  Dry granular materials consist of a vast ensemble of discrete solid
particles, interacting through complex frictional forces at the contact points.
The particles are so large that these systems are believed to be completely
athermal. Here, we arrest the dynamics of a flowing granular material in a
steady-state flow configuration, enabling an isolated examination of aging at
the particle contacts without granular rearrangements. Our findings reveal that
the evolution of interparticle forces within the arrested athermal granular
network results in the spontaneous increase of the system's yield stress. This
strengthening process is logarithmic in time with a rate that depends on
temperature. We demonstrate that the material's stress relaxation exhibits
similar time- and temperature-dependent behavior, suggesting a shared origin
for aging and stress relaxation in these systems governed by thermal molecular
processes at the scale of the grain contacts.
