Block Island Meteorite on Mars (APOD 2009 August 13)

[Chris Peterson]

Iron will react with the oxygen in water (H-O-H) to create ferris oxide, why not he oxygen in carbon dioxide (O-C-O)?

Iron doesn't simply react with the oxygen in water. What happens is that water and O2 react to produce hydroxide ions. This reduction reaction requires free electrons, which may be provided by iron, starting the oxidization process. The key point, however, is that you need both water and oxygen, and you need a fair bit of water: iron in fairly dry air does not oxidize.

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Iron will react with the oxygen in water (H-O-H) to create ferrous oxide, why not he oxygen in carbon dioxide (O-C-O)?

The process requires oxygen, or other strong oxidants and/or acids. Carbolic acid from CO2 & rainwater might work in a pinch but note:

• <<In August, 2008, the Phoenix Lander conducted simple chemistry experiments, mixing water from Earth with Martian soil in an attempt to test its pH, and discovered traces of the salt perchlorate, while also confirming many scientists theories that the Martian surface was considerably basic, measuring at 8.3.>>

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Rust is a general term for a series of iron oxides, usually red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture. Rust consists of hydrated iron(III) oxides Fe2O3·nH2O and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3). Rusting is the common term for corrosion of iron and its alloys, such as steel. Given sufficient time, oxygen, and water, any iron mass eventually converts entirely to rust and disintegrates.
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The rusting of iron is an electrochemical process that begins with the transfer of electrons from iron to oxygen. The rate of corrosion is affected by water and accelerated by electrolytes, as illustrated by the effects of road salt on the corrosion of automobiles. The key reaction is the reduction of oxygen:

O2 + 4 e- + 2 H2O → 4 OH-

Because it forms hydroxide ions, this process is strongly affected by the presence of acid. Indeed, the corrosion of most metals by oxygen is accelerated at low pH. Providing the electrons for the above reaction is the oxidation of iron that may be described as follows:

Fe → Fe2+ + 2 e−

The following redox reaction also occurs in the presence of water and is crucial to the formation of rust:

4 Fe2+ + O2 → 4 Fe3+ + 2 O2−

When in contact with water and *oxygen, or other strong oxidants and/or acids* , iron will rust. If salt is present as, for example, in salt water, it tends to rust more quickly, as a result of the electro-chemical reactions. Iron metal is relatively unaffected by pure water or by dry oxygen. As with other metals, a tightly adhering oxide coating, a passivation layer, protects the bulk iron from further oxidation. Thus, the conversion of the passivating iron oxide layer to rust results from the combined action of two agents, usually oxygen and water. Other degrading solutions are sulfur dioxide in water and *carbon dioxide in water* . Under these corrosive conditions, iron(III) species are formed. Unlike iron(II) oxides, iron(III) oxides are not passivating because these materials do not adhere to the bulk metal. As these iron(III) compounds form and flake off from the surface, fresh iron is exposed, and the corrosion process continues until all of the iron(0) is either consumed or all of the oxygen, water, carbon dioxide, or sulfur dioxide in the system are removed or consumed.

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Q: "The Mars Exploration Rover Opportunity has been studying a lot of meteorites. That made me wonder, why study meteorites on Mars when we can study them in hand on Earth? How are Mars meteorites interesting?"
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Are meteorites on Mars actually interesting?
By Emily Lakdawalla | Nov. 9, 2009

<<It's true that it's far easier to study meteorites on Earth than on Mars. Opportunity examines the meteorites it finds on Mars not to learn more about meteorites, but to learn more about Mars.

The meteorites at Meridiani planum have, in all likelihood, been sitting there for a very long time. During that time, they have interacted chemically and geologically with the surface and near-surface atmosphere, so their current composition can tell us about chemical behavior on Mars.

One of the meteorites, a metallic one named Block Island, has given us an important clue to Mars' past just by virtue of its large size. At the current thickness of Mars' atmosphere, a meteorite the size of Block Island would have disintegrated into much smaller fragments on impact with the Martian surface. So the meteorite must have fallen at a time when Mars had a significantly thicker atmosphere that would have decelerated the meteorite, slowing its crash into the ground.

Also, Opportunity's chemical analysis instruments observed varying composition on different parts of Block Island, which may indicate different states of alteration. Since meteorites are a fairly well-studied class of objects, we know a lot about what the starting composition must have been, so the current chemical composition can provide important clues to the chemistry that has operated on the Red Planet since the meteorite fell.>>