Did Mars Have an Ocean?

It is now generally recognized that Mars has had fluid flows, and a number of riverbeds, lake beds, etc have been identified, but there are also maps on the web of a proposed Northern Ocean. It has also been proposed that there has been polar wander, and this Northern Ocean was more an equatorial one when it was there about 3.6 billion years ago. The following is a partial summary from my ebook “Planetary Formation and Biogenesis”, where references to scientific papers citing the information can be found.

Various options include: (with bracketed volumes of water in cubic kilometre): a northern lake (54,000), the Utopia basin, (if interconnected, each with 1,000,000), filled to a possibly identified ‘shoreline’ (14,000,000), to a massive northern hemisphere ocean (96,000,000). Of particular interest is that the massive channels (apart from two that run into Hellas) all terminate within an elevation of 60 m of this putative shoreline.

A Northern Ocean would seem to require an average temperature greater than 273 degrees K, but the faint sun (the sun is slowly heating and three and a half billion years ago, when it is assumed water flowed, it had only about two thirds its current output) and an atmosphere restricted to CO2/H2O leads in most simulations to mean global temperatures of approximately 225 degrees K. There is the possibility of local variations, however, and one calculation claimed that if global temperatures were thirty degrees higher, local conditions could permit Hellas to pond if the subsurface contained sufficient water, and with sufficient water, the northern ocean would be possible and for maybe a few hundred years be ice free. A different model based on simulations, assuming a 1 bar CO2 atmosphere with a further 0.1 bar of hydrogen, considered that a northern ocean would be stable up to about three billion years. There is quite an industry of such calculations and it is hard to make out how valid they are, but this one seems not to be appropriate. If we had one bar pressure of carbon dioxide for such a long time there would be massive carbonate deposits, such as lime, or iron carbonates, and these are not found in the required volumes. Also, the gravity of Earth is insufficient to hold that amount of hydrogen and Mars has only 40% of Earth’s gravity. This cannot be correct.

This northern ocean has been criticized on the basis that the shoreline itself is not at a constant gravitational potential, and variations of as much as 1.8 km in altitude are found. This should falsify the concept, except that because this proposed ocean is close to the Tharsis volcanic area, the deformation of forming these massive volcanoes could account for the differences. The magma that is ejected had to come from somewhere, and where it migrated from would lead to an overall lowering of the surface there, while where it migrated to would rise.

Support for a northern sea comes from the Acidalia region, where resurfacing appears to have occurred in pulses, finishing somewhere around 3.65 Gy BP.  Accumulation of bright material from subsequent impacts and flow-like mantling was consistent with a water/mud northern ocean. If water flows through rock to end in a sea, certain water-soluble elements are concentrated in the sea, and gamma ray spectra indicates that this northern ocean is consistent with enhanced levels of potassium and possibly thorium and iron. There may, however, be other reasons for this. While none of this is conclusive, a problem with such data is that we only see the top few centimeters and better evidence could be buried in dust.

Further possible support comes from the Zhurong rover that landed in Utopia Planitia (Liu, Y., and 11 others. 2022. Zhurong reveals recent aqueous activities in Utopia Planitia, Mars. Science Adv., 8: eabn8555). Duricrusts formed cliffs perched through loose soil, which requires a substantial amount of water, and also avoids the “buried in dust” problem. The authors considered these were formed through regolith undergoing cementation through rising or infiltration of briny groundwater. The salt cements precipitate from groundwater in a zone where active evaporation and accumulation can occur. Further, it is suggested thus has occurred relatively recently. On the other hand, ground water seepage might also do it, although the water has to be salty.

All of which is interesting, but the question remains: why was the water liquid? 225 degrees K is about fifty degrees below water’s freezing point. Second, because the sun has been putting out more heat, why is the water not flowing now? Or, alternatively, as generally believed, why did it flow for a brief period than stop? My answer, somewhat unsurprisingly since I am a chemist, is that it depends on chemistry. The gases had to be emitted from below the surface, such as from volcanoes or fumaroles. The gases could not have been adsorbed there as the planet accreted otherwise there would be comparable amounts of neon as to nitrogen on the rocky planets, and there is not. That implies the gases were accreted as chemical compounds; neon was not because it has no chemistry. When the accreted compounds are broken down with water, ammonia forms. Ammonia dissolves very rapidly in water, or ice, and liquefies it down to about 195 degrees K, which is well within the proposed range stated above. However, ammonia is decomposed slowly by sunlight, to form nitrogen, but it will be protected when dissolved in water. The one sample of seawater from about 3.2 billion years ago is consistent with Earth having about 10% of its nitrogen still as ammonia. However, on Mars ammonia would slowly react with carbon dioxide being formed, and end up as solids buried under the dust.

