During the Noachian era, the entire Solar System was still an environment of unimaginable savagery -- the huge amount of smaller debris still orbiting the Sun after the planets had coalesced continued to rain down on the planets and moons at a high rate for the System's first 700 million years or so, before it was finally cleaned up.

During this period, objects as big as the 10-km asteroid that exterminated the dinosaurs (and half the other non-microscopic living species on Earth) would be crashing into Earth and Mars not just every few tens of millions of years, but every 100,000 years or so! And much, much bigger left-over asteroids would be hitting the planets at rarer intervals, blasting out craters hundreds of km wide.

During this era, both Earth and Mars usually would be cold and ice-covered under the fainter heat of the early Sun (indeed, most of Earth's oceans may have been covered with ice floes). But whenever such a huge object did crash into the planet, the staggering energy of that impact would blast huge amounts of melted -- and even vaporized -- rock up and outwards, to rain back down across the planet's surface.

A 100-km wide asteroid --- 1000 times the mass of the "dinosaur killer" that hit Yucatan -- hitting Mars would gouge out a 600-km wide crater, blasting enough molten or almost-molten rock to cover the planet's entire surface to an average depth of 7 meters (although most of this would land within a few hundred kilometers of the impact). Enough rock would be turned into incandescent vapor at 1300 deg C (2370 deg F) or hotter by the impact to condense soon afterwards as a 2-meter deep rain of lava all over the planet. And we can see 25 craters on Mars that big, or much bigger, that were produced during the 700 million years of the Noachian era -- one of them the gigantic Hellas Basin, 2400 km across!

Needless to say, such an event would vaporize all the frozen water ice on Mars' surface -- and the tremendous blast of heat from the impact conducted directly through the ground would boil (or, farther away, melt) a great deal of its more deeply buried permafrost. (A good deal of water would be added from the asteroid itself if it was made of water-rich "carbonaceous chondrite" rock.)

That 100-km-wide asteroid would heat Mars' surface to a roasting average of 530 deg C (980 deg F), turning the planet's atmosphere into a dense cloud of steam. About three months later, the surface and the atmosphere would finally cool off enough for scalding-hot rain to start condensing and pouring down across the surface -- rain which, as the planet's surface slowly cooled down, would continue evaporating back into the air in some regions and then refalling as rain again in others.

The result would be about ten straight years before Mars' surface cooled down below freezing again -- a decade during which such recycled rain would fall in deluges, pouring down a total of about 20 meters of rain onto Mars' surface. And the ground ice melted but not vaporized across Mars' surface by the impact would run down from its hills as mudslides, further adding to the erosion.

A bigger asteroid, of course, would have an even more staggering effect -- a 250-km wide one would gouge out a 1300-km wide crater, rain a 20-meter deep layer of vaporized rock condensing back into lava over the planet, and vaporize so much water that 150 meters of rain would fall across the entire planet's surface over the next century before Mars' surface finally cooled back down below freezing. And, as I say, Noachian Mars acquired a few craters much bigger than that -- the biggest could have kept Mars' surface above freezing, under a constant torrent of rain, for a millennium.

Nor must the effects of smaller impactors -- still much bigger than the Dinosaur Killer -- be forgotten; they could still produce impressive amounts of rain and mudslides on the Martian surface for hundreds of km away from their impact point, and they were far more frequent.

In his DPS paper, Colaprete presented the results of a detailed 2-D analysis of the meteorological effects of a 30-km wide object hitting Mars, as calculated by the computerized "General Circulation Model" of Martian weather developed by Ames Research Center. The rain from that smaller impact would be very unevenly distributed by Mars' terrain and weather patterns: some areas would get virtually none, but other large areas would get up to four meters' worth. And there were at least 40 impacts that big or bigger on Noachian Mars.

All this would add up. Colaprete's DPS paper concluded that the total rainfall produced by the rain of giant impactors hitting Mars during the Noachian era would be over a kilometer, running in short but repeated episodes along the same channels and carving them deeper and deeper. This by itself, he thinks, would be quite enough to carve the valley networks, without any other cause. And such rain could also explain why virtually all of Mars' surface features were eroded at rates hundreds -- or thousands -- of times faster during the Noachian than they have been during the planet's billions of thin-atmosphere years afterwards.

This model may also explain the paradox of Mars' lack of water-produced surface minerals better than any other, too -- for while these downpours of rain would be savage, they would also take up a very tiny part of Mars' total Noachian history.

The surfaces of Mars -- and Earth -- would have spent most of that period ice-covered under the dimmer light of the early Sun. Teresa Segura's vision of Noachian Mars is "a cold and dry planet, an almost endless winter broken by [brief] episodes of scalding rains followed by flash floods." And while the repeated flash floods would be violent enough to gradually carve the valley networks, Mars' surface would be exposed to liquid water for such a tiny fraction of that 700 million years that its minerals would undergo very little chemical weathering by water.

Colaprete tells "SpaceDaily" that -- despite the fact that Mars' surface may have had over a kilometer of total rain fall on it during the Noachian -- this still means that this water would have run across it for a total of no more than few thousand years, a short enough time that even a mineral as vulnerable to destruction by water as olivine might survive mostly unaltered. It would certainly not be enough to produce any large surface deposits of carbonates or clays, or to significantly weather the basalt covering Mars' southern highlands.