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"Egg Rock" is a smooth, shiny nickel-iron meteorite found sitting on the surface of Mars on 30-Oct-2016. It is described the next day in Arizona State University's Red Planet Report website: Curiosity: Egg Rock, a small meteorite.

According to Gizmodo's Curiosity Just Found A Freaky Metal Meteorite On Mars:

As Deborah Byrd notes in EarthSky, the surface of Mars is peppered with meteorites, so discoveries like this aren't all that uncommon. Meteorites can last for millions of years on the Red Planet, free from the oxidising and weathering effects of moisture and oxygen.

According to Wikipedia, Martian soil (and presumably blowing dust) contains plenty of perchlorate, a strong oxidizer. Perchlorate on Mars's surface is discussed in this answer, and both perchlorate and peroxide are discussed in this question, with reference to deleterious chemical effects on space suits.

Why then would metallic meteorites remain unoxidized on Mars?

Images from EarthSky, credit NASA/JPL/ASU, captions "October 30, 2016 image via Curiosity rover on Mars" (click for larger size):

"Egg Rock" meteorite on Mars "Egg Rock" meteorite on Mars "Egg Rock" meteorite on Mars

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As it turns out, perchlorate is actually rather unreactive under Martian surface conditions, and this actually explains the unusual abundance of perchlorate there:

In any case, most of the chlorine has been oxidized as far as possible because the chlorine within perchlorate has a maximal +7 oxidation number. From a thermodynamic perspective, this makes perchlorate a strong oxidizing agent but kinetically, perchlorate is very inert at typical planetary surface temperatures. The energetically stable tetrahedral structure of oxygen atoms in perchlorate around the central, electrophilic chlorine atom renders perchlorate unreactive [Brown and Gu, 2006]. Hence, perchlorate, once formed, is prone to persist in soil provided that liquid water does not flush the perchlorate away, given that perchlorate salts are generally highly soluble. In contrast, if an environment is very wet, perchlorate is liable to be leached out of soil and end up diluted to trace concentrations in groundwater or rivers. In anaerobic conditions on Earth, the perchlorate is then microbially degraded back to chloride [Coates and Achenbach, 2004]. In short, perchlorate will accumulate in soil if its rate of deposition exceeds the rate of dissolution, and it will then persist if conditions remain free of perchlorate reduction. Thus, the oxidizing aridity of Mars, combined with perchlorate being an unreactive cul-de-sac of oxidation chemistry, probably explains the unusual abundance of perchlorate in Martian soil.

-- Atmospheric origins of perchlorate on Mars and in the Atacama; Catling, et al.

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  • $\begingroup$ Excellent! That's quite a useful and informative reference, and it's open access too. I'm looking forward to reading it. The Atacama region is home to a large number of optical telescopes as well as the ALMA sub-millimeter array of radio telescopes. Lots of shiny metal things there, nearly-zero water too! $\endgroup$
    – uhoh
    Commented Jan 31, 2018 at 14:48
  • $\begingroup$ Possible additional explanation for "the unusual abundance of perchlorate there": Help understanding the false MRO observations of hydrated minerals on Mars $\endgroup$
    – uhoh
    Commented Nov 23, 2018 at 10:02
  • $\begingroup$ A corollary to this is that the highly oxidizing (thermodynamically) perchlorate may not be as damaging to organic materials/organisms as one might think. Might it be turned into a controllable oxygen source using appropriate enzymes? $\endgroup$
    – Oscar Lanzi
    Commented May 11, 2020 at 0:02

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