Rocky Planet Atmospheres

Where did the rocky planets get their atmospheres from? This question is not trivial. Planets accrete by some mechanism whereby dust particles form larger objects and sooner or later these form planets. However, when they are small, they are either in a vacuum, or earlier they are in the gas that is falling into the sun and which will make the sun. If they are in a vacuum there is no gas to accrete. If they are in the gas streaming into the star they will absorb some gas more or less in proportion to what is in the gas stream, with some preference of heavier gas per unit concentration. However, that preference will not mean much because the concentration of hydrogen is so high it will swamp out most of the rest. When the rocky planet gets big enough, it will form an atmosphere from the accretion disk gas, so these two mechanisms predict either no atmosphere (accretion after the disk gas is gone) or gas that is predominantly hydrogen and helium.

When the sun ejected its accretion disk, it continued to send out a flux of high-energy UV radiation. What is expected to happen then is this would boil the hydrogen atmosphere into space, and this hydrodynamic outflow would take most of the other gases with it. None of the rocky planets in our solar system has enough gravity to hold hot hydrogen and helium for long. So any gas accreted so far is either underground or lost to space. The rocky planets start without an atmosphere, except maybe residual heavy gas that was not blown away by the strong UV. The only gases that are likely to have been so held are krypton and xenon, and they have an excess of heavy isotopes that indicate they may be such residues.

The next possibility is the gases were trapped underground and emitted volcanically after the extreme UV from the sun had stopped. Now the hydrogen and helium could leak away to space slowly and leave everything else behind. But we know that our atmosphere is not a remnant of gas from the accretion disk held by gravity or absorption because if it were, neon is about as common as nitrogen in those gases, and they would be absorbed at about the same rate and both would be held equally by gravity. If our atmosphere was delivered that way, it should contain at least 0.6 bar of neon, which is many orders of magnitude greater than what we see. Neon is a very rare gas on Earth.

Attempts to answer this question have mixed results, and tend to divide scientists into camps, wherein they defend their positions vigorously. One school of thought has the gases were forced into a magma ocean that arises from the heat of the collisions of entities about the size of Mars. I disagree with this. Should this have happened, the time taken to get the collisions going (originally estimated as 100 million years, subsequently reduced to about 30 million years with some unspecified correction to the calculations to accommodate the planet being here when the Moon-forming collision occurred) the gas would have long gone. And if the calculations were so wrong and it did happen, we are back to the neon problem.

The usual way out of this is to argue the gases came from carbonaceous chondrites, which are supposedly bits knocked off asteroids from the outer part of the asteroid belt. Such chondrites sometimes have quite reasonable amounts of water in them, as well as solids containing carbon and nitrogen. The idea is that these hit the earth, get hot, and the water oxidises the carbonaceous material to liberate carbon dioxide and nitrogen gas. Ten years ago I published the first edition of my ebook “Planetary Formation and Biogenesis”, which contained evidence that this could not be the source of the gases. The reasons were numerous and some of them complex, but one simple reason is the three rocky planets all have different proportions of the different elements. How can this happen if they came from a common source?

Now, a paper has appeared (Péron and Mukhopadhyay, Science 377: 320 – 324) that states that the krypton gas in the Chassigny meteorite, shows Mars accreted chondritic volatiles before nebular gases. I have a logic problem with this: the nebula gases were there before Mars even started forming. There was never any time that there was a Mars and the nebular gases had yet to arrive. They then found the krypton and xenon had isotope ratios that fell on a line between cosmogenic and what they assigned as trapped Martian mantle gases. There is a certain danger in this because the rock would have been exposed to cosmic rays, which lead to spallation and isotope alteration. Interestingly, the xenon data contradicts a previous report by Ott in 1948 (Geochim Cosmochim Acta 52: 1937 – 1948), who found the xenon was solar in nature. It may be that these differences can be simply explained because these are taken from a meteorite and only very small amounts of the meteorite are allowed to be taken. The samples may not be representative. Interestingly Péron and Mukhopadhyay consider the meteorite to have come from the Martian interior, based on the observation by Ott that the sample had been heated to a high temperature and was presumably of volcanic nature. The problem I see with that is that Ott came to the same conclusion for a number of other meteorites that have quite different isotope ratios. It is usually wrong to draw major conclusions from an outlier result. Anyway, based on the argument that Ott thought this meteorite was igneous, this latest paper concludes that its rare gases came from the interior of Mars, and hence show the volatiles did not come from carbonaceous chondrites.

In my opinion, the conclusion is valid, but not for the right reasons. What annoys me is the example that a previous researcher thought the sample might have been volcanic rock is assume to have come from deep in the interior now, while the previous results that do not fit the proposition are put to one side. I think that small differences from two tiny samples show you should not draw conclusions. I know there are funding pressures on scientists to publish papers, but surely everything in their work and previous work they quote should be self-consistent or reasons be found for discrepancies.

Your Water Came from Where?

