m (7 feet) thick.^[4]. Therefore, the reddish dust is essentially an extremely thin veneer on the Martian surface and does not represent the bulk of the Martian subsurface in any way.

The reddish dust

Martian dust is reddish mostly due the spectral properties of nanophase ferric oxides (npOx) that tend to dominate in the visible spectrum. The specific npOx minerals have not been fully constrained, but nanocrystalline red hematite (α-Fe₂O₃) may be the volumetrically dominant one,^[5] at least at the less than 100 µm sampling depth^[6] of infrared remote sensors such as the Mars Express OMEGA instrument. The rest of the iron in the dust, perhaps as much as 50% of the mass, may be in titanium enriched magnetite (Fe₃O₄).^[7] Magnetite is usually black in color with a black streak^[8], and does not contribute to the reddish hue of dust.

The mass fraction of chlorine and sulfur in the dust is greater than that which has been found (by the Mars Exploration Rovers Spirit and Opportunity) in the soil types at Gusev crater and Meridiani Planum. The sulfur in the dust also shows a positive correlation with npOx.^[9] This suggests that very limited chemical alteration by thin brine films (facilitated by the formation of frost from atmospheric H₂O) may be producing some of the npOx.^[9] In addition, remote sensing observations of atmospheric dust (which shows slight compositional and grain size differences from surface dust), indicates that the bulk volume of dust grains consists of plagioclase feldspar and zeolite, along with minor pyroxene and olivine components.^[10] Such fine material can be generated easily via mechanical erosion from feldspar-rich basalts, such as rocks in the southern highlands on Mars.^[10] Collectively, these observations indicate that any chemical alteration of dust by aqueous activity has been very minor.

The occurrence of npOx in dust

There are several processes that can yield npOx as an oxidation product without the involvement of free O₂. One or more of those processes may have dominated on Mars, since atmospheric modeling over geologic time scales indicates that free O₂ (generated mostly via the photodissociation of H₂O)^[11] may have always been a trace component with a partial pressure not exceeding 0.1 µPa.^[12].

One O₂-independent process involves a direct chemical reaction of Fe²⁺ (commonly present in typical igneous minerals) or metallic Fe with H₂O to produce Fe³⁺(aq), which typically leads to hydroxides such as goethite (FeO•OH)^[11] under experimental conditions.^[13] While this reaction with H₂O is thermodynamically disfavored, it may be sustained nevertheless, by the rapid loss of the H₂ byproduct.^[12] The reaction can be further facilitated by dissolved CO₂ and SO₂, which lower the pH of brine films increasing the concentration of the more oxidative H⁺.^[13]

However, higher temperatures (c. 300 °C) are usually needed to decompose Fe³⁺ (oxy)hydroxides such as goethite into hematite. The formation of palagonitic tephra on the upper slopes of the Mauna Kea volcano may mirror such processes, as consistent with the intriguing spectral and magnetic similarities between palagonitic tephra and Martian dust.^[14] In spite of the need for such kinetic conditions, prolonged arid and low pH conditions on Mars (such as diurnal brine films) may lead to the eventual transformation of goethite into hematite given the thermodynamic stability of the latter.^[13]

Fe and Fe²⁺ may also be oxidized by the activity of hydrogen peroxide (H₂O₂). Even though the H₂O₂ abundance in the Martian atmosphere is very low,^[12] it is temporally persistent and a much stronger oxidant than H₂O. H₂O₂-driven oxidation to Fe³⁺ (usually as hydrated minerals), has been observed experimentally.^[13] In addition, the pervasiveness of the α-Fe₂O₃ spectral signature, but not of hydrated Fe³⁺ minerals reinforces the possibility that npOx may form even without the thermodynamically disfavored intermediaries such as geothite.^[5]

There is also evidence that hematite might form from magnetite in the course of erosion processes. Experiments at the Mars Simulation Laboratory of Aarhus University in Denmark show that when a mixture of magnetide powder, quartz sand, and quartz dust particles is tumbled in a flask, some of the magnetite converts to hematite, coloring the sample red. The proposed explanation for this effect is that when quartz is fructured by the grinding, certain chemical bonds get broken at the newly exposed surfaces; when these surfaces come in contact with magnetite, oxygen atoms may be transferred from quartz surface to magnetite, forming hematite.^[15]

Red skies on Mars

Recent, approximately true-color in situ images from the Mars Pathfinder and Mars Exploration Rover missions indicate that the Martian sky may also appear reddish to humans. Absorption of sunlight in the 0.4-0.6 µm range by dust particles may be the primary reason for the redness of the sky.^[16] An additional contribution may come from the dominance of photon scattering by dust particles at wavelengths in the order 3 µm^[3], which is in the near-infrared range, over Rayleigh scattering by gas molecules.^[17]

References

1. ^ ^{a b} Kieffer, Hugh H., Bruce M. Jakosky, and Conway W. Snyder (1992), "The planet Mars: From antiquity to the present," in Mars, University of Arizona Press, Tucson, AZ, p. 2 ISBN 0816512574
2. ^ Fergason et al. (2006), doi 10.1029/2005JE002583
3. ^ ^{a b} Lemmon et al. (2004), doi 10.1126/science.1104474
4. ^ Ruff and Christensen, 2002 doi 10.1029/2001JE001580
5. ^ ^{a b} Bibring et al. (2006), doi 10.1126/science.1122659
6. ^ Poulet et al. (2007), doi 10.1029/2006JE002840
7. ^ Goetz et al. (2007)
8. ^ Mindat entry
9. ^ ^{a b} Yen et al. (2005), doi 10.1038/nature03637
10. ^ ^{a b} Hamilton et al. (2005) doi 10.1029/2005JE002501
11. ^ ^{a b} Catling and Moore (2003), doi 10.1016/S0019-1035(03)00173-8
12. ^ ^{a b c} Chevrier et al. (2007), doi 10.1038/nature05961
13. ^ ^{a b c d} Chevrier et al. (2006) doi 10.1016/j.gca.2006.06.1368
14. ^ Morris et al. (2001), doi 10.1029/2000JE001328
15. ^ Clara Moskowitz (September 2009). "How Mars Turned Red: Surprising New Theory". Yahoo News. http://news.yahoo.com/s/space/20090921/sc_space/howmarsturnedredsurprisingnewtheory. Retrieved 2009-09-21. ^{[dead link]}
16. ^ Bell et al. (2006), doi 10.1029/2006JE002687
17. ^ Thomas et al. (1999), doi 10.1029/98JE02556