Key to the possibility of Martian biology is the availability of liquid water. The issue hinges on the physics of water under an atmosphere whose total pressure is only slightly higher than the triple point of water. The general consensus, that liquid water cannot exist on the Martian surface, was first challenged in 1998. This paper offers a more detailed analysis.
While orbital images from Mariner 9 onward have shown evidence of ancient water flows, recent Global Surveyor and Odyssey orbiters have produced images of apparently active erosion. The 1997 Pathfinder Lander measured surface atmospheric temperatures well above freezing, while temperatures one meter higher were as low as -40° C. The low vapor capacity of the atmosphere just above the surface acts as a lid, or barrier, to evaporation. This could allow ice to melt into liquid water instead of subliming to vapor. In 2000, in a demonstration made in a simulated Martian environment, water ice melted and remained liquid. However, many questions have been raised about the physics of this experiment. In this paper, numerical simulation provides values for the thermodynamic quantities controlling the phase of water. The binary diffusion coefficient of water vapor in CO2, and the law of binary gas diffusion are applied. Fluxes of water vapor under Martian conditions, including wind speeds, are calculated for various distances from surface ice sources. Evaporative heat loss is compared to the heat available from the sun. The paper provides the theoretical, if counterintuitive, basis for the existence of liquid water on the present Martian surface.