If early Mars was as barren and cold as it is today, massive asteroid and comet impacts would have produced enough heat to melt subsurface ice, said Stephen Mojzsis, a professor at University of Colorado Boulder in US.

The impacts would have produced regional hydrothermal systems on Mars similar to those in the Yellowstone National Park in US, which today harbour chemically powered microbes, some of which can survive boiling in hot springs or inhabiting water acidic enough to dissolve nails.

Scientists have long known there was once running water on Mars, as evidenced by ancient river valleys, deltas and parts of lake beds, said Mojzsis.

In addition to producing hydrothermal regions in portions of Mars' fractured and melted crust, a massive impact could have temporarily increased the planet's atmospheric pressure, periodically heating Mars up enough to "re-start" a dormant water cycle.

Much of the action on Mars occurred during a period known as the Late Heavy Bombardment about 3.9 billion years ago when the developing solar system was a shooting gallery of comets, asteroids, moons and planets.

Unlike Earth, which has been 'resurfaced' time and again by erosion and plate tectonics, heavy cratering is still evident on Mercury, Earth's moon and Mars, Mojzsis said.

Researchers used a supercomputer cluster for some of the 3D modelling used in the study.
They looked at temperatures beneath millions of individual craters in their computer simulations to assess heating and cooling, as well as the effects of impacts on Mars from different angles and velocities.

The study showed the heating of ancient Mars caused by individual asteroid collisions would likely have lasted only a few million years before the red planet - about one and one-half times the distance to the Sun than Earth – defaulted to today's cold and inhospitable conditions.

The study was published in the journal Earth and Planetary Science Letters.

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