6 Martian sunrises, as seen by the HiRISE orbiter. Once again, not artist’s renditions.
Morning on another world.
Under the Milky Way
Sun-like stars become brighter over the course of their long lives, which means that the Sun once provided Earth with far less heat than it does now - so much less, in fact, that life should have been impossible.
This is known as the faint young Sun paradox, and it was first identified by astronomers Carl Sagan and George Mullen back in 1972. In the early days of the solar system, the Sun should only have been at only about 70% its current luminosity. Such a relatively paltry amount of heat would have made it impossible for the Earth to maintain a liquid ocean, and yet we know for a fact that it did — and now that we’re pretty much certain Mars had a liquid ocean back then as well, we can add that as a second mystery to the list.