Towards the end of the "Expanding Earth Video" topic the subject was raised of whether Mars has also expanded and what could that mean for the possibility of finding evidence of past or present life on Mars. Because many people might be interested in this topic, yet might not have read the previous discussion, I thought it would be good to open a topic on Mars specifically. For those people, I should at least give a few arguments on why some think the Earth has expanded:

(1) It was realized by Hilgenberg, Carey and others that the continents can form a completely interlocking cover over the whole globe if the early radius of the Earth was 50-60 per cent of the present radius. Such perfect matching of continents is unlikely to have arisen by chance. Some proponents of the theory favour slow expansion (since the Earth was formed), others fast expansion (since about 200-300 million years ago) and others a combination of the two.

(2) While evidence for seafloor spreading is definite and measurable, the compensating process of subduction in the plate tectonics theory is largely conjectural. There is no 'smoking gun' of subduction.

(3) The sea floor is in all places not older than 200 million years or so. If sea floors were continually being formed for billions of years, as suggested in the plate tectonics theory, and if older sea floors were to disappear by subduction, one would expect to find here and there relics of older sea floors billions of years old. These have not been found. (This general observation tends to support the fast expansion theory of Carey and others).

(4) If the ocean basins formed relatively late in Earth's evolution through expansion, and if the amount of water on the Earth has stayed roughly constant, then the continents would gradually have become more and more exposed over time as water drained into the ocean basins. Fossil evidence supports this general trend.

(5) The expanding earth theory explains why the continents are very even in thickness. Plate tectonics has no explanation for this.

Many other points were mentioned in the previous thread, both for and against the expanding Earth theory. There is also a vast literature on the subject.

There is some pretty suggestive evidence that Mars like Earth has expanded. For some basic facts on Mars see http://scienceworld.wolfram.com/astronomy/Mars.html
The southern half of Mars is the ancient "highlands" region, raised about 1-4 km above the mean level of the planet. The northern half is the much younger "lowlands". The highlands show much more cratering than the lowlands, which is consistent with the lowlands being relatively newer crust analogous to Earth's ocean crust. Except for the absence of liquid water the configuration is not unlike the continent-ocean pattern of the Earth. There is even a likely spreading centre in the lowlands region in the Tharsis ridge, as well as canyons which could stand in for Earth's ocean trenches. I don't know if there are 'shelves' analogous to the continental shelves on Earth, or if the 'coastlines' fit together on Mars like they do on Earth.

Mars could be an interesting 'lab' to prove the reality of subduction, since the absence of ocean water there makes observations in some ways easier. (Of course, plate tectonics supporters will take the absence of subduction on Mars as evidence that Mars does not have plate tectonics!)

Many recent reports have dealt with evidence of rainfall on early Mars. In the June 6 issue of Science there is a story "Running Water Eroded a Frigid Early Mars" by Richard Kerr, stating that computer simulations of rain falling on Mars' surface seem to produce patterns of runoff similar to what is actually seen. Some additional links on this topic were given at the end of the Expanding Earth Video thread. See also

http://www.collegian.psu.edu/archive...ihealth-03.asp

http://northonline.sccd.ctc.edu/jmasura/earlymars.htm

Could a warm early Mars be connected to Mars expansion? As with the Earth it would be nice to know what caused its expansion. One possible cause of expansion is a secular decrease in G. Suppose that 4 billion years ago the radius of Mars was only 60 per cent of its present value, or 2,038 km. Suppose that G at that time was about 1.4 times the present value. Assuming that there was no increase in the mass of Mars (= 6.4 x 10^23 kg), Mars' surface gravity 4 billion years ago would have been

g = GM/R^2 = 14.4 m/sec^2 .

This is even greater than Earth's present surface gravity of 9.8 m/sec^2.

As discussed earlier, the solar luminosity (SL) would also have been greater if G was greater. A hotter Sun could have made rain on Mars a possibility. The problem is that in Dirac's decreasing G hypothesis, for instance, the increase in SL seems to be too great. Yet this could be offset somewhat if SL were reduced according to present solar models. (These models suggest that the luminosity of the early Sun was 30 per cent less than today, and so the existence of other evidence indicative of a warmer Earth back then leads to the 'faint young Sun paradox')

Finally, it is also reasonable to suppose that Mars at least started off with an atmosphere. If the Earth's atmosphere arose secondarily (e.g., by comets), then Mars would also have received these same gases. Whatever atmosphere Mars had could have easily been retained when Mars was smaller, due to its higher surface gravity . Taken together, the evidence for rainfall on early Mars could be considered as consistent with a decreasing G scenario.

The possibility for life evolving on ancient Mars is then considerably bolstered in decreasing G scenarios, since all the necessary elements would have been there: atmosphere, liquid water, ample sunlight, plus reduced compounds emitted volcanically. Either chemoautotrophic or photoautotrophic models might be entertained. Even in expanding Mars scenarios with expansion occurring at constant G and constant mass, the surface gravity would have been greater at earlier stages and this alone could have been significant for life.

Some of the points I mention were thoroughly debated in the EE Video topic. I raise them again not to antagonize anyone, but merely to introduce the topic to Mars researchers and hopefully to widen the debate.