The questions Phoenix is designed to address don't need mobility.

Mobility is expensive. Rovers cost twice as much per kg as stationarly landers and have much smaller payloads. they are much more complex to design and operate. if you don''t need it you don't have it.

Especially when this is a minimum cost scout mission!

The questions Phoenix will address have been waiting since Viking, there have been four other attempts to address them, all failures. It is time they were answered. MSL won't address them, it has its own quite different mission goals with a different instrument suite. And MSL is a much later mission with no assurance of success.

Jon