| MadSci Network: Astronomy |
A core solidifies when a planet loses heat (1). Part of what makes Earth such a great place to live is that it's nice and warm. This is partly due to its distance from the Sun and partly due to an atmosphere that not only blocks out harmful UV rays but acts as an insulating blanket. Mars is farther away from the Sun and doesn't have much of an insulating atmosphere.
Most of Earth's heat, however, results when the molten metal in Earth's core changes from liquid into crystalline solid. Heat is released during the transition (2). Other heat comes from the decay of radioactive elements like uranium, thorium, and potassium. As they turn into other elements, heat is released (2).
So, to make Mars' core molten again, you would need to add heat. A big impact could do that - obliterate the planet and have it reform, (a) capturing new material that forms the molten core, releasing heat as the core (again) solidifies and (b) capturing other material that decays and releases heat in the process.
You might also be able to "melt" the interior by adding a tremendous atmosphere that could insulate the planet, but I think Mars getting smacked by an object and reforming is more practical!
(1) http://csmres.jmu.edu/geollab/Fichter/PlateTect/heathistory.html
(2) http://www.wonderquest.com/fast-flying-bugs-slow-cooling-earth.htm#slow-cooling-earth
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