The Sun is not the only source of global warming. Scientists have a habit of putting on blinders and not thinking outside of their specialty
As events in Iceland should remind people, the Earth has a molten core, and the interior of the planet gets warmer as you dig down, not colder. Mars is not big enough to generate heat in its core, which is why it relies completely on sunlight, the atmosphere all froze and/or dispersed into space.
The Earth’s core is not hot because of heat from the big bang – it is hot because the intense pressure of gravity creates heat from the nuclear forces of the matter being compressed.
Heat energy in the Earth is due to residual heat from planetary accretion and radioactive decay. There are some speculations about nuclear reactions in the core, but I think such reactions are unlikely.
Lord Kelvin (William Thomson) greatly underestimated the age of the Earth from his thermal modeling because he didn’t know about radioactivity.
Thank you for the wikpedia summary.