Amateur astronomers have made an unexpected discovery: massive plumes spouting up at least 125 miles from the surface of Mars. Two plumes, up to 620 miles wide, were spotted over the Terra Cimmeria area in the red planet's Southern Hemisphere by at least 18 people over two 10-day periods in March and April 2012, Discovery News reports. A witness tells the BBC one plume looked like a "projection sticking out of the side of the planet." They appeared to form during cold mornings and changed "from double blob protrusions to pillars or finger-plume-like morphologies," according to a study in Nature. As to what's causing them, "Frankly, I'm puzzled," says a lead scientist. Clouds made of carbon dioxide and ice crystals have been spotted by the Hubble Space Telescope in the past, but they only reached 62 miles from the planet's surface, CBS News reports. Clouds of dust reached just 37 miles high.
Scientists have two theories, but "both explanations defy our current understanding of Mars' upper atmosphere," they say. The plumes could be caused by "a reflective cloud of water-ice, carbon dioxide-ice, or dust particles," the study author explains, but clouds shouldn't be able to form at such altitudes where the atmosphere is so thin. Others note the plumes may be bright auroras "1,000 times stronger than the strongest aurora" on Earth, but that, too, is "highly unrealistic." Whatever the cause, "this type of feature must be very rare, to have been observed only once out of all of the time that telescopic observers and spacecraft have been observing Mars," a scientist says. NASA's MAVEN spacecraft, now in Mars' orbit, may be able to shed light on the mystery, researchers say. (Meanwhile, 100 Earthlings have been named finalists for a possible 2024 Mars mission.)