At this time, giant asteroids were still crashing down from space at rates thousands of times those we experience today, blasting craters hundreds of km in diameter and forming the cratered highlands of Mars, Mercury, and the Moon.

No Earth rocks survive from that time - plate tectonics has wiped the slate clean - so we rely on evidence from beyond the Earth to tell us about conditions in the dawn of the Solar System.

The Moon and Mercury appear to have been airless rock balls at this time, and the record of impacts is preserved in their cratered surface, but Mars had an atmosphere, which interacted with the impacts and the warmth of the young Sun to produce weather patterns and erosion.

Many of the early craters on Mars are partly eroded and filled-in, perhaps by Earth-like streams, rivers, and lakes.

The big question which has been asked for decades is "how Earthlike was that Early Mars?". Embedded in this is the more fundamental question "Was there life on Early Mars?".

Those questions were addressed in an all-day session at the busy AGU convention, which commenced with a series of poster presentations in the morning and then moved to an oral session with brief contributions from the key scientists working in the field today, and then a broad-ranging open discussion which was able to reach a remarkable consensus on most of the issues. At the end, the conventional view of a "warm and wet" early Mars took a back seat to a new cold and dry Early Mars, with occasional damp moments.

The main problem with the observation of rivers on Early Mars is that the planet is today far too cold and dry for rivers to flow, although there are short gullies about 1 km long on steep poleward-facing slopes. These were originally discovered by Mike Malin and Ken Edgett of the Mars Global Surveyor imaging team, and were interpreted as the escape of groundwater on modern Mars.

Since then, a variety of authors have proposed instead that the gullies are formed by springtime melt of snowpack in favourably oriented valleys that are warmer than average during the spring thaw. For a few hours, on a few days each year, the gullies flow with a thin trickle of water. This may be enough, over millions of years of geologic time, to carve the gullies we see today.

In fact, most of the gullies are inactive today, but Mars orbit is more variable than the Earth's and over millennia, the obliquity - the angle of the spin axis to the sun - can vary from as little as 5 degrees to as much as 45 degrees, or perhaps even more in the distant past.

At present it is around 25 degrees, rather similar to Earth's 22.5 degrees. At higher angles, the climate is more strongly seasonal. The eccentricity of its orbit also varies with time in a coupled oscillation.

Today, Mars' orbit is quite elliptical and at perihelion - the closest approach to the sun in the southern summer - Mars may receive 10% more sunlight than at its furthest limit.

In the past, perihelion has varied between northern and southern summer and has been more or less extreme. These variations in seasonality and timing of perihelion make estimates of Mars past climate quite difficult since average values do not reflect the possibilities that fortuitous combinations of perihelion and ecc!

entricity can achieve. On a planet as slow and cold as Mars, a few days each millennium may add up over time to a lot of erosion.

The main focus of the meeting was on Noachian Mars, when most of the best river-like channels were formed. These were originally mapped over 25 years ago by the Viking Missions and Virginia Gulick (NASA Ames) summarised observations of the geographical and temporal distributions of the valleys.

Mike Carr (USGS) followed with a discussion of the later outburst flood channels (Hesperian in age, and dating to ~ 3.0 billion years ago), emphasising that in his view the massive floods required a very cold Mars so that the frozen skin could be strong enough to hold-in the massive pressures required to form the powerful flood channels.

He also commented that new Mars Odyssey Themis images in visible light and infra-red were showing that the earlier Noachian channels are more extensive than formerly believed, perhaps implying more extensive erosion on early Mars.

Tony Colaprete (NASA Ames) introduced the session and together with a presentation by Jim Kasting they discussed the concept of a possible "Warm and Wet" early Mars on which rain may have fallen to carve the river valleys.

Together with Raymond Pierrehumbert, they noted that atmospheric models could just about achieve the necessary conditions for rain, but that it was difficult to make early Mars that warm and wet on average, since the early Sun was less efficient than it is today and gave out 20% less heat.

Over the last decade, climate models have become more sophisticated, modelling clouds of CO2 ice particles, and this has made them less effective in warming early Mars. However, they concluded that climate models could probably deliver a warm and wet early Mars, IF the geological evidence required it.

Chris McKay (NASA Ames) followed with a less optimistic presentation that in his view the rivers on Mars were more likely the product of snow-melt than of rain, and that the melt probably occurred in only a few favourable seasons each decade or millennium depending on the amount of snow and the orbital parameters.

The rest of the time, a thin dusting of snow fell in the winter, and collected in low-lying hollows, but evaporated directly to the thin atmosphere without melting. Chris asked for an atmosphere of about 100 millibars for early Mars and average temperatures of -35 Centigrade to sustain this scenario.

