It is a well-known nuisance that when you leave a freshly sliced apple exposed to air, it quickly turns brown. It’s a nuisance of the same kind that must frustrate the Statue of Liberty—her once-coppery skin turned green from only two years of exposure to air. The process that causes these seemingly dissimilar metamorphoses is called oxidation. Oxidation occurs when an atoms in a substance lose electrons, changing the overall chemical makeup and structure. It doesn’t necessarily cause decay; oxidation is essentially just atomic reshuffling. Key to oxidation processes are atmospheric elements like water and oxygen.
Another common kind of oxidation is rust. Rust occurs when iron metals oxidize to form other compounds. Hematite is one of these results of oxidation, rust-red in powdered form and created by the oxidation of other iron-containing compounds, like magnetite. When magnetite rusts, one of its iron atoms loses an electron, chemically changing into hematite. The presence of water is an important catalyst for this reaction—if you’ve ever left an old bike out in the rain, you’ll notice it gets rusty pretty fast.
Hematite is the abundant mineral that gives Mars its distinctive reddish color. The planet is covered by a layer of fine red hematite powder, so much so that you can see the hue from Earth on a clear night. But there are no oceans or lakes of water on Mars, and the Martian atmosphere is practically parched. Without water to facilitate rusting and oxidation, the origins of this ubiquitous mineral are unknown. NASA’s Martian rovers have found no indication of rusty Martian bicycle tracks, so there’s one scenario we can rule out.
Mars has a very sparse atmosphere, about 1/100th of Earth’s, and so the surface receives very little protection from solar ultraviolet (UV) radiation. UV light is very powerful: it can split apart molecules to create extremely reactive new molecules called radicals, and it can even interact with our genes to cause mutations. Perhaps the abundance of UV light on Mars, in the absence of water, can facilitate the oxidation of magnetite into hematite.
To test this hypothesis, I placed a few magnetite samples into a chamber with a simulated Martian atmosphere: a very cold, low-pressure CO2 atmosphere with no water vapor. Then I switched on a powerful UV lamp and waited for two weeks. I carried out this research with the Mars Group at the Niels Bohr Institute at Copenhagen University.
There were two possible outcomes.
Possibility 1: Oxidation was detected on the samples at the end of the experiment. This could imply:
- UV radiation can physically knock electrons off of iron, and alter the ratios of iron and oxygen from magnetite to hematite. No water is necessary. This could mean that the dust on Mars is actively, however slowly, being formed. Mars may not be such a static “dead” planet as we may have thought.
- UV radiation can interact with CO2 to create an oxidant which then oxidizes iron, changing magnetite to hematite. No water is necessary. Again, this could mean that the dust on Mars is actively, however slowly, being formed.
- The magnetite in our samples had already been oxidized through exposure to Earth’s atmosphere before we introduced them to our chamber. I’ll explain this more in the results section.
Possibility 2: No oxidation was detected on the samples at the end of the experiment. This could imply:
- UV radiation cannot induce oxidation alone. It may have no role in converting magnetite to hematite. Alternatively, it may have a partial role combined with mechanical weathering, or other processes. Perhaps the only way UV radiation could have played a role in oxidation is under a completely different set of atmospheric conditions. If Mars previously had an Earth-like atmosphere containing more water and oxygen, UV radiation could photolyze H2O or O2 to create radicals that would steal an electron from iron atoms.
- UV radiation can indeed induce oxidation without the presence of water, but over very long timescales—the experiment did not run for enough time. This would imply that the dust on Mars could be very old, which suggests that the planet has been unchanging for millions of years.
- The analysis did not catch the oxidation.
The results are now in.
After the experiment, the samples were analyzed using x-ray photoelectron spectroscopy (XPS). This mouthful is a technique sensitive to the surfaces of materials, giving the chemical composition of the top 10 nanometers. This means that the results we found were only for the surfaces of our samples, not the bulk. XPS can also tell us the chemical environment of an atom, meaning we can determine the oxidation state of elements. The analysis was carried out by Dr. Nico Bovet at Copenhagen University’s NanoGeoScience Center.
The powder samples showed signs of oxidation even before they went in the chamber. The little bump in the graph around 719 eV indicates iron in the oxidized state. After the experiment, they didn’t show any noticeable change.
As for the solid samples, neither of them show that same bump around 719 eV — indicating that no iron oxidized after the experiment.
So there you have it. Our results indicate that UV radiation cannot, in two weeks, induce the oxidation of magnetite to hematite in an atmosphere analogous to Mars’.
In science, getting “no” for an answer is common, and it’s not necessarily a bad thing. Any result of the scientific process is important, and leads to new questions and new directions of research. It’s possible that UV radiation does play a role in oxidation on longer timescales, or in combination with other Martian processes. We will just have to ask more questions and do more science to find out.