Not really. That might have been true 10 years ago.
No.
All I'm saying is: you can bet the hardware is in a well-shielded heavy metal box, and today all it takes is about 1/4 of a cubic inch to squeeze in another GB of RAM or flash.
I wonder why they didn't think about that. A nice thick, heavy metal box. Easy! Perhaps you should go and work for NASA?
Let's ignore the earth's magnetosphere for the moment and make some massive assumptions.
The pressure on the ground is about 10^5 Pa. That means there's 10^4 Kg of stuff above you to absorb radiation from space. That equates to 10m of water, 1.25m of steel ot about 90cm of lead. Quite a lot.
Mars is about 1.5 Au from the sun, so receives about 0.4 times the radiation.cos
The atmosphere is about 600Pa, by comparison.
Radiation hardening is a very well established field. Using some degree of shielding is just one of the many techniques in use. On Mars, it is simply not enough on its own.
It is very, very difficult to make a rad-hard processor, and then very thoroughly test it. Yo can't just keep shrinking the feature size, because is it goes down, the effect of radiation increases. Not only that but as the amount of crystal per transistor shrinks, the chance of unrecoverable lattice damage increases, due to the lack of redundancy.
There are faster Rad-hardened DSPs, but those are, well, DSPs and only actually really fast for DSP like tasks.
There also are almost certainly faster ones available now. But it's been in transit for a year, and they certainly weren't building it with a brand-new untested processor for which thay had to write all the software on the way after they launched it.
So, given the constraints, it's a pretty great CPU to have on board.
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