Does this help a northers sea? If this is correct, there should be substantial deposits of nitrogen rich solids below the dust. If we went there to dig, we would find out.

How Did We Escape the RNA World?

In my ebook “Planetary Formation and Biogenesis” I argue that life had to start with nucleic acids because only nucleic acids can provide a plausible mechanism for reproduction, and, of course, that is exactly what they do now – they reproduce. The RNA world may not qualify as life as more is required, but if this step is not achieved there can be no life. The first reproducing agent had to be RNA because ribose is the only sugar that occurs at least partially in a furanose form. (The furanose is a five-membered ring; the pyranose is a six-membered ring and is generally more stable.) Why do we need the furanose? In my ebook I show various reasons, but the main one is that the only plausible experiment so far to show phosphate esters could have formed naturally lead to AMP and ATP. While the ribose is only about 20% furanose, NO pyranose formed phosphate esters.

Later, DNA was used primarily for reproduction for a very simple reason: it uses 2-deoxyribose. The removal of the 2-hydroxyl from ribose makes the polymer several orders of magnitude more stable. So why did this not be part of the starting mix? Leaving aside the fact we do not really know how to get 2-deoxyribose in any synthesis that could reasonably have happened in some sort of pond without help (complicated laboratory chemical syntheses are out!) there is a more important reason: at the beginning high accuracy in reproduction is undesirable. The first such life forms (i.e. things that reproduce) are not going to be very useful. They were chosen at random and should have all sorts of defects. What we need is rapid evolution, and we are more likely to get that from something that mutates more often. Further, RNA can act as a catalyst, which speeds up- everything.

Bonfio (Nature, 605, p231-2) raises two questions. The first borders on silly: why did proteins as enzymes replace most of RNA catalytic activity? The short answer is they are immensely better. They speed things up by factors of billions, and they are stable, so they can be reused over and over again. So why did they not arise immediately? Consider the enzyme that degrades protein; it has 315 properly sequenced amino acids. If we limit ourselves to 20 different ones, and allow for the initial ones being either left- or right-handed, except for glycine, the probability of random selection is 2 in 39^315. That is, 39 multiplied by itself 315 times. To put that in perspective, there are just 10^85 elementary particles in the visible universe. It was simply impossible. But that raises the second, and extremely interesting question: how could ordered protein selection emerge with such horrendous odds against?

What happens now is that messenger RNA has three nucleotide sequences “recognized” and “transfers” this information to transfer RNA which selects an amino acid and attaches it to the growing chain, then goes back to the messenger RNA to get the next selection information. That is grossly oversimplified, but you might get the picture. The question is, how could this emerge? The answer appears to include non—canonical nucleotides. RNA comprises mainly adenine, guanine, cytosine and uracil, and these are the “information” holders, but there are some additional entities present. One is adenosine with a threonylcarbamoyl group attached. The details are not important at this level – merely that there is something additional there. The important fact is there is no phosphate linkage so this is not in the chain. At first sight, these are bad because they block chain formation. Thus for every time this hydrogen-bonded to a uracil, say, it would block the chain synthesis and stop reproduction. However, it turns out that they may assist peptide synthesis. The non-canonical nucleotide at the terminal point of a RNA strand attracts amino acids. This becomes a donor strand, and it transfers to a similar RNA with a nascent peptide, and we have ordered synthesis. It is claimed that this can be made to happen under conditions that could plausibly occur on Earth. The peptide synthesis involves the generation of a chimeric peptide – RNA intermediate, perhaps the precursor of the modern ribosome. Of course, we are still a long way from an enzyme. However, we have (maybe) located how the peptides could be synthesised in non-random way, and from the RNA we can reproduce a useful sequence, but we are still a very long way from the RNA knowing what sequences will work. The assumption is, they will eventually self-select, based on Darwinian principles, but that would be a slow and very inefficient process. However, as I note in the ebook, the early peptides with no catalytic properties are not necessarily wasted. The most obvious first use would be to incorporate them in the cell wall, which would permit the formation of channels able to bring in fresh nutrients and get rid of excess water pressure. The evolution of life probably a very long time during which much stewing and testing was carried out until something sufficiently robust evolved.

