Complex sentence: Scientists have never recorded an active volcano eruption on the surface of Mars, however, the European Space Agency's Mars Express orbiter photographed lava flows that must have occurred within the past two million years, suggesting a relatively recent geologic activity.
Split into simpler sentences: Scientists have never recorded an active volcano eruption on the surface of Mars. However, the European Space Agency's Mars Express orbiter photographed lava flows that must have occurred within the past two million years, suggesting a relatively recent geologic activity.