One interesting question when considering why Earth has life is from where did we get our water? This is important because essentially it is the difference between Earth and Venus. Both are rocky planets of about the same size. They each have similar amounts of carbon dioxide, with Venus having about 50% more than Earth, and four times the amount of nitrogen, but Venus is extremely short of water. If we are interested in knowing about whether there is life on other planets elsewhere in the cosmos, we need to know about this water issue. The reason Venus is hell and Earth is not is not that Venus is closer to the Sun (although that would make Venus warmer than Earth) but rather it has no water. What happened on Earth is that the water dissolved the CO2 to make carbonic acid, which in turn weathered rocks to make the huge deposits of lime, dolomite, etc that we have on the planet, and to make the bicarbonates in the sea.

One of the more interesting scientific papers has just appeared in Nature Astronomy (https://doi.org/10.1038/s41550-021-01487-w) although the reason I find it interesting may not meet with the approval of the authors. What the authors did was to examine a grain of the dust retrieved from the asteroid Itokawa by the Japanese Space agency and “found it had water on its surface”. Note it had not evaporated after millions of years in a vacuum. The water is produced, so they say, by space weathering. What happens is that the sun sends out bursts of solar wind which contains high velocity protons. Space dust is made of silicates, which involve silica bound to four oxygen atoms in a tetrahedron, and each oxygen atom is bound to something else. Suppose, for sake of argument, the something else is a magnesium atom. A high energy hydrogen nucleus (a proton) strikes it and makes SiOH and, say Mg+, with the Mg ion and the silicon atom remaining bound to whatever else they were bound to. It is fairly standard chemistry that 2SiOH → SiOSi plus H2O, so we have made water. Maybe, because the difference between SiOH on a microscopic sample of dust and dust plus water is rather small, except, of course, Si-OH is chemically bound to and is part of the rock, and rock does not evaporate. However, the alleged “clincher”: the ratio of deuterium to hydrogen on this dust grain was the same as Earth’s water.

Earth’s water has about 5 times more deuterium than solar hydrogen, Venus about a hundred times. The enhancement arises because if anything is to break the bond in H-O-D, the hydrogen is slightly more probable to go because the deuterium has a slightly stronger bond to the oxygen. Also, being slightly heavier, H-O-D is slightly less likely to get to the top of the atmosphere.

So, a light bulb moment: Earth’s water came from space dust. They calculate that this would produce twenty litres of water for every cubic meter of rock. This dust is wet! If that dust rained down on Earth it would deliver a lot of water. The authors suggest about half the water here came that way, while the rest came from carbonaceous chondrites, which have the same D/H ratio.

So, notice anything? There are two problems when forming a theory. First, the theory should account for everything of relevance. In practice this might be a little much, but there should be no obvious problems. Second, the theory should have no obvious inconsistencies. First, let us look at the “everything”. If the dust rained down on the Earth, why did not the same amount rain down on Venus? There is a slight weakness in this argument because if it did, maybe the water was largely destroyed by the sunlight. If that happened a high D/H ratio would result, and that is found on Venus. However, if you accept that, why did Earth’s water not also have its D/H ratio increased? The simplest explanation would be that it did, but not to extent of Venus because Earth had more water to dilute it. Why did the dust not rain down on the Moon? If the answer is the dust had been blown away by the time the Moon was formed, that makes sense, except now we are asking the water to be delivered at the time of accretion, and the evidence on Mars was that water was not there until about 500 million years later. If it arrived before the disk dust was lost, then the strongest supply of water would come closest to the star, and by the time we got to Earth, it would be screened by inner dust. Venus would be the wettest and it isn’t.

Now the inconsistencies. The strongest flux of solar wind at this distance would be what bombards the Moon, and while the dust was only here for a few million years, the Moon has been there for 4.5 billion years. Plenty of time to get wet. Except it has not. The surface of the dust on the Moon shows this reaction, and there are signs of water on the Moon, especially in the more polar regions, and the average Moon rock has got some water. But the problem is these solar winds only hit the surface. Thus the top layer or so of atoms might react, but nothing inside that layer. We can see those SiOH bonds with infrared spectroscopy, but the Moon, while it has some such molecules, it cannot be described as wet. My view is this is another one of those publications where people have got carried away, more intent on getting a paper that gets cited for their CV than actually stopping and thinking about a problem.

Asteroids

If you have been to more than the occasional science fiction movie, you will know that a staple is to have the trusty hero being pursued, but escaping by weaving in and out of an asteroid field. Looks like good cinema, they make it exciting, but it is not very realistic. If asteroids were that common, according to computer simulations their mutual gravity would bring them together to form a planet, and very quickly. In most cases, if you were standing on an asteroid, you would be hard pressed to see another one, other than maybe as a point like the other stars. One of the first things about the asteroid belt is it is mainly empty. If we combined all the mass of the asteroids we would get roughly 4% of the mass of the Moon. Why is that? The standard theory of planetary formation cannot really answer that, so they say there were a lot there, but Jupiter’s gravity drove them out, at the same time overlooking the fact their own theory says they should form a planet through their self-gravity if there were that amny of them. If that were true, why did it leave some? It is not as if Jupiter has disappeared. In my “Planetary formation and Biogenesis”, my answer is that while the major rocky planets formed by “stone” dust being cemented together by one other agent, the asteroid belt, being colder, could only manage dust being cemented together with two other agents, and getting all three components in the same place at the same time was more difficult.

