I am aware that using magic outside of the Earth Biosphere is lethal for mages. Does this still hold true on Mars, or has the red planet enough life on it now to support magic users?
Yes, but at a penalty of -8.
Run and Gun p. 168:
Space locations function as astral voids, meaning spellcasters receive a dice pool penalty when they attempt any test that uses their Magic attribute. The penalty ranges from –8 (for places with decent concentrations of life for non-Earth locations, such as the Moon or a large space station) to –18 (for deep space).
The book further reports that the corps are aware that the manasphere can be changed and are working on improving the situation, but so far the results are mixed. As a GM I find this an excellent excuse to make my own changes as needed.
Further on p. 169:
Anyone attempting to assense away from the safety of Earth must make a Willpower + Logic (5) Test. If they fail, their Logic is permanently reduced to 1 (though it can be raised again with Karma).
If you try to assense in deep space you automatically fail the test and also loose two from your magic rating.