This paradox was originally coined in reference to the Earth. Studies of how climate worked had identified the importance of the greenhouse effect in maintaining global temperatures within reasonable limits. (Without greenhouse activity in the atmosphere,  temperatures on Earth would be about 25 Centigrade cooler than they are now. Except for the tropics, the entire Earth would freeze over).

The Paradox emerged when this understanding of climate was merged with the knowledge of Stellar evolution. Our Sun, like most main sequence stars, began life as a cooler star and has gradually warmed over time as the fusion of hydrogen to helium has proceeded, building up a core of helium at the centre of the Sun and pushing the fusion zone nearer to the surface, where the energy can get out more easily. It is generally acknowledged that the early Sun (the Faint Young Sun) had only 70% the energy output that it has today.

Fortunately, the surface temperature of a planet warmed by solar energy varies only as the 1/4 power of the energy (Stefan's Law), so the early Earth's surface temperature would have been 91% of the present value.

Unfortunately, the temperature is measured in degrees above absolute zero! Thus the present global mean temperature of ~10 Centigrade (283 K) would have been reduced by 25 degrees to -15 C (258 K), leaving the early Earth as an unpleasant iceworld.

The Paradox on Earth:

We know from geological evidence that the Earth has been more or less its present temperature for at least the last 2.8 billion years and there is no evidence for world-wide glaciations on the early Earth.

The Solution to the Paradox is that in the past there was a higher level of greenhouse gas (notably CO2), with increased greenhouse effect. In fact, models now suggest that the rate of biological consumption of CO2 is controlled by temperature and acts in a feedback cycle to maintain the planet at comfortable temperatures. The current excitement about the greenhouse effect is caused because human activity is potentially breaking that feedback chain and artificially warming the planet.

The Paradox on Mars:

Mars' orbit is 1.52 times the distance from the Sun as is the Earth. Since radiant energy drops as the square of the distance, Mars today receives only 43% the energy per square metre as does the Earth. This is a constraint for solar powered orbiters and landers, especially in polar regions. Mars today has a mean surface temperature ~218 K (-55 C) and the thin atmosphere (6.5 mbar - 0.65% that of Earth) consists of 96% CO2, which freezes out at the cold winter pole. In the past, early Mars would have had only 30% of Earth's current insolation, due to the Faint Young Sun. It's mean temperature would have been a mere ~196 K (- 77 C) and CO2 would have frozen out extensively across the surface, yielding an atmosphere of less than 1mb!

Mars today is too cold  and low pressure for liquid water to form. And early Mars would have been colder, drier, and with a thinner atmosphere. Yet we see erosion networks and channels on Mars that appear to have been caused by flowing liquids. By analogy with Earth, this should have been water.

The conventional solution to the Paradox is to invoke a thick early greenhouse atmosphere of between 0.5 and 5 bars CO2 which would have warmed early Mars to Earth-like temperatures. Unfortunately, this atmosphere cannot be stable and would probably collapse in a few months or years. In addition, if a mechanism exists to avoid immediate collapse, it is not clear why Mars no longer has this atmosphere (see the Atmospheric Stability Paradox).

The White Mars solution to the Paradox is that the fluid eroding the channels on Mars was not water but was a fluidised cloud of avalanche debris, made up of a mixture of Mars dust, rocks, ice, and supported by CO2 vapour.This entire process took place in a thin atmosphere at prevailing low temperature. No unusual past climate needs to be invoked.

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

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