The exact conditions of early Mars are distinctly uncertain, but we can draw information from the present and past state of the surface and atmosphere of Mars and the other terrestrial planets, and use analogues from elewhere in the solar system to study the atmospheres of cold planetary bodies.

There are several main lines of evidence about the past state of the Martian atmosphere:-

The Faint Young Sun (See also this linked page)

Observations of other stars, and studies of the nuclear processes that drive them, strongly suggest that the early Sun was less energetic than it is now. As best we can tell, some 4.5 billion years ago (4.5 Ga), the Sun emitted only 70% of its present energy. Fortunately, the equilibrium surface temperature of a "black body" (a perfect absorber/emitter of light and heat) is given by the 4th root of the power (T~P ^0.25 ), so the temperature of the Planets wouldn't have been very different. In fact, the equilibrium black body temperature (in Kelvins - degrees above absolute zero) would have been 92% of the present value. Mars now has a mean temperature of 218 K (-55 C). In the past it would have had a mean temperature around 200 K (-75 C). Winter at the South Pole on Earth is frequently this cold, but the Earth has plenty of warm areas to maintain life and to sustain liquid water.

 If we compare Mars and Earth, due to its wider orbit Mars is further from the Sun (1.52 AU). Since solar energy falls off as 1/(distance squared), then Mars has only 43% the radiant energy per square metre as has Earth. Equatorial Mars receives as much energy per square metre as does Northern Canada or Siberia. Early Mars would have received only 30% the energy input - making equatorial Mars akin to Greenland or the fringes of Antartica. By itself, that doesn't sound too bad. However, Mars has almost no atmosphere, other than CO2. Unlike Earth's Nitrogen/Oxygen atmosphere, the freezing point of the atmosphere is commonly reached on Mars! The atmospheric pressure on Mars is determined by the temperature at the winter poles of Mars  - currently ~ 147 K (-125 C). The reason for this is that on Mars when it gets cold, the atmosphere itself forms snow and falls to the surface. The present day atmospheric pressure on Mars of 6.5 millibars (0.0065 times Earth's 1 bar atmosphere), is determined by the equilibrium between solid CO2 ice/snow and CO2 vapour. On early Mars, the poles would have been ~133 K (-140 C) and less than 1 millibar of atmosphere would have been stable.

The Preservation of most, but not all craters

Large areas of Mars appear almost undisturbed since the epoch of cratering that is also visible on the Earth's Moon and on Mercury. Earth and Venus have more active surfaces that have removed most of the impact craters. We know from this that Mars has probably never had a global ocean and pervasive hydrological/sedimentary cycle like Earth (rain = erosion, rivers = transport, oceans = sedimentation)

On the other hand, there are several lines of eveidence that some sort of localised erosive/depositional systems took place. Many of the large (~50 km) old craters are degraded with almost no remaining rim wall and a flat infilled floor. Some are associated with dendritic drainage systems that look a lot like river systems on Earth. All these aspects are quite old (at least 3.5 Ga, based on crater densities). In some areas small craters are obliterated by unknown causes, implying surface sediment transport and/or erosion of the craters.

The most significant evidence of surface erosion and transport is the Outburst Flood channels. There is no doubt that catastrophic flows of some sort have been channeled down these valleys. The conventional interpretation is that the cold dry surface of Mars was repeatedly inundated by watery floods that burst out from under frozen ground, carved great channels, then froze in "lakes" that were later buried. Unfortunately, these floods need to have recurred an improbable number of times on a planet where water recycling is almost impossible. I prefer an alternive model of the "floods" in which cold dry avalanches poured down the valleys, lubricated by CO2 vapour.

The present state of Mars atmosphere

Mars is cold and dry now. The Sun was fainter in the past. It would take an unusual train of events for Mars to have been warmer and wetter in the past. There's no shortage of ice. The problem is temperature. Plenty of planetary scientists have worked on atmospheric models of early Mars, trying to make it warm and wet. The trouble is most of the hard evidence disagrees with a warm wet past.

The Sister Planets (see also this linked page)

This is the real strong constraint, in my opinion. I don't think enough work has been done on matching the history of all the terrestrial planets with one single model. So far as we can tell, all the terrestrial planets began with similar inventories of water and CO2 (on a size-for-size basis). Venus has boiled off its water because the atmosphere got so hot that the oceans turned to steam. What remains is a thick, hot greenhouse of CO2 (~90 atmospheres worth!) On Earth, all that CO2 is locked up as carbonate minerals and the water is liquid. On Mars, all the water and most of the CO2 are locked up as ice in the regolith, and to a minor extent in the polecaps.

There are plenty of scientific papers that have looked at Earth, or Venus, or Mars in isolation and stated that "early Mars was warm and wet" or "early Earth was not too hot for life to develop". However, most of these did not apply the same model to all the Terrestrial planets to see what happened. The same processes (clouds, greenhouse gases etc.) that are invoked to keep early Mars warm and allow flowing water, would also act on the Earth. To make early Mars like modern Earth we need about 3 times as much insulation in the atmosphere to compensate for the faint young Sun and the wider orbit. If we do the same to  Earth's early atmosphere, then Earth becomes like modern Venus (hotter, in fact!)

The observational fact that Earth is not (and probably never has been) like Venus, is a very strong constraint that Mars has never been appreciably warmer than it is today. In fact, early Mars should have been a cold iceworld - hence my evocative description of "White Mars" (with apologies to Kim Stanley Robinson).

      Created: May 2002
      Last modified: May 2002
      Authorised by:  Head, Earth Sciences

      Maintained by: Nick Hoffman
      Email: nhoffman@unimelb.edu.au