New Scientist: For the first 2 billion years after Earth’s formation, the Sun was 25% less bright than it is now, so Earth’s surface temperature would have been about −10 °C. However, the geologic record shows that Earth was covered in liquid water, not ice. Robin Wordsworth and Raymond Pierrehumbert of the University of Chicago believe that Saturn’s moon Titan, which has liquid on its surface, can help explain liquid water on early Earth. Titan has a liquid surface because of the high concentration of hydrogen and nitrogen in its atmosphere. Because of the pressure they are under, they act as greenhouse gases, trapping heat and warming the surface. Wordsworth and Pierrehumbert suggest that if early on Earth’s atmosphere was made up of 10% hydrogen and nitrogen that was at a level two to three times its current concentration, then the surface temperature would have been 10–15 degrees higher, allowing for liquid water. They admit that the geologic evidence doesn’t show those concentrations, but they point to several factors that could have allowed it. They include the lack of oxygen, increased volcanism, and the potential lack of hydrogen-consuming microbes. However, there is much that the model still has to explain and a lack of hard evidence to support it.