A new theory says that salty ocean currents may be an overlooked source for the Earth's magnetic field. A magnetic field is important to protect the planet's surface from damaging ultraviolet rays. But it also protects the atmosphere from the eroding effects of the energized particles of the solar wind - though it is imperfect protection - the magnetosphere is responsible for causing some leakage by funneling some of the energy into the upper atmosphere and heating it up.
This still does not rule out life developing on desert planets, especially planets with higher gravity and thus thicker atmospheres. I wonder if a large habitable moon circling a gas giant could benefit from the giant's magnetic field?
However, if it turns out that our oceans contribute to our planet's protective magnetic shield in no small manner, then it adds impetus for us to search for planets with large bodies of water.
Tidal Mixing and the Rise of Life
Water, as mentioned in earlier posts, may also be critical when combined with tectonic activity (whether from internal forces or from external tidal forces generated from orbiting a large gas giant) to the rise of life. For example, as mentioned in an earlier post (Life Outside the "Zone"), some scientists feel that the Moon was essential to the origin of life on the Earth due to the tidal mixing which helped to mix, mainly from erosion caused by the tides, chemicals from the soil with the oceans, creating the chemical soup from which life arose.
"The Earth's magnetic field remains a charged mystery." Institute of Physics News. 14 June 2009. Web. 16 June 2009. <http://www.iop.org/News/news_35352.html>.
Ryskin, Gregory. "Secular variation of the Earth's magnetic field: induced by the ocean flow?" New J. Phys. Vol. 11. 2009. (23pp) <http://www.iop.org/EJ/abstract/1367-2630/11/6/063015>.