View source for What makes Mars red


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File:Mars - August 30 2021 - Flickr - Kevin M. Gill.png|Mars in true color,[a] as captured by the Hope orbiter. The Tharsis Montes can be seen at the center, with Olympus Mons just to the left and Valles Marineris at the right.
	This image was originally posted to Flickr by Kevin M. Gill at https://flickr.com/photos/53460575@N03/51831155718. It was reviewed on 24 January 2022 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

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The red color of Mars has impressed us for a long time.  Scientists have suggested several chemicals that could account for it.  They all center around iron and iron rust.  Rust can be several iron compounds.  As early as 1934, Rupert Wildt suggested an iron oxide was responsible.  In the 1960’s, an iron ore called limonite was advanced by Sagan in the US and by Sharonov in the U.S.S.R.  Limonite contains ferric oxide and some bound water.  Others suggested the minerals hematite or goethite.<ref>Glasstone, S.  1968.  The Book of Mars,  NASA.</ref>
We have tried many ways of tying down the exact chemical responsible for Mars’s distinctive color.  Telescopic observations with spectroscopes have found anhydrous hematite (termed “nanophase NpOx”) on the surface.<ref> Morris, R. V. et al. Evidence for pigmentary hematite on Mars based on optical, magnetic, and Mossbauer studies of superparamagnetic (nanocrystalline) hematite. J. Geophys. Res. Solid Earth 94, 2760–2778 (1989).</ref> <ref>  Bell, J. F. III, McCord, T. B. & Owensby, P. D. Observational evidence of crystalline iron oxides on Mars. J. Geophys. Res. Solid Earth 95, 14447–14461 (1990).</ref>  Anhydrous means without water.  However, other studies have detected some water.  Some of the first spacecraft observations revealed some water in the Martian dust spectrum.<ref>Pimentel, G. C., Forney, P. B. & Herr, K. C. Evidence about hydrate and solid water in the Martian surface from the 1969 Mariner Infrared Spectrometer. J. Geophys. Res. 79, 1623–1634 (1974). </ref> <ref> Murchie, S. et al. Spatial Variations in the Spectral Properties of Bright Regions on Mars. Icarus 105, 454–468 (1993).</ref>     NASA’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) that is orbiting Mars aboard the Mars Reconnaissance Orbiter also discovered water.<ref> Murchie, S. L. et al. Visible to Short-Wave Infrared Spectral Analyses of Mars from Orbit Using CRISM and OMEGA. in Remote Compositional Analysis: Techniques for Understanding Spectroscopy, Mineralogy, and Geochemistry of Planetary Surfaces (eds. Bell I. I. I., J. F., Bishop, J. L. & Moersch, J. E.) 453–483 (Cambridge University Press, 2019). https://doi.org/10.1017/9781316888872.025.</ref>  Studies of meteorites from Mars showed signs of water.<ref>Bishop, J. L., Pieters, C. M., Hiroi, T. & Mustard, J. F. Spectroscopic analysis of Martian meteorite Allan Hills 84001 powder and applications for spectral identification of minerals and other soil components on Mars. Meteorit. Planet. Sci. 33, 699–707 (1998).</ref> <ref>Beck, P. et al. A Noachian source region for the “Black Beauty” meteorite, and a source lithology for Mars surface hydrated dust? Earth Planet. Sci. Lett. 427, 104–111 (2015).</ref>
Instruments landed on Mars have all detected iron compounds on the surface.  Everything is covered with the red-yellow dust.  Mars Exploration Rovers (MER), both Spirit and Opportunity, showed the existence of  the iron minerals hematite  and goethite, as well as the ubiquitous presence of undetermined iron oxide phase (“nanophase NpOx”) in the fine dust.<ref>Morris, R. V. et al. Mössbauer mineralogy of rock, soil, and dust at Meridiani Planum, Mars: Opportunity’s journey across sulfate-rich outcrop, basaltic sand and dust, and hematite lag deposits. J. Geophys. Res. Planets 111, E12S15 (2006).</ref>  <ref>Morris, R. V. et al. Mössbauer mineralogy of rock, soil, and dust at Gusev crater, Mars: Spirit’s journey through weakly altered olivine basalt on the plains and pervasively altered basalt in the Columbia Hills. J. Geophys. Res. Planets 111, E02S13 (2006).</ref>  <ref> Goetz, W. et al. Indication of drier periods on Mars from the chemistry and mineralogy of atmospheric dust. Nature 436, 62–65 (2005).</ref>
Spirit and Opportunity were also equipped with a series of magnet arrays designed to analyze airfall dust. The analysis of the magnetic targets using their spectra and imaging systems identified two distinct ferric iron endmembers in the dust: one comprising strongly magnetic and dark-colored magnetite, and the other an unidentified bright-colored (oxy)hydroxide exhibiting weak magnetic properties.  These findings support iron that was altered by water.<ref>Madsen, M. B. et al. Overview of the magnetic properties experiments on the Mars Exploration Rovers. J. Geophys. Res. Planets 114, E06S90 (2009).</ref> <ref>
NASA landed the Mars Science Laboratory (MSL) rover named Curiosity in Gale Crater.  It yielded several key chemistry and mineralogy measurements of Martian dust and soils. The Chemistry and Camera (ChemCam) instrument used  its laser-induced breakdown spectroscopy (LIBS) capability to analyze the composition of airfall dust. In each of the initial laser shots from a series of 50 shots on dusty rock surfaces and calibration targets that collected dust over the years, ChemCam consistently found hydrogen.  This supports the idea that water is bound with iron.<Meslin, P.-Y. et al. Soil Diversity and Hydration at Gale Crater. Mars. Sci. 341, 1–9 (2013).</ref> <ref>Lasue, J. et al. Martian Eolian Dust Probed by ChemCam. Geophys. Res. Lett. 45, 10,968–10,977 (2018).</ref>


