Over the past several years NASA’s Mars rovers have provided us with an up-close look at the surface of the red planet. The images they have beamed back to Earth show a desolate landscape of barren craters, windswept rock outcrops, and red dust. But Mars hasn’t always look like this. Liquid water once flowed across the surface of the planet – 3.5 billion years ago a vast, deep ocean may have covered some of the planet’s northern regions, and glacial meltwater may have flowed across the surface of Mars as recently as several hundred million years ago (relatively recent in geological time).
While Mars’ oceans and rivers are long gone, the water is still there in the form of ice deposits buried just beneath the dusty Martian terrain. In 2003 NASA’s Odyssey spacecraft collected circumstantial evidence that large amounts of frozen water lay just beneath the surface. Later, the Phoenix Lander uncovered and photographed patches of shiny ice beneath the planet’s ubiquitous red dust. Instruments aboard Phoenix melted and analyzed the icy soil, confirming the existence of frozen water beneath the surface of Mars.
Some scientists even speculate that liquid water may still exist on Mars, trapped under the planet’s polar ice caps or deep underground. Water is crucial to life on Earth, and the existence of water, liquid or frozen, on Mars supports the tantalizing, but still far-fetched possibility that life may have developed on the red planet at some point. It’s pretty unlikely that little green men ever lived on Mars, but it’s possible that simple, single-celled organisms could have evolved sometime during the planet’s past. But even though water is present, there is very little evidence that the organic compounds that are the building blocks of life exist on Mars. In a more practical sense, plentiful water could be invaluable to a future manned mission to Mars. Astronauts could use water for drinking, to produce breathable oxygen, and even to grow plants for food.
Aerial photographs of the surface of Mars taken by the Surveyor Orbiter beginning in 1997 revealed gullies etched into the sides of sand dunes and rocky slopes. These gullies looked suspiciously similar to gullies carved by running water here on Earth, and their size, shape, and color even changed over the course of several years. This seemed to suggest that liquid water may still occasionally flow on Mars, even though the planet’s surface is far too cold to maintain liquid water for any significant amount of time. Perhaps, scientists speculated, water periodically emerges from deep underground and remains liquid just long enough to cascade over the surface, carving or reshaping a gully before freezing solid.
But last week NASA released images showing that at least some of these gullies are not caused by water flows after all, but by solid carbon dioxide – dry ice. The High Resolution Imaging Science Experiment (HiRISE) aboard the Mars Reconnaissance Orbiter photographed seven gullies over a short period of time to track any changes in their appearance. Changes in all seven gullies corresponded to a build-up of carbon dioxide frost during the planet’s winter. Exactly how the carbon dioxide frost carves the gullies remains uncertain. The frost may become so thick that it shears off a slope, dragging other material with it. Or carbon dioxide gas released by the frost may loosen the surrounding soil, triggering small landslides.
The Martian landscape is far more dynamic than we could have imagined: dust storms sweep across the planet; gullies are created, expanded, and diminished with the changing seasons; frozen water evaporates as it is exposed by violent meteorite collisions. And we’ve literally just scratched the surface.
Check out the image gallery, New Views of Mars, that accompanies this post.