The First Atmosphere

Ι have now published the second edition of my ebook “Planetary Formation and Biogenesis”. It has just under 1290 references, each about a different aspect of the issue, although there is almost certainly a little double counting because references follow chapters, and there will be some scientific papers that are of sufficient importance to be mentioned in two chapters. Nevertheless, there is plenty of material there. The reason for a second edition is that there has been quite a lot of additional; information from the past decade. And, of course, no sooner did I publish than something else came out, so I am going to mention that in this post. In part this is because it exemplifies some of what I think is wrong with modern science. The paper, for those interested, is from Wilcoski et al. Planet Sci J. 3: 99. It is open access so you can read it.

First, the problem it attempts to address: the standard paradigm is that Earth’s atmosphere was initially oxidised, and comprised carbon dioxide and nitrogen. The question then is, when did this eventuate? What we know is the Earth was big enough that if still in the accretion disk it would have had an atmosphere of hydrogen and helium. If it did not accrete until after the disk was expelled, it would have no atmosphere initially, and an atmosphere had to come from some other process. The ebook shows the evidence and in my opinion it probably had the atmosphere of hydrogen. Either way, the accretion disk gets expelled, and assuming our star was the same as others, for the first few hundred million years the star gave off a lot of extremely energetic UV radiation, and that would be sufficient to effectively blow any atmosphere away. So under that scenario, for some number of hundred million years there would be no atmosphere.

There is an opposing option. Shortly after the Moon-forming event, there would be a “Great Bombardment” of massive impactors. There are various theories this would form a magma ocean and there is a huge steam atmosphere, but there is surprisingly little evidence for this, which many hold onto no matter what. The one piece of definite evidence are some zircons from the Jack Hills in Australia, and these are about 4.2 – 4.3 billion years old – the oldest of any rock we have. Some of these zircons show clear evidence that they formed under temperatures not that different from today. In particular, there was water that had oxygen isotope ratios expected of water that had come from rain.

So, let me revisit this paper. The basic concept is that the Earth was bombarded with massive asteroids and the iron core hit the magma ocean, about half of it was sent into the atmosphere (iron boils at 2861 degrees C) where it reacted with water to form hydrogen and ferrous oxide. The hydrogen reacted with nitrogen to form ammonia.

So, what is wrong with that? First, others argue that iron in the magma ocean settles to the core. That, according to them, is why we have a core. Alternatively, others argue if it comes from an asteroid, it emulsifies in the magma. Now we have the iron doing three different kind of things depending on what answer you want. It can do one of them, but not all of them. Should iron vapour get into the atmosphere, it would certainly reduce steam and make hydrogen, but then the hydrogen would not do very much, but rather would be lost to space because of the sun’s UV. The reaction of hydrogen with nitrogen only proceeds to make much ammonia when there is intense pressure. That could happen deep underground. However, in atmospheric pressure at temperatures above the boiling point of iron, ammonia would immediately dissociate and form nitrogen and hydrogen. The next thing that is wrong is that very few asteroids have an iron core. If one did, what would happen to the asteroid when it hit magma? As an experiment, throw ice into water and watch what happens before it tries to reverse its momentum and float (which an asteroid would not do). Basically, the liquid is what gets splashed away. Rock is a very poor conductor of heat, so the asteroid will sink quite deeply into the liquid and will have to melt off the silicates before the iron starts to melt, and then, being denser, it will sink to the core. On top of that it was assumed the atmosphere contained 100 bars of carbon dioxide, and two bars of nitrogen, in other words an atmosphere somewhat similar to that of Venus today. Assuming what was there to get the answer you want is, I suppose, one way of going about things, in a circular sort of way. However, with tidal heating from a very close Moon, such an atmosphere with that much water would never rain, which contradicts the zircon data. What we have is a story that contradicts the very limited physical evidence we have, which has no evidence in favour of it, and was made up to get the answer wanted so they could explain where the chemicals that formed life might have come from. Needless to say, my ebook has a much better account, and has the advantage that no observations contradict it.

Did a Galactic-Scale Collision Lead to Us?

Why do we have a planet that we can walk around on, and generally mess up? As most of us know, the atoms we use, apart from hydrogen, will have originated in a nova or supernova, and some of the planet possibly even from collisions of neutron stars. These powerful events send clouds of dust into gas clouds, but then what? We call it dust, but the particle size is mainly like smoke. Telescopes like the Hubble space telescope have photographed huge clouds of gas and dust in space. These can be quite large, thus the Orion molecular cloud complex is hundreds of light years across. These giant clouds can sit there and do very little, or then start forming stars. The question then is, what starts it? The hydrogen and helium, which are by far the predominant components, with hydrogen masses about ten thousand times as much as anything else except helium, are always colliding with each other, and with dust molecules, but they always bounce back because there is no way to lose their kinetic energy. The gas has been around for 13.6 billion years, so why does it collapse suddenly?

