Faulty radio-chemistry
Hi Nick,
My guess is that the 14N has long been converted by neutron bombaredment to 14C.
That's neutrons, not photons.
Mars has had neither a protective magnetosphere nor a protective atmosphere for a long long time.
I got up this morning, the sky was blue, the plants green, the sun shinning.
On Mars the sky was dirty pink, the surface red, and the sun could barely be seen through the dust.