Introduction to special issue

Water, Water, Not Everywhere?

Science  21 Sep 2007:
Vol. 317, Issue 5845, pp. 1705
DOI: 10.1126/science.317.5845.1705
CREDIT: ZUBER ET AL.

The story of water on Mars has a checkered past. Historically thought to be criss-crossed by alien canals, the arrival of the first spacecraft showed that the red planet is bone dry. A flotilla of satellites has since traveled to Mars. Water ice has been seen dusting the ground, in hydrous minerals, in glaciers draped on volcano walls, and, the vast majority, locked up in the polar caps. Such a frosting gives a clue that water was once abundant on Mars and the planet's early climate was wet. Since then, water has been lost from the surface and atmosphere. It is possible that some may have survived underground, occasionally seeping to the surface, and so signs of flowing water have been sought extensively. Such places would be top destinations for future landers to search for signs of extant or past life.

The primary mission of the latest spacecraft to reach the red planet, the Mars Reconnaissance Orbiter (MRO), is to study its hydrologic history. It is opening up new vistas with its exceptionally keen eyesight. Regional landforms were mapped before, but MRO can now spot boulders less than a few meters in size, allowing Martian global geology to be surveyed on a human scale over vast areas. Among several other instruments, it also contains a radar sounder for exploring subsurface ice deposits and is accurately measuring Mars' gravity. This special issue presents scientific results from MRO detailed images of the ice-capped poles and regions thought to have been shaped by liquid water, both recently and in the past.

The papers in this issue show that evidence for liquid water is rare and difficult to discern (see also the News story by Kerr, p. 1673). Images of supposed ancient ocean floors and riverbeds show no obvious signs that liquid water was ever present. Reexamination of some landforms implies that they have been formed by flowing lava, not water. The only locations where features seem to indicate the presence of liquid water today or in the recent past are on the rims of craters and some gullies, suggesting that heat from impacts may have been the trigger for trickles rather than a revealed water table. Radar and gravity data show that the cap on Mars' south pole now holds the largest reservoir of relatively pure water ice on the planet, and layers there and in the north polar cap reveal seasonal oscillations in climate.

These papers thus provide a tour around the martian world of water in the first 100 days or so of the main MRO mission. More will be revealed as it continues to probe the surface, including Mars' mineralogy, and focuses several instruments on its atmosphere, dust, and clouds. But there is a lot to drink already.

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