My first self-published ebook was about how to form a theory. The origin of this has an interesting history: Elsevier asked me to write a book, and while I know what they thought they were going to get, I sent back a proposal that I thought they could never accept, largely to get them off my back. They accepted it, at that stage, so I had to write. The problem for me was, it took somewhat longer than I expected; the problem for them was the time taken, the length, and then, horrors, they found out I was not an academic with lots of students forced to buy the book. The book was orphaned, but I was so far on I thought I might as well self publish it. The advocated methodology is that of Aristotle, and oddly enough, most of his scientific bloopers arose because he ignored his own instructions! So, let me show what I made of it on one of my projects: how did Mars ever have flowing rivers? Why I chose that is a story best left for a later post.
The first step is to state clearly what you know. In this case, Mars has some quite long what seem like riverbeds, and they start sometimes from the coldest parts of Mars. The longest goes from highlands 60 degrees south and stops somewhere near the equator, and these can only reasonably be explained by fluid flow. Almost certainly water is the only fluid there in sufficient volume, so it had to be at least part of the flow. However, water freezes at 0 degrees Centigrade, the average temperature on Mars now is about minus 60 degrees C, and when the rivers were flowing the sun had only about 2/3 its current heat output.
The next step is to ask questions. To start, how did water flow, starting from high altitude high latitude sites, where the temperatures would be well below that of the rest of the planet? Could we dissolve something in the water to lower the freezing point? Dissolving salts in the water depresses the freezing point, but even the aggressive calcium chloride will not buy you more than forty degrees, so that is not adequate by itself. There are worse problems with this explanation: where did these salts come from, and how could salts get into snow on the southern highlands?
The standard explanation is that there must have been a greenhouse effect, and many have argued for a very significant carbon dioxide atmosphere. There are three problems with this explanation. The first is, it won’t work. Anything less than ten atmospheres pressure is inadequate, and at three atmospheres, the carbon dioxide liquefies. You cannot get sufficient pressure. The second is, the winters on Mars are very long, and carbon dioxide would snow out on the poles, thus reducing the pressure, and because of the albedo of the snow, not all of it would revolatalize, so as the years progressed, the planet would quickly become what it is like now. The third problem is, if there were that much carbon dioxide, where did it go? From isotope fractionation, it appears that about half of the original material that stayed in the atmosphere has been lost to space. Some more could well be frozen out on the poles. However, if there were enough to sustain liquid water for extended periods of time, there should be a lot of carbonates, and there are not. Now it is true we do not know how much could be buried, so maybe that argument is a bit on the weak side. On the other hand, there is plenty of other evidence that the atmosphere of Mars was always thin, although not as thin as now, as there had to be enough to keep water liquid. A number of estimates put it in the 100 millibar range. Further, if it lasted for periods of a few hundred thousand years it could not have been carbon dioxide, at least not initially as otherwise most would have snowed out. Of course it could have been continuously replenished by volcanic action, but if so, there must be very large deposits of carbon dioxide at the poles and that does not appear to be the case. So by asking such simple questions, we have made progress.
The next question is, how did the gases and water get to Mars? This is a rather convoluted question, but the simple answer is, the river flows lasted for only a few hundred thousand years and they started about 1.5 billion years after Mars formed. They also corresponded to significant periods of volcanic eruptions, so the most likely answer for the gases is they came from volcanic eruptions. Most of the water would have too, however it is possible that there were ice deposits near the surface following accretion. The next question is, how did the gases get below the surface of Mars to be erupted?
If we think about them being adsorbed during accretion, then, with the exception of water and ammonia, because the heats of adsorption are very similar for various gases, they would be adsorbed approximately proportional to their concentrations in the disk gases. That would mean, predominantly hydrogen and helium, although these would have been subsequently lost to space. However, neon would also be a very common gas, and to a lesser degree argon, but both neon and argon (apart from argon 40, which is a decay product of potassium 40) are very rare on Mars, so that was not the mechanism.
A commonly quoted mechanism is the volatiles arrived on the rocky planets through comets. That is not valid, at least for Earth, the reason being that the deuterium levels on comets are too high. Another suggestion is they arrived on carbonaceous chondrites. That too does not ring true, first because there would have had to be a huge number more of them, but not silicaceous asteroids, and second, the isotopes of some other elements rule that out. As far as Mars goes, there is the additional point that since it had no plate tectonics, and it had a rocky surface approximately three million years after formation, there is no mechanism to get the gases below the surface.
The only way they could get there is to be accreted as solids. Water would bind chemically to silicates; carbon would probably be accreted as carbides, or as carbon; nitrogen would be accreted as nitrides. The gases are then formed by the reaction of water with the carbides or nitrides, so the amount of gas available depends on how many of these solids were formed, and how much water was accreted. The lower levels of these gases on Mars is due to the fact that the material in the Mars feeding zone never got as hot as around Earth during stellar accretion. The higher temperature in the Venusian accretion zone is why it also has about three times the nitrogen as Earth: nitrides were easier to form at higher temperatures. Water binding to silicates happened after the disk cooled, but before the dust accreted to planets, and Mars has less water because the better aluminosilicates never phase separated because the temperatures earlier were never hot enough. Venus got less water because the disk never got as cool as around Earth and the silicates could not absorb so much.
When water reacts with nitrides and carbides it makes ammonia and methane, and these are most stable under high pressure, which is easily obtained in the interior of planets. If so, this hypothesis predicts that the initial atmosphere would comprise ammonia and methane. This is usually considered to be wrong because ammonia in the atmosphere is quickly decomposed by UV radiation, however, the ammonia will not stay in the atmosphere. Ammonia is rapidly absorbed by water, and even snow, and it will liquefy ice even at minus 80 degrees C. That gets it out of the atmosphere quickly and now there is a simple mechanism why water would flow, and also why it would later stop flowing near the equator and form ice deposits: as it got warmer, the ammonia would evaporate off. The atmosphere would start as methane, but would gradually be oxidised to carbon dioxide, which is why the atmosphere had such a short life. The carbon dioxide would react with ammonia, and eventually the ammonium carbonate would be converted to urea and the water would stop flowing. Thus in this theory under the soil of Mars, provided it has not reacted further, there is just the fertilizer settlers would need.