To make things slightly more complicated, the cloud does not collapse on itself. Rather, sections collapse to form stars. The section that formed our solar system would probably have been a few thousand astronomical units across (an astronomical unit, AU, is the distance between Earth and the Sun), and this is a trivial fraction of such giant clouds. So what happens is sections collapse, leaving the cloud with “holes”, a little like a Swiss cheese.

For us, about 4.6 billion years ago such a piece of a gigantic gas cloud started to collapse upon itself, which eventually led to the formation of the solar system, and us. Perhaps we should thank whatever caused that collapse. A common explanation is that a nearby supernova sent a shockwave through the gas, and that may well be correct for a specific situation, but there is another source of disruption: galactic collisions. We have observed these elsewhere, and invariably such collisions lead to a good generation of stars. Major galaxies do not collide that often because they are so far away from each other. As an example, in about five billion years, Andromeda will collide with the Milky Way. That may well initiate a lot of formation of new stars as long as there is plenty of gas and dust clouds left.

However, there are some galactic collisions that are a bit more frequent. There is something called the Sagittarius Dwarf Spheroidal Galaxy which is approximately a tenth the diameter of the Milky Way. It comprises four main globular clusters and is spiralling around our galaxy on a polar orbit about 50,000 light years from the galactic core and passes through the plane of the Milky Way periodically. It apparently did this about five to six billion years ago, then about two billion years ago, and one billion years ago. Coupled with that, a team of astronomers have argued that star formation in the Milky Way peaked at around 5.7, 1.9 and 1 billion years ago. The argument appears to be that such star formation arose about the same time that the dwarf galaxy passed through the Milky Way. In this context, some of our nearest stars fit ths hypothesis. Thus Tau Ceti, EZ Aquarii,  and Alpha Centauri A and B are about 5.8 billion years old, Procyon is about 1.7 billion years old, while Epsilon Eridani is about 900 million years old.

However, if we look at other local stars, we find Earth, Lacaille 9352 and Proxima Centauri are about 4.5 billion years old, Epsilon Indi is about 1.3 years old, Alpha Ophiuchi A is about 750 million years old, Sirius is about 230 million years old, and Wolf 359 is between 100 – 300 million years old. Of course, a galaxy passing through another galaxy will consume a lot of time, so it is not clear what to make of this. There is always a temptation to correlate and assume causation, and that is unsound. On the other hand, the more massive Milky Way may have stripped some gas from the smaller galaxy, and a wave of gas and dust on a different orbit could have long term effects.

In case you think the stars in a galaxy are on well-behaved orbits around the centre, that is wrong. Because the galaxy formed from the collision and absorption of smaller galaxies the motion is actually quite chaotic, but because stars are so far apart by and large they ignore each other. Thus Kapteyn’s Star orbits the galactic centre and is quite close to our Sun, except it is going in the opposite direction. We “meet again” on the other side of the galaxy in about 120 million years. So to summarize, we still don’t know what caused this solar system to form but we should be thankful that we got what we did. Our system happens to be just about right for our life to form, but as you will see, when it comes out, from the second edition of my ebook “Planetary Formation and Biogenesis” there are a lot of things that could have gone wrong. Let’s not help more things to go wrong.

Could Aliens Know We Are Here?

While an alien could not see us without coming here, why pick here as opposed to all the other stars? We see exoplanets and speculate on whether they could hold life, but how many exoplanets could see our planet, if they held life with technology like ours or a little better? When I wrote the first edition of my ebook “Planetary Formation and Biogenesis” I listed a few techniques to find planets. Then, the most had been found through detecting the wobble of stars through the frequency changes of their line spectra to which a Doppler shift was added. The wobble is caused by the gravity of the planets. Earth would be very difficult to see that way because it is too small. This works best with very large planets very close to stars.

While there are several methods for discovering planets that work occasionally, one is particularly productive, and that is to measure the light intensity coming from the star. If a planet crosses our line of sight, the light dims. Maybe not a lot, but it dims. If you have seen an eclipse of the sun you will get the idea, but if you have seen a transit of Venus or of Mercury you will know the effect is not strong. This is very geometry specific because you have to be able to draw a straight line between your eye, the planet and part of the star and the further the planet is from the star, the smaller the necessary angle. To give an idea of the problem, our planetary system was created more or less on the equatorial plane of the accretion disk that formed the sun, so we should at least see transits of our inner planets, right? Well, not exactly because the various orbits do not lie on one plane. My phrase “more or less” indicates the problem – we have to be exactly edge-on to the plane unless the planet is really close to the star, when geometry lends a hand because the star is so big that something small crossing in front can be seen from wider angles.