Although this is only 10% of Earth's atmospheric pressure (1 bar), it is over 10 times the modern pressure on Mars, but it is much more conservative than the 2 bars required before rain becomes possible. Modern Mars also has an average temperature of -60 Centigrade, so this is "warm" by Mars standards but still pretty chilly.

Conway Leovy proposed that wind erosion alone could account for most of the features we see now, with an atmosphere similar to McKay's acting for over 3 billion years.

However, general discussion on this point was not supportive. Everyone recognized that wind has modified the earlier-formed features, but the shape and pattern of the river channels appears to require running water.

Teresa Segura offered an interesting new slant on the early Mars atmosphere. Based on a freshly-published paper, she described how large impacts would blast rock and ice into the atmosphere, vapourising them, and delivering a pulse of heat to the air and ground, as rock fragments fell back to the ground.

Since Mars odyssey has shown that much of Mars is covered in shallow permafrost, there is plenty of ice available to melt and boil by this process. She proposed that each giant impact made Mars hot and wet for a few decades, before the rain poured out of the skies again, eroded channels, and refroze.

There was intense debate on this issue. Mike Carr showed that the size of impactors that Teresa had modelled were too large for the time when the river channels were formed.

There certainly had been such giant impacts earlier in Mars history but these had been almost completely erased, perhaps by processes as described by Teresa, but that during the formation of the river channels we see now, impacts were much smaller.

Others noted that Teresa had modelled only the contribution of water, and not that possible from Carbon Dioxide, and that if Mars had more CO2 available at that time, then her model would work for smaller impacts.

Nick Hoffman (University of Melbourne), discussed the role of CO2 in forming gas-supported floods, rather than water-borne ones. His "White Mars" models of a CO2-rich Mars have been gaining currency over the last few years, emphasising that we need to study both CO2 and water in order to understand Mars.

He showed that Mars-like channels can form in a variety of unusual environments on Earth, such as the ocean floor or in volcanic eruptions, and suggested that we need to understand these flows better before we can be certain about the fluids involved on Mars.

He elaborated on Teresa's impact model, showing that use of CO2 in the crust (where it should be present as a liquid) allows smaller and more frequent impactors to energise the atmosphere for short bursts, between long episodes of total freezout - a snowball Mars.

Nick also showed active flows on modern Mars, down the Malin and Edgett gullies. These flows occur during the spring thaw of snowpack, but based on CO2 snow, not water ice.

These flows occur at temperatures of -130 centigrade and have been imaged by the Mars Global Surveyor camera in two successive springs. A gas-based flow of dust, sand, and dry ice, avalanches down the gullies, supported on a cushion of CO2.

John Dohm and Ken Tanaka (USGS) discussed the role of volcanic activity on Mars. Clearly, these can deliver water and CO2 to the surface and atmosphere from volcanic gases, and also supply the warmth to locally thaw frozen ground to energise an outburst flood.

Ken showed how early volcanic eruptions fluidized vast quantities of volatiles in the crust and led to devastating collapse of terrain over thousands of km, but later volcanism led to much smaller valleys, diminishing with time.

In general discussion, a quick consensus was reached. Nobody was comfortable with a permanently warm and wet early Mars. The "warmest" average temperatures were probably around Chris McKay's -35 Centigrade and many people preferred colder conditions.

The valley networks were seen as the slow product of occasional trickles of water in occasional seasons, rather than permanent streams and rivers.

The question remains whether, or rather to what degree, impact events & volcanism are responsible for those brief trickles, and for the slight amelioration of climate that permits the dusting of snow to be a little thicker than normal in those crucial "wet" years.

But it is clear that the description of "warm and wet" is not really appropriate for early Mars. Certainly, conditions were less cold and less dry than modern Mars, but they were by no means warm and wet. Conditions on Mars at the best of times were worse than the best locations in Antarctica today - the Dry Valleys region where similar rivers flow from annual snowmelt.

Calling these conditions "warm and wet" is like describing your refrigerator as a sauna. Sure, it's warmer than your deep freeze, but that won't help you get a sun tan!

And where does this leave the question of life on Mars? Well, the odds got stacked a little higher against surface life on Mars. But biology is tenacious. As Chris McKay pointed out, in the Dry Valleys of Antarctica most of the ground surface is a barren desert, but 10 metres away from desolation is a seasonal stream that feeds an ice-covered lake in which microbes thrive.

If life ever evolved on Mars, it is just possible that it could have adapted to the surface conditions in similar, but less dependable, water courses that flowed for a few seasons, every few thousand years.

Some powerfully resistant spores would be required to weather out the long dry freezouts, but who knows, something might have coped. However, the evidence is going to be very hard to find after all these billions of years, and the chance of anything still surviving is remote indeed, especially if some of these flows are based on Carbon Dioxide, not water.

Nick Hoffman is an Australian based researcher at Melbourne University who has promoted a Co2 model to explain Mars known as the White Mars theory.

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