The ExoMars Rover, launching in 2022, is designed to search for signs of past life through surface, sub-surface and atmospheric measurements using a wide array of instruments. One of them is the multispectral stereo imager, PanCam, containing a pair of Wide Angle Cameras (WACs), each with an 11-position filter wheel, and a High Resolution Camera (HRC). Two solar filters, SO1 and SO2, centered at 925nm and 935nm can be utilised to measure water vapour content in the atmosphere by measuring the 936 nm absorption feature. Through direct imaging of the rising and setting Sun as well as imaging the scattered light at the horizon, vertical distribution of water vapour can be retrieved in the lower Martian atmosphere. Using radiative transfer techniques and a sophisticated retrieval tool, NEMESIS (Non-linear optimal Estimator for MultivariatE Spectral analysIS), a vertical height profile can be computed. Currently, we are investigating the accuracy of these measurements using a PanCam instrument model and the Planetary Spectrum Generator tool. We are also investigating if similar measurements of water vapour in the lower atmosphere can be carried out using the MastCam-Z instrument onboard the Perseverance Rover at Jezero Crater.