Nevertheless, the Kepler telescope has seen many such exoplanets. Interestingly, the Kepler telescope, besides finding a number of stars with multiple planets close to the star has also found a number of stars with only one planet at a good distance from the star. That does not mean there are no other planets; it may mean nothing more than that one is accidentally the only one whose orbital plane lies on our line of sight. The others may, like Venus, be on slightly different planes. When I wrote that ebook, it was obvious that suitable stars were not that common, and since we were looking at stars one at a time over an extended period, not many planets would be discovered. The Kepler telescope changed that because when it came into operation, it could view hundreds of thousands of stars simultaneously.

All of which raises the interesting question, how many aliens, if they had good astronomical techniques, could see us by this method, assuming they looked at our sun? Should we try to remain hidden or not? Can we, if we so desired?

In a recent paper from Nature (594, pp505 – 507 2021) it appears that 1,715 stars within 100 parsecs of the sun (i.e. our “nearest neighbours”) would have been in a position to spot us over the last 5,000 years, while an additional 319 stars will have the opportunity over the next 5,000 years. Stars might look as if they are fixed in position, but actually they are speedily moving, and not all in the same direction. 

Among this set of stars are seven known to have exoplanets, including Ross 128, which could have seen us in the past but no longer, and Teegarden’s star and Trappist-1, which will start to have the opportunity in 29 years and 1642 years respectively. Most of these are Red Dwarfs, and if you accept my analysis in my ebook, then they will not have technological life. The reason is the planets with composition suitable to generate biogenesis will be too close to the star so will be far too hot, and yet probably receive insufficient higher frequency light to drive anything like photosynthesis.

Currently, an Earth transit could be seen from 1402 stars, and this includes 128 G-type stars, like our sun. There are 73 K stars, which may also be suitable to house life. There are also 63 F-type stars. These stars are larger than the sun, from 1.07 to 1.4 times the size, and are much hotter than the sun. Accordingly, they turn out more UV, which might be problematical for life, although the smaller ones may be suitable and the Earth-equivalent planet will be a lot further from the star. However, they are also shorter-lived, so the bigger ones may not have had time. About 2/3 of these stars are in a more restricted transit zone, and could, from geometry, observe an Earth transit for ten hours. So there are a number of stars from which we cannot hide. Ten hours would give a dedicated astronomer with the correct equipment plenty of time to work out we have oxygen and an ozone layer, and that means life must be here.

Another option is to record our radio waves. We have been sending them out for about 100 years, and about 75 of our 1402 stars identified above are within that distance that could give visual confirmation via observing a transit. We cannot hide. However, that does not mean any of those stars could do anything about it. Even if planets around them have life, that does not mean it is technological, and even if it were, that does not mean they can travel through interstellar space. After all, we cannot. Nevertheless, it is an interesting matter to speculate about.

Why Plate Tectonics?

How did plate tectonics start? Why has Earth got them and none of the rocky planets have, at least as far as we know? In my ebook “Planetary Formation and Biogenesis” my explanation as to one of the reasons for why plate tectonics are absent on Mars is that the Martian basaltic mantle appears to have about 17% iron oxide whle Earth has 7 – 11%. This means it cannot make eclogite whereas Earth’s basalt can. Eclogite is a particularly dense silicate and it is only made under serious pressure. 

To see the significance, we have to ask ourselves how plate tectonics works. The core generates hot spots, and hotter mantle material rises and has to push aside other rock, and we get what we call seafloor spreading, although it does not have to be underwater. The African rift valley is an example, in this case a relatively new example where the African plate is dividing, and eventually will have sea between Somalia and the Nubian zone. Similarly, the Icelandic volcanoes are due to “seafloor spreading”. Thus matter coming up pushes the surface plates aside, but then what? On Mars, the cold basalt has nowhere to go so it forms what is called a “stagnant lid”, and heat can only escape through volcanism. On Mars, this resulted in quite significant volcanism about three and a half billion years ago, then this more or less stopped, although not as much as some think because there is evidence of volcanic eruptions around Elysium within the last two million years. The net result is the “lid” gradually gets thicker, and stronger, which means the heat loss of the Martian mantle is actually much less than that of Earth.