Recent research has strongly indicated that ferrihydrite, a water-rich iron oxide mineral, plays a significant role in Mars' reddish hue.   The team of scientists mixed up chemicals they thought caused the color.  When they compared the spectrum of their mixture to the spectra of Mars, they found a good match.  
This mineral forms in environments with liquid water, suggesting that Mars may have had a wetter past.  The presence of ferrihydrite is a key finding, as it provides clues about the planet's ancient climate and potential habitability.  This discovery of a water iron oxide means that liquid water with a neutral pH could have been on Mars.   For a long time, hematite, another iron oxide, was considered the primary source of Mars' red color.  While hematite is undoubtedly present on Mars, this study  suggests that ferrihydrite may be more prevalent.  It's important to note that various other iron oxides may also contribute to the Martian surface's color.  The composition of Martian dust is complex, and it's likely a combination of different iron-bearing minerals. 
<ref>https://www.nasa.gov/centers-and-facilities/goddard/nasa-new-study-on-why-mars-is-red-supports-potentially-habitable-past/#:~:text=Martian%20dust%20is%20known%20to,those%20iron%20oxides%2C%20ferrihydrite%2C%20is</ref>
<ref>https://www.sci.news/space/mars-color-13699.html#:~:text=Called%20ferrihydrite%2C%20this%20iron%20mineral,transition%20to%20the%20current%20hyper%2D</ref>
<ref>A. Valantinas et al. 2025. Detection of ferrihydrite in Martian red dust records ancient cold and wet conditions on Mars. Nat Commun 16, 1712; doi: 10.1038/s41467-025-56970-z</ref>

The following is a more detailed description of the iron compounds that can be red.

Iron can exist in different states.  One state, called the ferric oxidation state, is red.  It forms the color of rust.   Rust  is not a single chemical compound, but instead it is  a complex mixture. The primary ingredient  of rust is iron(III) oxide (Fe₂O₃). This is what gives rust its  reddish-brown color.  Other iron oxides, such as iron(II) oxide (FeO) and magnetite (Fe₃O₄), can also be present. 

Hydrated iron oxides are another.  They contain water molecules  Their chemical formula is   (Fe₂O₃·nH₂O). The "n" represents a variable number of water molecules.  A related kind is  Iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3).  
Some factors affect how well this process goes.  Water is a crucial catalyst for rust formation.  The availability of oxygen affects the oxidation.  And,  salts, such as sodium chloride (NaCl), can increase rusting.  That is why the use of salt on icy roads causes cars to rust out easily. 
<ref>https://www.metaltek.com/blog/what-is-rust-and-how-to-prevent-it/#:~:text=The%20chemical%20reaction%20between%20iron,oxide%2C%20commonly%20known%20as%20rust.</ref> <ref>https://www.geo.mtu.edu/KeweenawGeoheritage/HoughtonEC/Iron_Formation.html</ref> <ref>https://www.rutgers.edu/news/how-rocks-rusted-earth-and-turned-red#:~:text=%E2%80%9CAll%20of%20the%20red%20color,said%20lead%20author%20Christopher%20J.</ref>

We often say that iron is oxidized from the ferrous form to the ferric form.  Ferris forms are found in minerals from volcanic eruptions.  The ferris minerals can then be changed.  This is done with the process of oxidation–that means the iron has combined with oxygen.  Oxygen has to be present, but the reaction goes much faster with water.  Oxygen takes electrons away from the iron; thus, increasing the negative charge on the oxygen.  That negative charged oxygen causes the oxygen to stick (or be bonded) to the iron.  Oxidized iron can have a brown to reddish color.  
The red color arises from the way Fe₂O₃ interacts with light. Specifically, the electronic transitions within the iron ions absorb certain wavelengths of light, leaving the red wavelengths to be reflected.
<ref>https://pubchem.ncbi.nlm.nih.gov/compound/Ferric-Oxide-Red#section=FTIR-Spectra</ref>

So, at this point in time, we believe the red color of Mars is due to the dust containing a hydrated iron oxide.  Volcanic rocks were modified with water and oxygen.  The rocks eventually broke up and formed some dust over time.  The red dust was then deposited all over the planet.  Millions or billions of years were available for these processes to happen.

==See Also==
*[[Iron]]
*[[Mars Reconnaissance Orbiter]]
*[[Curiosity]]
*[[Spirit]]
*[[Opportunity]]
==References==
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