Ancient Mars was home to giant volcanoes capable of huge eruptions, scientists said last week.

The finding raises fresh questions about conditions on Mars in its early years, a time when scientists believe the planet was much more Earthlike, with a thick atmosphere, warmer temperatures and water on its surface.

Major volcanic eruptions probably would have triggered climate shifts that toggled Martian temperatures between cold spells when ash blocked sunlight and heat waves when greenhouse gases filled the skies, according to scientists. Super-volcanoes may have made it more difficult for life to evolve on the planet’s surface, although underground steam vents and the release of water into the atmosphere may have created niches for microbes to thrive, said geologist Joseph Michalski of the Planetary Science Institute in Tucson.

These types of volcanoes, also known as caldera volcanoes, are collapsed structures rather than steep, cone-shaped or domed mountains like Mars’s Olympus Mons, a so-called shield volcano that stands nearly three times as tall as Mount Everest, the highest peak on Earth.

Scientists say super-volcanoes erupt with about 1,000 times the force of such typical volcanoes as Mount St. Helens in Washington state. The eruption of Mount St. Helens in 1980 blasted the top off the mountain, killed 57 people and, according to the U.S. Geological Survey, shot ash, steam, water and debris about 80,000 feet into the air.

Michalski and colleagues came across the caldera while they were studying Martian impact craters, not looking for volcanoes.

“We made the discovery by accident,” he said. “As I went through [images from Mars orbiters over the past 15 years] of this one region, I found a number of them that were simply not impact craters,” he said.

“One was clearly a volcano. . . . It is quite possible there are many more of these,” Michalski said.

Because the emission of gases from volcanoes helps create a planet’s atmosphere, understanding the volcanic history of Mars is crucial to figuring out what the planet — the fourth from the Sun — was like in its early years.

Additional evidence may come from NASA’s Mars Curiosity rover, which is heading toward a three-mile-high mound of deposits called Mount Sharp.

The rover touched down inside a giant impact basin near the planet’s equator in August 2012 to assess if Mars ever had the chemistry and environment that might support and preserve microbial life.

“There are thousands of layers of rocks in Mount Sharp and they contain a long record of geologic history,” Michalski said. “There could be interlayered rocks that are ash beds, and we predict that and we hope that the rover can test it,” he said.