On Mars during the Late Hesperian and Amazonian, most workers are agreed that surface conditions were glacial. Surface water could not be impounded and then released to drive the floods. It is agreed by most workers that the fluid for the floods was stored underground and then burst out abruptly, undermining the chaos zones and carrying debris down the outburst channels and depositing it in the (mostly northern) lowlands. However, there is a volumetric problem. To flow as a flood, even a debris-rich one, several times more liquid is required than debris. The storage capacity of rocks is limited to about 40% of its volume, at a maximum. Therefore many floods must have repeated ly emerged from an underground storage area, carried a portion of the debris each time, and the source area must have recharged with water. It is estimated that at least 100 floods poured down each outburst channel. Each of these flood event was hundreds or thousands of times the flow rate of the Mississippi river.

The paradox: How was the water recharged, given such a cold thin atmosphere?

The conventional solutions to the Paradox demonstrate the main weakness of the argument. Many seperate ideas exist. None are very practical and all involve an element of luck or hopefulness.

1)    The water returned as rain or frosts that permeated the ground without eroding it

Problems:- Too slow to get the huge volumes for repeated flows. Also, the chronology of different channels is different. Why was the atmosphere only locally active, not globally? One would expect some areas to have been rained/snowed on more and show signs of stream erosion, or galaciers. There are none.

Problems:- The melting would have occurred once for each intrusion, not hundreds of times. Once the water was melted, we might see the intrusions break out as volcanics at surface - we don't. Also, if enough volume was melted as required by the water floods, then much larger areas of ground should have collapsed.

Problems:- There is only a weak association of channels with volcanoes, and the main chaos zones are unrelated.

Problems:- Mars has low geothermal gradients and it is hard to get a liquid zone extensive enough to act this way. For the process to work, a high pressure aquifer must underly the entire Northern plains, with enough pressure to pump water back up the crustal dichotomy (several km elevation). A single imnpact crater would puncture the seal and lead to huge local floods erupting out of the northern plains. We don't see a single event like this - everything flows into the plains, not outwards. We would also expect the Chaos zones to line up along tectonic fractures near the crustal dichotomy. They don't.

In a variant on this model, vapour-phase subsurface transport is invoked. This is just too difficult to arrange.

The White Mars solution to the Paradox: No water is involved, so no water need be recycled. In addition, the natural (5% to 15%) content of CO2 ices in the regolith transforms into enough CO2 vapour to carry the entire flow all the way to the northern plains. Each chaos zone supplies its own volatiles. Each block that collapses is already charged with overpresured solid that readily explodes when exposed to the atmosphere. There is no requirement for resupply of volatiles at all.

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

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