2026ApJ...998...71N

We present James Webb Space Telescope (JWST) NIRSpec observations of the starless dark cloud Barnard 68 (B68) that reveal the spatially resolved signature of cosmic-ray-excited molecular hydrogen (CRXH₂) emissions for the first time. Following up on our initial detection of CRXH₂ emissions from B68, we now exploit JWST's sensitivity and spatial multiplexing to map CRXH₂ rovibrational lines across 16 sight lines through the cloud. By disentangling the CRXH₂ and UV-pumped H₂ components, we isolate the para-H₂-dominated spectrum attributable to cosmic-ray (CR) excitation. We find that there are significant spatial variations in the ratio of the CRXH₂ line intensity to the line-of-sight H₂ column density; these cannot be accounted for by dust extinction alone and demonstrate a clear attenuation of the CR flux with increasing shielding column. Modeling B68 as a Bonnor-Ebert sphere, we constrain both the unshielded CR ionization rate, ${\zeta }_{{{\rm{H}}}_{2}}$, and how it decreases with shielding column. At a reference depth of N(H₂) = 3 × 10²¹ cm^–2, we infer ${\zeta }_{{{\rm{H}}}_{2}}\approx 1.4\times 1{0}^{-16}$ s^–1, a factor of ≈3 higher than the average value derived from H${}_{3}^{+}$ absorption studies. These results provide the most direct probe to date of CR penetration into cold, dense gas, offering new constraints on both the microphysics of CR-H₂ interactions and the attenuation of low-energy CRs in molecular clouds. Our findings establish CRXH₂ emission as a powerful new diagnostic of the CR environment in interstellar space.