Lava flows carved valleys on Mars, US scientists said on Thursday in the US amid a long-running debate over whether water or volcanoes formed part of the red planet's landscape.
The lava left behind telltale coils as seen on some parts of Earth, like on the Big Island of Hawaii and in submarine lava flows near the Galapagos Rift on the floor of the Pacific Ocean, said the findings in the journal Science.
Lead author Andrew Ryan of Arizona State University focused on the Athabasca Valles nears Mars' equator, and performed his analysis using more than 100 high resolution images beamed back by the NASA's Mars Reconnaissance Orbiter.
Ryan said the large spiral coils there, in the Elysium volcanic province of Mars, range from five to 30 metres wide and could not have been formed by ice- or water-related processes.
"That's bigger than any known lava coils on Earth," said Ryan, who was surprised by their size but not by the fact that the coils had been missed by scientists studying Mars' landscape in the past.
"The coils become noticeable in the full-resolution HiRISE (High Resolution Imaging Science Experiment camera on the Mars Reconnaissance Orbiter) image only when you really zoom in," he said.
"They also tend to blend in with the rest of the light-gray terrain - that is, until you stretch the contrast a bit," he added.
"I don't find it surprising that these were overlooked in the past. I nearly missed them too."
The coils, which resemble the circular lines on a snail's shell, likely formed when streams of lava flowed past each other at different speeds or in different directions.
So far Ryan, a graduate student in ASU's School of Earth and Space Exploration, and his co-author Philip Christensen, a professor of geological sciences at ASU, have counted nearly 200 lava coils in the Cerberus Palus region, and they believe there are more.
"Lava coils may be present in other Martian volcanic provinces or in outflow channels mantled by volcanic features. I expect that we'll find quite a few more in Elysium as the HiRISE image coverage grows over time," Ryan said.
The US space agency launched the Mars Reconnaissance Orbiter in 2005 to circle the red planet and take pictures that would allow scientists to search for evidence of water on the surface and study how long it may have persisted.
The orbiter's camera has been able to increase by a factor of 10 the number of close-up spots surveyed and can now identify objects as small as a dinner table, NASA said.