There is a further reason why I do not believe Jupiter removed most of the asteroids. The distribution currently has gaps, called the Kirkwood gaps, where there are very few asteroids, and these occur at orbital resonances with Jupiter. Such a resonance is when the target body would orbit at some specific ratio to Jupiter’s orbital period, so frequently the perturbations are the same because in a given frame of reference, they occur in the same place. Thus the first such gap occurs at 2.06 A.U. from the sun, where any asteroid would go around the sun exactly four times while Jupiter went around once. That is called a 4:1 resonance, and the main gaps occur at 3:1, 5:2, 7:3 and 2:1 resonances. Now the fact that Jupiter can clear out these narrow zones but leave all the rest more or less unchanged strongly suggests to me there were never a huge population of asteroids and we are seeing a small residue.

The next odd thing about asteroids is that while there are not very many of them, they change their characteristics as they get further from the star (with some exceptions to be mentioned soon.) The asteroids closest to the sun are basically made of silicates, that is, they are essentially giant rocks. There appear to be small compositional variations as they get further from the star, then there is a significant difference. How can we tell? Well, we can observe their brightness, and in some cases we can correlate what we see with meteorites, which we can analyse. So, further out, they get significantly duller, and fragments that we call carbonaceous chondrites land on Earth. These contain a small amount of water, and organic compounds that include a variety of amino acids, purines and pyrimidines. This has led some to speculate that our life depended on these landing on Earth in large amounts when Earth was very young. In my ebook “Planetary Formation and Biogenesis”, I disagree. The reasons are that to get enough, a huge number of such asteroids would have to impact the Earth because they are still basically rock, BUT at the same time, hardly any of the silicate based asteroids would have to arrive, because if they did, the isotopes of certain elements on Earth would have to be different. Such isotope evidence also rules these out as a source of water, as does certain ratios such as carbon to chlorine. What these asteroid fragments do show, however, is that amino acids and other similar building blocks of life are reasonably easily formed. If they can form on a lump of rock in a vacuum, why cannot they form on Earth?

The asteroid belt also has the odd weird asteroid. The first is Ceres, the largest. What is weird about it is that it is half water. The rest are essentially dry or only very slightly wet. How did that happen, and more to the point, why did it not happen more frequently? The second is Vesta, the second largest. Vesta is rocky, although it almost certainly had water at some stage because there is evidence of quartz. It has also differentiated, and while the outer parts have olivine, deeper down we get members of the pyroxene class of rocks, and deeper down still there appears to be a nickel/iron core. Now there is evidence that there may be another one or two similar asteroids, but by and large it is totally different from anything else in the asteroid belt. So how did that get there?

I rather suspect that they started somewhere else and were moved there. What would move them is if they formed and came close to a planet, and instead of colliding with it, they were flung into a highly elliptical orbit, and then would circularise themselves where they ended up. Why would they do that? In the case of Vesta, at some stage it suffered a major collision because there is a crater near the south pole that is 25 km deep, and it is from this we know about the layered nature of the asteroid. Such a collision may have resulted in it remaining in orbit roughly near its present position, and the orbit would be circularised due to the gravity of Jupiter. Under this scenario, Vesta would have formed somewhere near Earth to get the iron core. Ceres, on the other hand, probably formed closer to Jupiter.

In my previous post, I wrote that I believed the planets and other bodies grew by Monarchic growth, but that does not mean there were no other bodies growing in a region. Monarchic growth means the major object grows by accreting things at least a hundred times smaller, but of course significant growth can occur for other objects. The most obvious place to grow would be at a Lagrange point of the biggest object and the sun. That is a position where the planet’s gravitational field and the sun’s cancel, and the body is in stable or metastable orbit there. Once it gets to a certain size, however, it is dislodged, and that is what I think was the source of the Moon, its generating body probably starting at L4, the position at the same distance from the sun as Earth, but sixty degrees in front of it. There are other metastable positions, and these may have also formed around Venus or Mercury, and these would also be unstable due to different rocky planets. The reason I think this is that for Vesta to have an iron core, it had to pick up bodies with a lot of iron, and such bodies would form in the hotter part of the disk while the star was accreting. This is also the reason why Earth has an iron core and Mars has a negligible one. However, as I understand it, the isotopes from rocks on Vesta are not equivalent to those of Earth, so it may well have started life nearer to Venus or Mercury. So far we have no samples to analyse that we know came from either of these two planets, and I am not expecting any such samples anytime soon.