Red Dragon will presumably have to shed or re-stow solar arrays before any landing. What will be the source of electrical power after landing?
Solar panels for both Dragon versions are on the trunk of the craft, the unpressurized external (relative to the capsule) component that separates the capsule from the upper stage of the rocket that launches it. There's a bunch of reasons that the trunk must be jettisoned before landing:
- The trunk is aerodynamic for takeoff, though only with the panels stowed (on Dragon 1, they are stowed folded inside fairings on each side; I'm not sure about Dragon 2). It is not aerodynamic for landing, nor is it heat-shielded (it actually covers the capsule's heat shield) so it certainly can't survive (re-)entry anywhere with an atmosphere.
- The small Draco thrusters used for maneuvering and orbital adjustments can be used with the trunk attached - otherwise, the Dragon 1 wouldn't be able to reach the ISS safely - but I'm not sure if the large SuperDraco thrusters that Dragon 2 uses to land propulsively can be used while the trunk is attached (without damaging it).
- While the trunk can withstand considerable vertical compression force (it certainly does so during liftoff), it has no landing legs or similar. Dragon 2's landing legs are behind its heat shield, which is behind the trunk, and they aren't long enough to land on even if they could be deployed with the trunk still attached.
- The trunk adds weight (equipment and structure) that will probably not be needed after landing. Of course, part of that equipment is the solar panels, but other parts are just dead weight. Jettisoning it means there's that much less work for the heat shield, parachutes, and/or SuperDracos to do.
So, altogether, the trunk definitely has to go before landing (unless Dragon 2 gets massively redesigned to permit the trunk to survive landing). However, that doesn't necessarily mean that the capsule will only be on battery power. Dragon 1 has 11 cubic meters (388 cubic feet) of internal capacity (couldn't find the numbers for Dragon 2, but Elon Musk has described it as "about the size of an SUV"), and Falcon Heavy can get the capsule to a Mars orbit with several (numbers I've seen have said 2-4) tons of payload. That's plenty of space and mass to carry solar panels, a radioisotope thermal generator, or even a small nuclear fission reactor, probably with volume and mass left over for a scientific payload.
It could use a large radioscopic isotype generator, if it used this along with several rechargeable and non rechargeable batteries, it could possibly remain with power for several months.