Mars’ iconic dust, which gives it its signature red color, has a different source of formation than previously thought. The results of a new study suggest that it originated in the planet’s early past, when it was still wet.
Why Mars has a red color
Mars is easily recognized in the night sky by its distinctive red hue. Thanks to the fleet of spacecraft studying the planet, we know that this color is due to dust rich in iron oxide particles. Or, more simply, rust. It formed when the iron contained in Mars rocks reacted with liquid water or oxygen at some point, similar to what happens on Earth. Over billions of years, iron oxides turned to dust and were blown by winds across the planet. This process continues to this day.
But iron oxides are different, and the exact chemical composition of Martian rust is highly debated because the mechanism of its formation allows us to judge the environmental conditions on the planet at that time. And closely related to this is the question of whether Mars was ever habitable.
Previous studies of iron oxide, a component of Martian dust, are based only on spacecraft observations and have found no evidence of water in it. Therefore, researchers have long assumed that this particular type of iron oxide should be hematite, formed in dry surface conditions by atmospheric reactions in the billions of years following Mars’ early wet climate.
Wet and rusty Mars
However, this view has now been called into question. An international team of scientists believes that the red color of Mars is due to a very different mineral. They came to this conclusion in a laboratory experiment. The scientists ground the volcanic basalt to a size 1/100 the thickness of a human hair (this is the typical size of Martian dust grains) and then mixed it with different types of iron oxides.
Subsequent spectral analysis with the same techniques used by the Martian spacecraft showed that the red color of the planet best corresponds to ferrihydrite.
Ferrihydrite is a mineral that forms when iron comes in contact with cold water and retains traces of it. Its presence in the dust suggests that Mars “rusted” much earlier than previously thought — back in the days when the planet had a humid climate and streams of water flowed across its surface.
While this discovery doesn’t change basic ideas about the causes of Mars’ color, it provides a glimpse into its past and an understanding of the evolution the planet has undergone. Scientists say they have high hopes for missions to bring samples of Martian soil back to Earth. Analyzing them will give an accurate measure of how much ferrihydrite the dust contains, which in turn will improve our understanding of how wet the planet was at the time of its formation.
According to ESA
