The Mars Orbiter Camera on board the Mars Global Surveyor spacecraft has returned images of numerous dark streaks that are the result of down-slope mass movement occurring under present-day martian climatic conditions. We systematically analyze over 23,000 high-resolution images and demonstrate that slope streaks form exclusively in regions of low thermal inertia (confirming earlier results), steep slopes, and, remarkably, only where peak temperatures exceed 275 K. The northernmost streaks, which form in the coldest environment, form preferentially on warmer south-facing slopes. Repeat images of sites with slope streaks show changes only if the time interval between the two images includes the warm season. Surprisingly (in light of the theoretically short residence time of H2O close to the surface), the data support the possibility that small amounts of water are transiently present in low-latitude near-surface regions of Mars and undergo phase transitions at times of high insolation, triggering the observed mass movements.