Volcanic Dike Spawns Giant Flood

Ample evidence lies all over Mars for lots of subsurface water and ice. Such water clearly existed in the past and may even exist today. In places, scientists suspect the water may be confined under pressure because its natural outflow is blocked by impenetrable layers of rock or simply by a capping layer of permanently frozen ground. But if anything breaks into that layer, any water present will gush out with dramatic results.

Five kilometers (3 miles) wide and about 500 meters (1,600 feet) deep, this breach in the north wall of Mangala Fossae was carved by floodwaters released when the Mangala fault widened. The probable trigger for widening was molten magma pushing up into the fault, driven by volcanic activity in the vast province of Tharsis just to the north and east. One researcher attributes the features in Mangala Valles wholly or almost entirely caused to volcanic action, although most scientists see the eroding agent as water.

How much water emerged, and how often, remain unanswered questions, although scientists can take an educated guess. Terraces that stairstep up and down the downstream channel walls imply several episodes of outflow. Counting craters in the channel using the High Resolution Stereo Camera on Mars Express, scientists estimate the first and largest flow occurred about 3.5 billion years, followed by three lesser episodes of flooding around 1 billion, 500 million, and 200 million years ago.

 

Mass movement of a different kind has left its traces on this crater, which lies just inside the rim of a larger one. This smaller crater is about 40 km (25 mi) wide. Both craters lie outside the bounds of the Mangala valley floods, and are probably much older.

To the heat-sensing infrared eye of THEMIS, this crater holds two bright white spots. But since infrared THEMIS images record daytime ground temperatures, this means the white spots are not in fact light-colored, but instead are warmer than their surroundings.

So is this a volcanic hot spot poking through? Or residual heat from the impact? Neither, really, as a look at visible wavelengths (right) shows. The "bright" spots would actually appear dark if you could see them with the human eye.

What's happening here is that crater contains patches of sand and other tiny particles of rock. At the same time, over this part of Mars (and many others) a coating of dust perhaps a few inches deep has settled.

When winds blow across the surface they create turbulent eddies that agitate the particles and stir them into motion. This kicks the dust up from the surface, where the winds carry it off. Left behind is the darker bare ground that grows warmer in the sunlight.

And the process is largely self-sustaining, so long as the sand patches remain. The dark ground causes updrafts, which in turn can develop into dust devils. These whirlwinds help keep the sandy patches freer of dust, hence darker and warmer — and thus brighter to the thermal eye of THEMIS.

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