FEROCIOUS asteroid bombardments on Mars could have triggered scalding rain and flash floods that carved out the planet鈥檚 giant river valleys billions of years ago. Mars must have been wet in the past for these valleys to have formed. The big question has always been how the planet could ever have been warm enough for this rain to fall. Asteroids impacts could be the answer.
Mars lies around 75 million kilometres farther from the Sun than Earth does and average temperatures today are about -50 掳C. But 3.5 billion years ago, asteroids regularly bombarded the planet. A single impact would have heated the planet for thousands of years, causing torrential downpours of scalding rain that led to flooding, according to Teresa Segura at the University of Colorado, Boulder, and her team. Segura noticed that Martian river valleys were all about the same age as the craters made by giant asteroid impacts. 鈥淲hen the impacts stopped occurring, the rivers also stopped being formed,鈥� she says.
To understand the connection, she developed a computer model of massive asteroids crashing on Mars and found that the impact vaporised the asteroid and all the matter from the crater. Vaporised rock and water from the asteroid, the planet鈥檚 surface and the polar ice caps enveloped the atmosphere, heating it up. The vaporised rock would have condensed first and fallen to the ground, followed by rain. The hot rocks would also melt frozen groundwater, forcing it to the surface (Science, vol 298, p 1977).
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A single asteroid 250 kilometres wide would have made it rain for 20 years, says Segura. She calculates that the barrage of asteroids during this period of Martian history would have generated rainfall across the planet of over 2 metres per year, creating rivers, and causing floods and mudslides that reshaped the landscape. 鈥淭his was a global phenomenon,鈥� says Segura. 鈥淩ivers didn鈥檛 have to be near the craters that formed them.鈥�
Chris McKay of the NASA Ames Research Center in Moffet Field, California, has another theory. It was snow, not rain, that fed the rivers and lakes on Mars, he told the American Geophysical Union meeting in San Francisco this week.
McKay points out that heavy rain would have eroded many of the smaller impact craters that are visible around the larger craters and in river valleys on Mars. 鈥淩ain is the most powerful erosive agent,鈥� he says. 鈥淭hese [small] craters would be erased on Earth in much less than a million years.鈥�
To explain how rivers could have formed without rain, McKay points to Lake Vanda and the Onyx River in the cold, dry valleys of Antarctica which get little rain. The snow that falls on the valley floor evaporates, causing no erosion. But glaciers melt slowly in the Sun鈥檚 heat and meltwater forms the Onyx River. 鈥淭his is a hydrological cycle without rain which has rivers and lakes,鈥� says McKay. 鈥淚t鈥檚 a miniature version of what I鈥檓 proposing for Mars.鈥�
This doesn鈥檛 happen on Mars today, not because it鈥檚 too cold, but because the low atmospheric pressure would cause glaciers to evaporate rather than melt. But if the pressure had been above 100 millibars, about a tenth of Earth鈥檚 atmospheric pressure, then glacier melts would be liquid, leading to rivers and lakes.
Climate modellers trying to show that Mars would have been warm enough for rain have been addressing the wrong question, says McKay. If they could show instead how the pressure could have been higher, it would explain how rivers formed even on a sub-zero Mars.
Whatever the answer, scientists say it鈥檚 still impossible to guess the chances that life arose on Mars.