Stuart Clark discusses the paradox that 2.5 billion years ago, Earth was warm enough to have liquid water on its surface when the faint young sun meant it should have been much cooler than today (16 February, p 44). Mars was once warm enough to have liquid water too, about 3.5 billion years ago, and has since cooled. Is this a coincidence? If not, it could rule out theories to explain the paradox described.
Clark asks how life got started when the early Earth was 20 degrees cooler than present, but parts of equatorial oceans could still have been 10 degrees above freezing – warm enough for life.
Also, the freezing point of the water could have been lower, due to more dissolved salt or other minerals, or the ingredients of a primeval organic soup.
And the primordial Earth could have generated more heat internally, before the radioactive content ...