Water on Mars

Liquid water cannot exist pervasively on the Martian surface today due to the low atmospheric pressure and surface temperature, although there is evidence for spurts of liquid flow that perhaps consist of a briny solution with reduced freezing temperature, according to Joseph Grebowsky of NASA's Goddard Space Flight Center in Greenbelt, Md. Water under current Martian atmospheric conditions can be ice or sublimate directly into vapor without staying in a liquid phase. Grebowsky is the project scientist for the mission.

Surface features and mineral compositions suggest ancient Mars had a denser atmosphere and liquid water on its surface, according to Grebowsky. "There are characteristic dendritic structured channels that, like on Earth, are consistent with surface erosion by water flows. The interiors of some impact craters have basins suggesting crater lakes, with many showing connecting channels consistent with water flows into and out of the crater. Small impact craters have been removed with time and larger craters show signs of erosion by water before 3.7 billion years ago. And sedimentary layering is seen on valley walls. Minerals are present on the surface that can only be produced in the presence of liquid water, e.g., hematite and clays," said Grebowsky.

Estimates of the amount of water needed to explain these features have been made that equated to possibly as much as a planet-wide layer one-half a kilometer (1,640 feet) deep or more, according to Grebowsky. If liquid surface water existed in the past, then Mars' atmosphere had to have had a different climate that was warmer and a pressure near or greater than the current terrestrial atmospheric pressure at the surface.

It's unknown if the habitable climate lasted long enough for life to emerge on Mars. "The only direct evidence for life early in the history of a planet's evolution is that on Earth," said Grebowsky. "The earliest evidence for terrestrial life is the organic chemical structure of a rock found on the surface in Greenland. The surface was thought to be from an ancient sea floor sediment. The age of the rock was estimated to be 3.8 billion years, 700 million years from the Earth's creation. No fossil evidence of life has yet been found from this period. The oldest claimed micro-fossils (found in Western Australia) date to 3.5 billion years ago. The existence of a potential life-nurturing climate on Mars ended near these times. A comparison between the two planet's life histories must be done with caution, due to the different chemical compositions of the surfaces (e.g., Mars' chemistry may have been more suitable early on than Earth's) and different volcanic and meteoroid impact histories. Also, the histories of life on either planet may not have been continuous. Catastrophic events could have killed off all life at one time only to have it start anew."

There are several theories of how Mars was stripped of its thick atmosphere. "Hydrodynamic outflow and ejection from massive asteroid impacts during the later heavy bombardment period (ending 4.1 billion to 3.8 billion years ago) were early processes removing part of the atmosphere, but these were not prominent loss processes afterwards," said Grebowsky. "The leading theory is that Mars lost its intrinsic magnetic field that was protecting the atmosphere from direct erosion by the impact of the solar wind."

The solar wind is a thin stream of electrically charged particles (plasma) blowing continuously from the sun into space at about a million miles per hour. "The interaction of the atmosphere with the solar wind leads to escape by sputtering of atoms and molecules out of the atmosphere, electromagnetic loss process of the planet's ionospheric particles, direct escape of hot plasma particles or by chemical processes that produce atoms with escape speeds," said Grebowsky.

"Studies of the remnant magnetic field distributions measured by NASA's Mars Global Surveyor mission set the disappearance of the planet's convection-produced global magnetic field at about 3.7 billion years ago, leaving the Red Planet vulnerable to the solar wind," said Grebowsky.

"MAVEN has been designed to measure the escape rates for all the applicable processes and will be able to single out the most prominent," said Grebowsky. It will also work with other missions to examine the past habitability of Mars. "Previous remote Mars observations from orbiting spacecraft have observed the geological features that have been used to estimate the amount of water that did exist and have analyzed the global distribution of water ice and surface chemistry to infer that water was lost through time. Mars Curiosity rover has the ability to analyze the chemical composition of the solid surface, which contains information of the atmospheric composition during the formation of the planet, in particular the isotope ratios, the lower atmosphere composition, and the current gas exchange with surface reservoirs. MAVEN is going to measure the current rates of loss to space and the controlling processes. Given the lower-atmosphere information and the nature of the escaping processes, one can extrapolate from current conditions into the climate of the past," said Grebowsky.