Wet periods on Mars too short for life
Ancient riverbeds on Mars were created by brief periods of hot rain and flash-flooding caused by sporadic meteor impacts - indicating that favourable conditions for life to develop were far too short, a new American study has found.
Professor Owen Toon and doctoral student Teresa Segura, astrophysicists at the University of Colorado in Boulder, USA, found that rivers on Mars were carved during short periods when it was warm and wet - the result of intermittent impacts by comets and asteroids.
Between impacts, the planet dried up and cooled down again, making it highly unlikely that life had time to form, they argue in the latest issue of the journal Science.
The team used spacecraft images of Mars and computer models to show that large comets and asteroids hit the planet between 3.8 and 3.5 billion years ago, coinciding with the period that major river channels formed on Mars.
"What sparked our interest was that the large craters and river valleys appeared to be about the same age," said Toon.
The researchers assumed the two events were probably related. "Hypotheses of a warm, wet Mars, based on the presumption that the valley networks formed in a long-lasting greenhouse climate, imply that Mars may once have been teeming with life," Segura wrote.
"In contrast we envision a cold and dry planet, an almost endless winter broken by episodes of scalding rains followed by flash floods."
Mars was formed around the same time as Earth, some 4 billion years ago. Water flow and bombardments started soon after: between 3.8 and 3.5 billion years ago, when the river valleys on Mars cut through the oldest terrain on the planet.
The bombardment was heavy, with about 25 huge asteroids - between 50 km and 240 km in diameter - hitting Mars every 10 to 20 million years, they said. These impacts had the energy equivalent of 100,000 million megatons of TNT, depending on the velocity of the incoming object.
There are at least 30 craters dating from this period which were caused by impacts larger than 100 km in diameter. No-one yet knows why the heavy bombardment ceased.
Conventional thinking had been Mars had an Earth-like period when it was warm and had rivers and oceans. But the new study suggests the impacts were infrequent and Mars was almost always a predominantly cold, dry planet, said Segura, now based at NASA's Ames Research Centre in California, USA.
The force of the impacts warmed up the planet by transferring energy during collision, as well creating super hot 'ejecta': a mixture of soil, rocks and water that was catapulted into the early atmosphere and heated it on re-entryand generated more energy on impact. This kept Mars' surface warm for hundreds of years at a time, they said.
As Mars cooled down after each impact, the water in the atmosphere would have condensed back into rain. But these events were isolated and did not cause a prolonged warm and wet climate. "We definitely see river valleys, but not tributaries - indicating the rivers were not as mature as those on Earth," said Toon.
The group believes that during the warm periods, Mars would have experienced rainfall of between one and two metres a year - similar to that of Earth today. But the rain did not last long, and the water eventually sank into the soil.
While Mars' atmosphere would have been temperate enough to support life for a number of years after each impact, the time span would have been insufficient for life to establish itself and prosper, they argued.