THE widely held idea that the Sun began life dimmer than it is today has been challenged by a pair of astrophysicists who say it may actually have been brighter. If they are right, it would solve a major planetary puzzle: how water could have run on Mars 3.8 billion years ago when, by rights, the planet’s surface should have been frozen solid.
There is now plenty of evidence that liquid water existed on Earth and on Mars soon after they formed. On Earth, greenhouse gases might just have warmed the planet enough for ice to melt even if the…
