Preferred orientations of cracks in surface clasts have been reported on Earth and on Mars. This paper uses a 2D radiative transfer algorithm to simulate the Martian atmosphere, in combination with an original geometric code to determine the levels of differential insolation received by cracks as a function of season, latitude, and crack morphology on the surface of Mars. We find significant variation in preferred orientation both as a function of latitude and relative crack depth. Crack shape has little effect on the overall preferred orientation. The effect of requiring a minimum energy threshold to mobilize water is found to also affect the preferred direction, suppressing the E–W direction and somewhat promoting cross-modes. Crack orientations observed along the Spirit traverse can only be recreated by restricting growth propagation to mornings in the local autumn/winter season.