Lakes of Mars
This week French scientists have concluded that water ice from Planum Borum, the polar ice cap, is exceptionally pure. Data from the Shallow Radar (SHARAD) instrument about the Mars Reconnaissance Orbiter points to 95 percent purity. The Martian North would be an excellent place for humans to settle and stay. a settler could bet her life on finding water beneath the surface.
Calcium carbonates found by Phoenix are more evidence of a warm, wet past for Mars. Abundant photos point to ancient lakes and stream beds. A lake bed is difficult to spot from orbit, so even more may exist. This adds to evidence from meteorites that early Mars had conditions suitable for life. Billions of years ago the planet may have been awash with water.
The "Faint Sun" hypothesis speculates that the early Sun was only about 75% as bright as today. Earth and Mars would have been frozen solid, making evolution of life unlikely. Abundant evidence from both planets shows that they were both warm enough for life. Since the Sun turns fuel to energy according to E=mc^2, a higher speed of light would have made solar luminosity nearly equal to today. Ancient lakes on Mars are more evidence that the speed of light has changed.