If we want to answer the question, how likely is there to be life on other planets, then these questions must be answered because life depends on these elements. Also, to answer questions such as how the catastrophic floods on Mars could have occurred also requires answers to where the materials for the floods came from. The questions are also of interest because they illustrate what I think is an ideal method for solving problems, and that is to ask questions and from the answers, eliminate what could not have happened. This gets to Conan Doyle’s mantra: if you have eliminated all but one, even if it seems unreasonable it must be what happened.
First, all material came from the accretion disk, and that got hotter and hotter as it fell into the sun. While the sun was accreting rapidly, temperatures a bit further away than where Earth is now reached about 1550 degrees Centigrade, and much of the material present at the end of rapid accretion made up the material from which the rocky planets formed. We know that because iron melted, we now have iron meteorites, and Earth has a large iron core, whereas Mars has much less iron in its core. (It would still accrete some iron, which would melt, but a lot would also oxidise and form rust if it were finely divided.) The gases in the disk at these temperatures and likely pressures would be mainly hydrogen, helium, water, carbon monoxide, nitrogen, neon, followed by a number of lesser gases. There came a point where the sun’s rate of accretion reduced by several orders of magnitude. The disk that remained now formed the rocky planets, and these planets formed rather quickly. Mars apparently formed within three million years, although of course bombardment led to some additional mass being added later.
So, where did the rocky planets’ atmospheres come from.
Option 1. The rocky planets formed and gravity started to hold atmosphere. We have the residue, after the hydrogen and helium was lost to space. This is obviously wrong. If our air arrived that way, then there should be roughly the same amount of neon in the air as nitrogen, but neon is extremely rare, although some of the krypton and xenon may have survived from this period. Accordingly, the early earth, and presumably the other rocky planets, were basically lumps of rock without atmosphere or oceans.
Option 2. The volatiles were delivered by comets. This was what was thought to be the case for some time, but while some comets probably have struck the earth, they are not considered to be a significant source of water or air. The reason is that the levels of deuterium in comets is about five times that of our seawater. You can concentrate the levels of deuterium by boiling off water, and you would have to lose the hydrogen to space, but you cannot concentrate the hydrogen. There have also been claims that the comets might have come from around Jupiter, because at least one of those has lower deuterium levels. That, however, would not get us any nitrogen, assuming the composition of Europa is typical of icy bodies in the Jovian region. It also presupposes a mass of comets that have essentially disappeared, but not into Jupiter, the obvious gravitational accretional centre.
Option 3 The volatiles were delivered by asteroids. There is some water, nitrogen and carbon in some asteroids, mainly carbonaceous chondrites, but these are in the minority. So, what is being asked for is that there were massive numbers of these asteroids further from the sun that were dislodged and struck Earth, but did not significantly affect the inner asteroids. Unfortunately, there is worse. Earth had to be struck by asteroids with very large amounts of water, large amounts of carbon, and modest amounts of nitrogen. Venus had to be struck by asteroids with modest amounts of water, about the same amount of carbon as Earth, and three times the amount of nitrogen. Mars had to be struck by asteroids with good amounts of water and carbon, but for some reason not as many, despite the fact that all asteroid striking anything have to cross Mars’ path, and seemingly very little nitrogen, although as later posts will show, there is ambiguity here. In other words, there were three different sorts of asteroids, and they each separately struck a different planet. Now, if you believe that, there is another problem. Isotope analysis of other elements (Drake, M. J., Righter, K., 2002. Determining the composition of the Earth. Nature 416: 39-44.) shows us that the rocks in asteroids can only have had a very minor part to play in Earth’s accretion. There are other compositional problems as well, and the overall conclusion is that the volatiles did not come from asteroids either.
Option 4 The volatiles were adsorbed onto dust in the accretion disk, and as the planet accreted, the dust got hotter, turned into rock, and the gases came out of volcanoes to form the atmosphere. Actually, there are two premises here. The first is that the gases were trapped on dust, and the second one is that the gases were emitted from volcanoes (or fumaroles). The first premise is wrong, because nitrogen, carbon monoxide and neon are each adsorbed on silicate dust to roughly the same degree, and so would end up being present in roughly the same amounts. Compared with nitrogen, Earth has roughly a hundred times more carbon, and about five orders of magnitude less neon. That is not the source of the atmosphere. On the other hand, there is good evidence that the volatiles on Mars came from volcanoes. Thus the large “normal” river systems and remains of lakes, etc, are of about the same age as the volcanic activity. For Mars, at least, it seems that for about half a billion years Mars had no atmosphere of significance, then volcanic activity started and there were rivers flowing. This happened on and off for a few hundred million years, then everything started to freeze out. The catastrophic flows actually occurred almost a billion years after the main rivers stopped flowing.
So, the problem now is, we have eliminated just about every possible physical mechanism for getting the gases there. Obviously, we have missed something in the above analysis, yet the clues are there. But to get further, we have to think about how rocky planets could form in the first place, and that will be material for a further Monday post. To get to an explanation for the catastrophic floods, or how the materials for life emerged, we have to go to a number of other places first.