Black plumes rising five miles above Neptune's moon Triton may be swirling funnels called "dust devils" instead of volcano-like geysers as many researchers believe, a study published Friday says.

"The majority theory is still that they're geysers. But we're having a lively debate," said planetary scientist Andrew Ingersoll of the California Institute of Technology in Pasadena.At least four plumes were discovered in photographs taken by the Voyager 2 space probe when it zoomed past Neptune and Triton in August 1989. The plumes of nitrogen ice, nitrogen gas and carbon dust rise five miles high, then flatten into clouds that drift 90 miles downwind.

"A lay person would call them volcanoes. But the proper technical term is geysers - if that's what they are," said Voyager's chief scientist.