There is an unfortunate tendancy amongst planetary scientists to tackle problems individually, rather than collectively. This resembles the task facing geoplogists on Earth before the Plate Tectonic paradigm showed that the geology of disparate parts of the Earth was linked by process and sometimes by proximity. The puzzles of Earth's geology then became a single large puzzle with many different views, rather than many independent puzzles to be solved seperately.

So it is with the Terrestrial Planets. As far as we can tell, they all received broadly similar volatile inventories and accreted in similar fashions (although the late-stage impact that formed the Moon has made the Earth a very special Planet in several ways). The question of the evolution of the atmospheres and surfaces of the Terrestrial planets needs to be treated as a single question, not as three seperate ones.

Over the last couple of decades, two threads have permeated atmospheric studies of the Terrestrial Planets. One is the desire to make early Mars Warm and Wet, by using special greenhouse cocktails and modelling clouds and atmospheric properties to keep it warm. The other is the desire to avoid a runaway greenhouse on the early Earth, by using special greenhouse cocktails and modelling clouds and atmospheric properties to keep it cool. Obviously, these are completely contradictory goals. To be fair, some workers have approached the early Earth from another starting point and, finding it to be a little on the cold side, have used Mars-like models to warm it. Others let it get hot, and then show that chemical reactions will consume the CO2 as carbonate and so reduce it to a comfortable temperature.

Let's look at the basic facts about Solar energy input.We'll measure everything relative to Earth at the present day, which receives 1 "unit" per square metre of solar radiation. This is enough (with the help of the Terrestrial greenhouse effect) to keep Earth warm and blue. Venus receives 1.93 times the energy per square metre that Earth does, and Mars receives 0.43 times as much. This is why Mars is cold and Venus is hot. Note that Earth is more like Venus than Mars is like Earth (Venus is 1.93 times earth, while Earth is 2.3 times Mars).

In the early days of the Solar System, the Sun would have put out only 0.7 times as much energy as it now does. Thus Earth would have received 0.7 units per square metre. Venus would have received 1.35 times as much as Earth now does, and Mars a mere 0.3 times.

The Paradox of the Terrestrial Planets: If we treat the greenhouse effects of early atmospheres in a very simplistic way, if early Mars had a greenhouse effective enough to reach modern Earth temperatures from its 0.3 times modern Earth solar input, then that greenhouse must have had the effect of multiplying its heat input by a factor of ~3.3. If early Earth had an equally effective greenhouse, its solar input of 0.7 would also be multiplied by 3.3, giving an effective heating rate of 2.31 - way more than Venus' current hothouse. Thus we see that If Mars had a powerful greenhouse atmosphere in the Noachian, sufficient to result in Earth-like temperatures and flowing water, then Earth would have been "cooked" by its equivalent greenhouse!

The conventional solution to this Paradox is to either ignore it by treating one planet at a time, or to invoke special atmospheres for each planet and avoid the problem. There is no evidence for this.

The White Mars solution to the Paradox is to acknowledge the whole family of Terrestrial Planets and make a single model that honours all their histories. In this model, the early earth is cool, but survivable. Venus is hot and doomed to runaway greenhouse conditions, but Mars is a chill iceworld in the past, colder even than it is today.
 

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

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