Currently, popular models for Type Ia supernovae (SNe Ia) fall into two gen
eral classes. The first comprises explosions of nearly pure carbon/oxygen (
C/O) white dwarfs (WDs) at the Chandrasekhar limit which ignite near their
centers. The second consists of lower mass C/O cores which are ignited by t
he detonation of an accreted surface helium layer. Explosions of the latter
type produce copious Fe, Co, and Ni K alpha emission from Ni-56 and Co-56
decay in the detonated surface layers, emission which is much weaker from C
handrasekhar-mass models. The presence of this emission provides a simple a
nd unambiguous discriminant between these two models for SNe Ia. Both mecha
nisms may produce 0.1-0.6 M . of Ni-56, making them bright gamma -ray line
emitters. The time to maximum brightness of Ni-56 decay lines is distinctly
shorter in the M < M-ch class of model (<similar to>15 days) than in the M
-ch model (similar to 30 days), making gamma -ray line evolution another di
rect test of the explosion mechanism. It should just be possible to detect
K-shell emission from a sub-M-ch explosion from SNe Ia as far away as the V
irgo cluster with the XMM observatory. A 1-2 m(2) X-ray telescope such as t
he proposed Constellation-X observatory could observe K alpha emission from
M < M-ch SNe Ia in the Virgo cluster, providing not just a detection but h
igh- accuracy flux and kinematic information.