On Earth, red rocks — sometimes called “red beds” — generally get their color from oxidized iron (Fe3+), which is the same form of iron that makes our blood red, or the rusty red color of metal left outside. Green spots like those observed in the Wallace Butte abrasion are common in ancient “red beds” on Earth and form when liquid water percolates through the sediment before it hardens to rock, kicking off a chemical reaction that transforms oxidized iron to its reduced (Fe2+) form, resulting in a greenish hue. On Earth, microbes are sometimes involved in this iron reduction reaction. However, green spots can also result from decaying organic matter that creates localized reducing conditions. Interactions between sulfur and iron can also create iron-reducing conditions without the involvement of microbial life.
