The BBC is reporting on peer-reviewed articles in Nature, which document a large frozen sea and geologically-recent volcanic activity.
The interesting part about this is that according to physicists, the reason Mars lacks a more substantial atmosphere is that its core has cooled and solidified. Such a core cannot generate the magnetic field needed to shield the atmosphere from solar wind. Unprotected, the atmosphere literally gets blown away.
Mars is believed to, at best, possess a very weak magnetic field induced by solar wind. But Venus, which has a very thick atmosphere, also apparently possesses only an induced magnetic field.
But apparently “Venus has no magnetic field, perhaps because of its slow rotation.” Like earth, Venus still has a molten core. Volcanism on a planet like Mars, presumed to have a cold solid core, is a little harder to explain.
If Mars has a warmer core than anyone expects, then the absence of a magnetic field is also apparently unexplained. And since the lack of a magnetic field explains the relative lack of atmosphere, then we also don’t know why or if Mars can’t be terraformed.