On Earth, what happens is that as the basaltic plates get pushed aside, one goes under another, and this is where then eclogite becomes relevant. As the plate goes down, the increased pressure causes the basalt to form eclogite, and because it is denser than its surroundings, gravity makes it go deeper. It is this pull subduction that keeps plate tectonics going.

So, what about Venus? The usual answer is that Venus had a stagnant lid, but at certain intervals the internal heat is so great there is a general overturn and there is a general resurfacing. However, maybe that is not exactly correct. Our problem with Venus is we cannot see the surface thanks to the clouds. The best we can manage is through radar, and recent (June, 2021) information has provided some surprises (Byrne, et al.,   https://doi.org/10.1073/pnas.2025919118).  Basically, what was found was evidence that many of the lowlands had broken into crustal blocks and these blocks are moving relative to each other, in the same way as pack ice moves. The cause would be mantle convection that stresses the crust. The Venusian crust has many landforms, including thin belts where crust has been pushed together to form ridges, or pulled apart to form troughs. However, these ones tend to encompass low-lying regions that are not deformed, but rather appear to be individual blocks that shift, rotate and slide past each other. The authors suggest this what Earth was like before plate tectonics got going.

As to why they started here and not there has no obvious answer. The fact that Earth rotates far more quickly will generate much stronger Coriolis forces. It may be that the absence of water on Venus removes a potential lubricant, but that seems unlikely if blocks of crust are moving. My personal view is that one key point is it needs something to force the crust downwards. Eclogite may pull it down, but something has to push the basalt down to force it to make eclogite. My guess here is that Earth has one thing the other rocky planets do not have: granitic continents. Granite floats on basalt, so if a basaltic mass was pushed against a significant granitic mass, the granite would slide over the top and its weight would push the basalt down. When it made eclogite, the denser basalt would continue its downward motion, pulling a plate with it. Is that right? Who knows, but at least it looks plausible to me.

Where to Look for Alien Life?

One intriguing question is what is the probability of life elsewhere in the Universe? In my ebook, “Planetary Formation and Biogenesis” I argue that if you need the sort of chemistry I outline to form the appropriate precursors, then to get the appropriate planet in the habitable zone your best bet is to have a G-type or heavy K-type star. Our sun is a G-type. While that eliminates most stars such as red dwarfs, there are still plenty of possible candidates and on that criterion alone the universe should be full of life, albeit possibly well spread out, and there may be other issues. Thus, of the close stars to Earth, Alpha Centauri has two of the right stars, but being a double star, we don’t know whether it might have spat out its planets when it was getting rid of giants, as the two stars come as close as Saturn is to our sun. Epsilon Eridani and Tau Ceti are K-type, but it is not known whether the first has rocky planets, and further it is only about 900 million years old so any life would be extremely primitive. Tau Ceti has claims to about 8 planets, but only four have been confirmed, and for two of these, one gets about 1.7 times Earth’s light (Venus get about 1.9 times as much) while the other gets about 29%. They are also “super Earths”. Interestingly, if you apply the relationship I had in my ebook, the planet that gets the most light, is the more likely to be similar geologically to Earth (apart from its size) and is far more likely than Venus to have accreted plenty of water, so just maybe it is possible.

So where do we look for suitable planets? Very specifically how probable are rocky planets? One approach to address this came from Nibauer et al. (Astrophysical Journal, 906: 116, 2021). What they did was to look at the element concentration of stars and picked on 5 elements for which he had data. He then focused on the so-called refractory elements, i.e., those that make rocks, and by means of statistics he separated the stars into two groups: the “regular” stars, which have the proportion of refractory elements expected from the nebular clouds, or a “depleted” category, where the concentrations are less than expected. Our sun is in the “depleted” category, and oddly enough, only between 10 – 30% are “regular”. The concept here is the stars are depleted because these elements have been taken away to make rocky planets. Of course, there may be questions about the actual analysis of the data and the model, but if the data holds up, this might be indicative that rocky planets can form, at least around single stars. 

One of the puzzles of planetary formation is exemplified by Tau Ceti. The planet is actually rather short of the heavy elements that make up planets, yet it has so many planets that are so much bigger than Earth. How can this be? My answer in my ebook is that there are three stages of the accretion disk: the first when the star is busily accreting and there are huge inflows of matter; the second a transition when supply of matter declines, and a third period when stellar accretion slows by about four orders of magnitude. At the end of this third period, the star creates huge solar winds that clear out the accretion disk of gas and dust. However, in this third stage, planets continue accreting. This third stage can last from less than 1 million years to up to maybe forty. So, planets starting the same way will end up in a variety of sizes depending on how long the star takes to remove accretable material. The evidence is that our sun spat out its accretion disk very early, so we have smaller than average planets.

