Main conclusions <p>Contrary to previous assumptions, dew and frost cannot be regarded as water sources for lithobionts in the McMurdo Dry Valleys (MDV) of Antarctica, which therefore solely rely on snowmelt water for growth. Therefore, and contrary to previous claims, MDV can be regarded as a good analogue for life on Mars.</p> Abstract <p>Rock-dwelling chlorolichens and cyanobacteria provide most of the total biomass of the ice-free zone of Antarctica, i.e., the McMurdo Dry Valleys (MDV), and yet, the water sources of MDV are not clear. In addition to snowmelt water that provides water to the lichen-dominated cryptoendolithic communities, many scholars also advocate the use of dew or frost as important water sources for chasmoendolithic cyanobacteria. The implications of these suggestions are large, especially due to the fact that the MDV serves as an important analogue for life on Mars. Based on 5-year long analysis of the four growing months (November, December, January, February) in three stations, we show that the likelihood of both sources to take place is low. Rock temperatures as measured by one of the stations (Marble Point) allowed us to also perform a detailed analysis. Rock temperatures never reached the dewpoint temperature, excluding the formation of dew. As for frost, the likelihood is extremely low during the growing season and an optimistic evaluation yields a possible occurrence of frost for 0.8 h per year. We, therefore, conclude that neither dew nor frost may serve as a meaningful water source for the chasmoendolithic cyanobacteria, which, therefore, rely on snowmelt water only. The endolithic communities of the MDV may be justifiably regarded as the best analogue for life on Mars.</p>