Does feroxyhyte occur on the surface of Mars?

Abstract

The red colour of Mars has long been attributed to the occurrence of ferric oxides on its surface and in atmospheric dust storms. Such a belief has been accentuated by results of Viking Lander experiments on the surface of Mars, which suggested the presence of maghaemite, γ-Fe₂O₃ (refs 1, 2). This dark brown, ferromagnetic iron (III) oxide phase not only conforms with the colour and magnetic properties of dust on the martian surface, but suggests that maghaemite acted as a catalyst in some of the biological experiments performed in situ on Mars³. Many of the physical properties and chemical reactions attributed to maghaemite, however, are also displayed by δ-FeOOH, an oxide hydroxide polymorph of ferric iron^4,5. On Earth, δ-FeOOH occurs as the mineral feroxyhyte in gleyed soils and in submarine manganese nodule deposits⁶. Some of the properties and paragenetic relationships of feroxyhyte are described here and it is suggested that δ-FeOOH is a prime candidate for the red-brown ferromagnetic phase coating the surface of Mars.