So, would the regular stars not have planets? No. If they formed giants, there would be no real selective depletion of specific elements, and a general depletion would register as the star not having as many in the first place. The amount of elements heavier than helium is called metallicity by astronomers, and this can vary by a factor of at least 40, and probably more. There may even be some first-generation stars out there with no heavy elements. It would be possible for a star to have giant planets but show no significant depletion of refractory elements. So while Nibauer’s analysis is interesting, and even encouraging, it does not really eliminate more than a minority of the stars. If you are on a voyage of discovery, it remains something of a guess which stars are of particular interest.

Why We Cannot Get Evidence of Alien Life Yet

We have a curiosity about whether there is life on exoplanets, but how could we tell? Obviously, we have to know that the planet is there, then we have to know something about it. We have discovered the presence of a number of planets through the Doppler effect, in which the star wobbles a bit due to the gravitational force from the planet. The problem, of course, is that all we see is the star, and that tells us nothing other than the mass of the planet and its distance from the star. A shade more is found from observing an eclipse, because we see the size of the star, and in principle we get clues as to what is in an atmosphere, although in practice that information is extremely limited.

If you wish to find evidence of life, you have to first be able to see the planet that is in the habitable zone, and presumably has Earth-like characteristics. Thus the chances of finding evidence of life on a gas giant are negligible because if there were such life it would be totally unlike anything we know. So what are the difficulties? If we have a star with the same mass as our sun, the planet should be approximately 1 AU from the star. Now, take the Alpha Centauri system, the nearest stars, and about 1.3 parsec, or about 4.24 light years. To see something 1 AU away from the star requires an angular separation of about one arc-second, which is achievable with an 8 meter telescope. (For a star x times away, the required angular resolution becomes 1/x arc-seconds, which requires a correspondingly larger telescope. Accordingly, we need close stars.) However, no planets are known around Alpha Centauri A or B, although there are two around Proxima Centauri. Radial velocity studies show there is no habitable planet around A greater than about 53 earth-masses, or about 8.4 earth-masses around B. However, that does not mean no habitable planet because planets at these limits are almost certainly too big to hold life. Their absence, with that method of detection, actually improves the possibility of a habitable planet.

The first requirement for observing whether is life would seem to be that we actually directly observe the planet. Some planets have been directly observed but they are usually super-Jupiters on wide orbits (greater than10 AU) that, being very young, have temperatures greater than 1000 degrees C. The problem of an Earth-like planet is it is too dim in the visible. The peak emission intensity occurs in the mid-infrared for temperate planets, but there are further difficulties. One is the background is higher in the infrared, and another is that as you look at longer wavelengths there is a 2 – 5 times coarser spatial resolution due to the diffraction limit scaling. Apparently the best telescopes now have the resolution to detect planets around roughly the ten nearest stars. Having the sensitivity is another question.

Anyway, this has been attempted, and a candidate for an exoplanet around A has been claimed (Nature Communications, 2021, 12:922 ) at about 1.1 AU from the star. It is claimed to be within 7 times Earth’s size, but this is based on relative light intensity. Coupled with that is the possibility that this may not even be a planet at all. Essentially, more work is required.

Notwithstanding the uncertainty, it appears we are coming closer to being able to directly image rocky planets around the very closest stars. Other possible stars include Epsilon Eridani, Epsilon Indi, and Tau Ceti. But even then, if we see them, because it is at the limit of technology, we will still have no evidence one way or the other relating to life. However, it is a start to look where at least the right sized planet is known to exist. My personal preference is Epsilon Eridani. The reason is, it is a rather young star, and if there are planets there, they will be roughly as old as Earth and Mars were when life started on Earth and the great river flows occurred on Mars. Infrared signals from such atmospheres would tell us what comprised the atmospheres. My prediction is reduced, with a good amount of methane, and ammonia dissolved in water. The reason is these are the gases that could be formed through the original accretion, with no requirements for a bombardment by chondrites or comets, which seemingly, based on other evidence, did not happen here. Older planets will have more oxidized atmospheres that do not give clues, apart possibly if there are signals from ozone. Ozone implies oxygen, and that suggests plants.What should we aim to detect? The overall signal should indicate the temperature if we can resolve it. Water gives a good signal in the infrared, and seeing signals of water vapour in the atmosphere would show that that key material is present. For a young planet, methane and ammonia give good signals, although resolution may be difficult and ammonia will mainly be in water. The problems are obvious: getting sufficient signal intensity, subtracting out background noise from around the planet while realizing the planet will block background, actually resolving lines, and finally, correcting for other factors such as the Doppler effect so the lines can be properly interpreted. Remember phosphine on Venus? Errors are easy to make.

