  Transverse momentum ($p_{T}$) generation in relativistic heavy ion collisions
is sensitive to the initial geometry and the final-state bulk evolution. We
demonstrate with hydrodynamic calculations that the mean $p_T$ ratio
($R_{\langle p_{T}\rangle}$) between the highly similar isobar
$^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr collisions is
insensitive to the bulk evolution and remains sensitive to the small difference
in the initial nuclear structure (neutron skin and deformation) between the Ru
and Zr nuclei.We further find that nuclear deformation can produce an
anticorrelation between $R_{\langle p_{T}\rangle}$ and eccentricity (or
elliptic flow) in central collisions. These findings suggest that the
$R_{\langle p_{T}\rangle}$ between the isobar systems can be used to measure
the neutron skin thickness and deformation parameters, which can in turn
constrain the nuclear symmetry energy slope parameter.
