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The interaction of the shock waves originated from supernova explosions with the circumstellar medium provides crucial information on the physics of shock heating. Astrophysical shocks at all scales, from those in the heliosphere up to the cosmological shock waves, are typically collisionless and electrons, protons, and ions are expected to be heated at different temperatures. Although optical observations of Balmer-dominated shocks in young SNRs showed that the post-shock proton temperature is higher than the electron temperature, the actual dependence of the post-shock temperature on the particle mass is still widely debated. We tackle this longstanding issue through the analysis of deep multi-epoch and high-resolution observations of SN 1987A, made with the Chandra X-ray telescope. We study the observed spectra in close comparison with a dedicated full 3-D hydrodynamic simulation. The simulation is able to reproduce self-consistently the whole broadening of the spectral lines of many ions altogether. We could therefore measure the post shock temperature of protons and selected ions in the shocked circumstellar medium. We found that the ion to proton temperature ratio is always significantly higher than one and increases linearly with the ion mass for a wide range of masses and shock parameters. This provides information about the heating processes in collisionless shocks.