Exoplanets: Do They Have Life?

One question that NASA seeks to answer is, is there life somewhere else? That raises the question, how can you tell? The simplest answer is, find something that only life can make. The problem with that is that life uses chemistry, and chemistry occurs anyway, so it is sometimes possible that what you find might be due to life, or it might be due to geophysical or geochemical action. Another problem is, some of the molecules that life makes more readily than simple geology does, say, may be difficult to find. When looking for minor traces, it is possible to find “signals” but misinterpret them. In an earlier post, I suggested the so-called signals for phosphine on Venus fell into that class, and what I have seen since reinforces that view. Many now think it was one of the signals from sulphur dioxide, which is known to be there.

One of the strongest indications we could find would be to find a number of homochiral chemicals. Thus when sugars are made chemically, say by condensing formaldehyde, they are either in the D or L configuration. Chirality can be thought of as “handedness”; your left hand is different from your right hand, and the same thing happens for chemicals used by life – life only makes one sort, the reason being that reproduction from nucleic acids only work if they can make a double helix, and that only works if there is a constant pitch, which in turn requires the linking group, the ribose, to be in one form – left or right handed. Amino acids are similar because enzymes only work in specific configurations, as do many of the other properties of proteins. The problem with chirality as evidence of life is that it is hard to measure. The usual method is to isolate the compound in a pure form in solution, pass polarised light through it, and measure the rotation of the polarization. But that really needs a chemist on the spot. Remote sensing is not really suitable. Forget that for exoplanets.

One approach has been to find a gas in the atmosphere typical of life. If you found an atmosphere with as much oxygen as Earth’s, it would almost certainly have life because oxygen cannot be accreted directly by a planet in the habitable zone. The bulk of Earth’s oxygen has probably come from photosynthesis, or the photolysis of water. The latter occurs in the absence of life, but when it does in the atmosphere, it forms ozone, which stops the reaction because water will be below the ozone. On Mars, some water has been photolysed on the surface, but  it formed peroxides or superoxides with iron oxide, or perchlorates with chlorides. So a lot of oxygen is indicative. Another gas is methane. Methane is given of by anaerobic bacteria, but it is also made geologically by reacting carbon compounds, such as the dioxide, with water and ferrous ions, which are common in the olivine-type minerals, which in turn are very common. Almost any basalt will react, in time. So methane is ambiguous.

Perhaps, we should look for more complicated molecules. There are still traps.  Recent work has shown that the chemicals that are part of the Krebs cycle, which is rather fundamental to life, actually can be made from carbon dioxide, iron, and some metal ions such as zinc. Even these are not characteristic of life, although the work may give further clues as to how life got underway, and why the chemicals used in the Krebs cycle “got involved”.

When NASA sent its Viking rovers to Mars, their approach was to treat soil samples with water and nutrients that microbes could metabolise, and then they looked to see if there were any products. One experiment detected radio-labelled gases from samples treated with carbon-14-labelled nutrients, and the idea was if the 14C got into the gas phase, where its radioactivity could be detected, it would mean life. Maybe not. If the nutrients landed on a superoxide, they would have been converted to gas. It is not easy doing this remotely.The one difference that characterises Earth when seen from space is its colour. However, the blue merely means oceans. It is possible that planets with oceans will also have what is required for life, but we could not guarantee that. If we recognised spectral signals from chlorophyll, that would be a strong indication, but whether such signals can be observed, even if there are plants there, is unclear. Again, this is not easy.

No Phosphine on Venus

Some time previouslyI wrote a blog post suggesting the excitement over the announcement that phosphine had been discovered in the atmosphere of Venus (https://ianmillerblog.wordpress.com/2020/09/23/phosphine-on-venus/) I outlined a number of reasons why I found it difficult to believe it. Well, now we find in a paper submitted to Astronomy and Astrophysics (https://arxiv.org/pdf/2010.09761.pdf) we find the conclusion that the 12th-order polynomial fit to the spectral passband utilised in the published study leads to spurious results. The authors concluded the published 267-GHz ALMA data provide no statistical evidence for phosphine in the atmosphere of Venus.

It will be interesting to see if this denial gets the same press coverage as “There’s maybe life on Venus” did. Anyway, you heard it here, and more to the point, I hope I have showed why it is important when very unexpected results come out that they are carefully examined.