"voyager 1"<nowiki>'s</nowiki> first images of io revealed a lack of impact craters, suggesting a very young surface. craters are used by geologists to estimate the age of a planetary surface; the number of impact structures increase with the age of the planetary surface. instead, "voyager 1" observed a multi-coloured surface, pockmarked with irregular-shaped depressions, which lacked the raised rims characteristic of impact craters. "voyager 1" also observed flow features formed by low-viscosity fluid and tall, isolated mountains that did not resemble terrestrial volcanoes. the surface observed suggested that, just as peale and colleagues had theorized, io was heavily modified by volcanism.