With a thicker atmosphere and active volcanoes, was there ever a Yellowstone on Mars?

Today, Mars is a cold and dry planet with a very thin, low-pressure atmosphere. It has water, but it’s all frozen, locked up in underground ice (like permafrost) and in polar ice caps. But billions of years ago Mars had a thicker atmosphere and a warmer and wetter climate. It also used to have active volcanoes. So, with all […]