8. Mars: the red planet
Liquid water on Mars
Cold, parched and wrapped in a thin, carbon-dioxide atmosphere, Mars today is a frozen, desiccated and inhospitable world. It cannot now rain on Mars, and liquid water cannot now remain on its surface. Yet, dry riverbeds, deep winding channels, and streamlined, washed-out landforms all provide compelling evidence for abundant liquid water on Mars in the distant past. In the planetís early history, rivers ran across its surface and powerful floods coursed down its valleys, emptying into the russet plains and perhaps forming ancient lakes or seas. The ancient, water-cut features take two main forms, dubbed the valley networks and outflow channels. The valley networks seem to have been derived from the gradual flow of liquid water. The outflow channels were gouged out of the surface by the powerful rush of short-duration floods.
Dry river beds
The valley networks have branching tributaries that connect into larger flows, so they look like dry river beds. The meandering flows increase in size downstream, and follow the local topography. They have lengths of up to hundreds of thousand of meters and widths of one to a few thousand meters, and they resemble river valleys on Earth. Of course, there is now no water in sight on Mars and these dry riverbeds were created long ago. They lie almost entirely in the ancient, heavily cratered highlands, and are rarely found in the younger, lowland plains. So, the valley networks are about as old as the highlands, dating back to 3.9 billion years ago.
Scientists have debated whether the running water fell as rain, during a sustained period of warm, wet climate, or flowed just below the Martian surface, warmed by internal heat. Recent evidence supports the view that the valley networks formed by collapse into cavities formed by water running under the frozen, ice-rich surface. The branching tributaries of some valley networks end abruptly in box canyons, which indicate flooding rather than rain. Valleys that seem to have been deeply cut by the continual flow of water also have features that suggest formation by collapse rather than rainfall. Yet, other evidence points to a time when Mars was wrapped in a thicker and warmer atmosphere than now, with possible rainfall, rivers, lakes and maybe even an ocean.
Ancient, water-charged torrents
Long, wide grooves have been gouged out of the equatorial regions of Mars, running downhill from the equatorial uplands to the lowland plains, measuring up to a million meters long. They tend to be narrow and deeply incised near their origins in the highlands and broad and shallow in the volcanic plains. Unlike valley networks, these enormous channels lack tributaries and are characterized by sculptured landforms such as scoured surface features, streamlined hills, and teardrop-shaped islands where the flowing water encountered an obstacle.
Possible ancient lakes and seas
As we all know, water collects within holes in the ground, ranging in size from potholes in winter roads to stream-fed lakes and ocean basins. And if water once flowed across the surface of Mars, it would similarly pool in low-lying depressions, such as impact craters and basins, deep canyons and the northern lowland plains. At one time, they could all have been filled with water, forming ancient lakes and seas with perhaps a thin layer of ice on top, but all that now remains is their dried-out floors and sediment.
Where did all the water go?
Everyone agrees that water once flowed on the surface of Mars in large quantities, but the exact fate of all that water remains unknown. If spread uniformly over the surface, the amount of water involved in the flooding of the outflow channels would, by itself, cover the entire planet to a depth of 500 meters. Yet, there is no liquid water residing on Mars today, and the amount of water vapor in its atmosphere is negligible.
Some of the water might have evaporated in more clement times, to be later lost to space, but most of the water that once flowed on Mars probably persists today, frozen just beneath the surface or remaining liquid at greater, warmer depths. Some of the water is most likely buried as ice, frozen into the soil as permafrost. That would account for the muddy debris ejected from some impact craters. Vast stretches of ice could be hidden under dust and sand in the northern regions, outside the polar caps, frozen into the places that the outflow channels emptied their flows. The rest might be liquid water buried beneath the ice. Spring-like seeps and small gullies suggest that liquid water has been released from under the frozen surface of Mars in recent times, perhaps even today.
Running water in modern times
Up until fairly recently scientists believed that water has not flowed on Mars for billions of years, but features identified in close-up images suggest that liquid water may still flow on Mars. The small, unassuming gullies have been carved into the steep, inside walls of some craters or valleys, with shapes that resemble gully washes on Earth. The Martian flow features emerge high up on the wall, run downhill in deep, winding channels, and fan out with an abrupt ending in an apron of dirt and rock.
The amount of water involved in each event can be estimated by measuring the volume of deposits with a conservative assumption of a 2-meter thickness, and by assuming that water accounted for just 10 percent of that volume. The rest of the debris flow is attributed to the dirt and rocks detected at the end of the gullies. It is thereby estimated that about 2,500 cubic meters of water have shaped each small gully, about the volume of an Olympic-sized swimming pool. That is equivalent to 2.5 million liters, and 660 thousand gallons, of water for each gully, enough to sustain future visitors to the planet.
Liquid water and life
(page 8 of 10)
Copyright 2010, Professor Kenneth R. Lang, Tufts University