  Low-energy cosmic rays ($<1$ TeV) are a pivotal source of ionisation of the
interstellar medium, where they play a central role in determining the gas
chemical composition and drastically influence the formation of stars and
planets. Over the last few decades, H$_3^+$ absorption lines observations in
diffuse clouds have provided reliable estimates of the cosmic ray ionisation
rate relative to H$_2$ ($\zeta^{\rm ion}_{{\rm H}_2}$). However, in denser
clouds, where stars and planets form, this method is often inefficient due to
the lack of H$_3^+$ rotational transitions. The $\zeta^{\rm ion}_{{\rm H}_2}$
estimates are, therefore, still provisional in this context and represent one
of the least understood components when it comes to defining general models of
star and planet formation. In this Letter, we present the first high-resolution
maps of the $\zeta^{\rm ion}_{{\rm H}_2}$ in two high-mass clumps obtained with
a new analytical approach recently proposed to estimate the $\zeta^{\rm
ion}_{{\rm H}_2}$ in the densest regions of molecular clouds. We obtain
$\langle\zeta^{\rm ion}_{{\rm H}_2}\rangle$ that span from $3\times10^{-17}$ to
$10^{-16}\rm~s^{-1}$, depending on the different distribution of the main ion
carriers, in excellent agreement with the most recent cosmic rays propagation
models. The cores belonging to the same parental clump show comparable
$\zeta^{\rm ion}_{{\rm H}_2}$, suggesting that the ionisation properties of
prestellar regions are determined by global rather than local effects. These
results provide important information for the chemical and physical modelling
of star-forming regions.
