# Why don't iron meteorites on Mars rust or oxidize? Why are they shiny?

Wikipedia's WikiNews' 2005 article Mars Rover’s Basketball-Shaped Discovery Stirs Controversy says:

The surface of the meteorite was found to be remarkably free of corrosion. Researchers are as yet unsure whether this means it arrived recently or that it has possibly been sandblasted thoroughly.

and links to Space.com's Mars Rover's Meteorite Discovery Triggers Questions

On earth we don't normally see clean, shiny iron surfaces because iron is easily oxidized in a short time even at room temperature by several oxidizing components of air. Some of these are present on Mars but of course at far lower in concentration, but there are other oxidizers including perchlorate. Yet both iron meteorites discovered on Mars that I'm aware of (Heat Shield Rock and Egg Rock) look clean as do others shown in that Sky & Telescope article.

Is it known why? Have they landed so recently that oxidation hasn't had the time to make much change, or are they sandblasted clean as Karen Steele's character Eve McHuron told Kirk one of the miners to do with a dirty cast-iron skillet on the storm-plagued planet Rigel XII.

Photos of these two meteorites taken by Curiosity can be seen in

• this answer to What is the name of meteoroids which hit Moon, Mars, or pretty much anything that isn’t the Earth?
• this answer to Who discovered “Egg Rock”? The Curiosity rover or people?

• There is further background (and possibly some answers) in Evidence for mechanical and chemical alteration of iron‐nickel meteorites on Mars: Process insights for Meridiani Planum
– uhoh
Mar 26 '20 at 0:50
• «Shiny» or not does not need to depend the degree of oxidation. You get shiny shoes, for example, because the polishing them eventually may smooth the surface to a roughness at magnitude equal to or below of the wavelength of observation. Like if you grind a parabola antenna to cook a meal (e.g, first part of youtube.com/watch?v=RWX5HH6HyPI). Beside albedo ... Mar 26 '20 at 2:09
• @Buttonwood show me some oxidized yet shiny iron from a natural environment. The question is about the chemical and physical processes associated iron oxidized by environmental conditions, not shoes and parabolae.
– uhoh
Mar 26 '20 at 2:10
• A rusty surface is not smooth, it this much porous that oxidation advances till the ground. Copper, on the other hand, passivates and is not «eaten away» this way. Mar 26 '20 at 2:14
• @uhoh As side note I speculate @John referred to pictures like «A rare pseudo-scalenohedral crystal habit» en.wikipedia.org/wiki/Hematite (the entry equally mentiones its use as jewelery as well as occurences on Mars), the title picture for the Czech (cs.wikipedia.org/wiki/Hematit) and Spanish (es.wikipedia.org/wiki/Hematita) wikipedia of $\ce{Fe2O3}$, or the «Perfekt gewachsener Hämatit-Skalenoeder von der Insel Hormus, Iran » on de.wikipedia.org/wiki/Hämatit of high reflectance but still share the rust-red streak. Mar 26 '20 at 20:41

tl;dr: Compared to Earth, the atmosphere on Mars is very thin; in addition, it contains much less of oxygen and water (i.e., is very dry). It is much colder there. These conditions may slow down oxidation to an irrelevant rate.

For an object of pure iron, note that according to the English Wikipedia about Mars:

"The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water."

This contributes as plausible contribution for a lesser rate of oxidation than on earth because the concentration of an oxidizer (i.e., oxygen) is much lower (0.146 vol%, according to the side table, ibid.) if one assumes absence of processes splitting the $$\ce{CO2}$$ to yield oxygen. To set these values into context (entry Earth):

"A dry atmosphere is composed of 78.084% nitrogen, 20.946% oxygen, 0.934% argon, and trace amounts of carbon dioxide and other gaseous molecules. [...] Water vapor content varies between 0.01% and 4% but averages about 1%."

Equally note that the reported surface temperatures on Mars (min 130 K, mean 210 K, max 308 K) are substantially lower, than on Earth (min 184 K, mean 287.16 K, max 330 K) -- equally slowing oxidation.

About water and atmosphere humidity: Provided the meteorites get water wetted, said perchlorates could start to work as oxidizer of iron. In comparison to this, dry perchlorate will remain inactive. Again, Mars' atmosphere (0.0210 vol% of water vapour, ibid.) seems to be considerably more conservative than Earth's.

You can't have oxidation alone, it is a electrochemical process where some other material in contact with the one oxidized has to be reduced. Corrosion of the meteorites equally could occur if these simultaneously contain grains of metals / alloys differing in the their electrode potential provided

• these grains were in close contact with each other, especially if their grain boundary were water wetted, and

• if one of the metals / alloys -- relatively speaking -- would be «less noble», i.e. a material with a more negative reduction potential, than any other grain / material in contact. The less noble grain than would become a galvanic anode and eventually be "eaten away", protecting the other, more noble one, to be oxidized.(table) The formation of these little, local electrochemical cells could be even more important if there were salty water droplets on the surface at room temperature and above accelerating corrosion. (This, by the way, a reason why steel bridges and copper house roofs exposed to the climate and atmosphere of salty seas are more prone to galvanic corrosion than similar constructions only exposed to fresh water.) Presence of phosphates in said meteorites however could slow down the oxidation of the meteorites (passivation, as assumed for the Iron pillar in Dehli).

Note: The initial answer assumed iron meteorites to contain discrete grains, occasionally altogether with grains of other metals. A comment by@KenFabian pointed out iron meteorites however mainly consist of (Fe,Ni) alloys. Their electrochemical properties may be different to their components (e.g, table), which the edit aims to include.

• This pretty much sums it up. There is very little oxygen in the Martian atmosphere. Whatever "trace oxygen" is in the atmosphere, it's at an even lower total pressure, so the partial pressure of oxygen is negligible. Mar 26 '20 at 4:44
• Note that metallic meteorites have never been found as pure iron; they are all Nickel-Iron alloys - mostly taenite or kamecite. Mar 26 '20 at 5:39
• @uhoh Your first quote (WikiNews' 2005 article) allows: a) metorites found are shiny, because they are young. b) They are old, but because they are sand blasted traces of oxidation are insignificant (Mars' dust storms). c) They are old, but low concentration of oxygen and water (wikipedia's entry about the Martian atmosphere reads almost like a low-pressure inert gas here) slow down oxidation to a minimal rate / there is no significant oxidation. Source accessed by mine do not indicate if the time since the impact of these meteroites on Mars was determined to exclude either a), b), or c). Mar 26 '20 at 11:14
• @uhoh Re ruling out one in (a,b,c) / narrowing range of age of these metorites: according to meteoritestudies.com/protected_dating.htm, it is possible to date them. While the principle is similar to the carbon clock, isotopes determined here by mass spectroscopy were Sr/Rb ratios, a technique dominant till about the 1980s. Maybe the mars rovers (will) use such an analyzer. Mar 26 '20 at 12:29
• $\ce{CO_2}$ can reduce to the monoxide and by doing that, oxidize iron. It is way less oxidizing than oxygen though. But the important key is the low total pressure. Mar 26 '20 at 15:08