Why is Mars red

Fourth planet from the Sun, Earth’s closest planetary neighbour and separated by a mere 54 million kilometres when at its closest, Mars is close enough for detailed inspection from Earth and comfortably within reach of unmanned probes. Mars shares many common features in the physical and historical formation of its landscape. It has been noticeably shaped by crust movement, volcanism, collisions with other objects and volatile atmospheric phenomenon. Although much is still unknown about the planet’s physical geography, for centuries, for those observing from afar, the planet has exhibited a strong red appearance and for this reason is known as the Red Planet. The precise cause of the red appearance of the planet has been an issue of debate for some time. However, unmanned probe missions in the last fifteen years seem to have verified the notion that the redness is caused by an abundance of iron oxide, the same compound that causes earthly rust.
The soil on Mars bears a high concentration of iron-bearing compounds and there are large iron-based deposits on the planet’s surface. However, the iron alone will not give the red appearance. In order to exhibit the red glow associated with rust, iron must come in to contact with oxidants. These are usually water and oxygen in the case of iron.

Initially, the redness was thought to derive from the prolonged contact between the iron in the Martian soil and traces of oxygen and water in the Martian atmosphere. As no substantial water deposits are seen on the surface of Mars today, this oxidation is thought to have occurred over aeons. The redness is also suggestive of large water deposits on the surface or in the atmosphere of Mars in times gone by. The great prevalence in the soil of iron is also attributed at least in part to the existence of greater water deposits on the surface in the past. Expanses of water and the channels they may have wrought in the iron-rich rocks observed on the Martian surface would have caused significant erosion. Smaller erosion deposits would rust more readily and eventually would be broken down to the extent that they would be dispersed across the planet’s surface by atmospheric phenomena, but mostly through rain.
However, this seemingly logical sequence of steps was thrown in to disarray in the late 1990s. In 1997, the Pathfinder probe sent back results from Mars indicating the concentration of iron exceeds that which could be obtained by the previous water erosion theory and alternative explanations were promptly sought.

One such suggestion was that iron was brought to the planet by collisions with meteorites, and the pock-marked nature of the Martian surface certainly seemed to lend weight to this idea. The theory went full circle when it was suggested that if the iron came from meteorites then it would not be necessary to have had water on the surface of Mars to create iron oxide; the iron oxide might exist as a consequence of actions from other oxidants. This latter theory was borne out when a compound commonly found in Martian soil was observed to oxidize under Martian condition in a laboratory simulation on Earth.