It turns purple from the presence of numerous microbes which have to have sunlight and have to have hydrogen sulfide, and we can detect their presence today -- we can see them -- but we can also detect their presence in the past.
变成紫色，是因为数不尽的微生物出现， 它们必须有阳光，而且还得有硫化氢。 现在我们能检测到它们的存在，也能看得见， 但是以前也能检测到。