A Ball on Mars

In New Zealand we are approaching what the journalists say is “The Silly Season”, the reason being that what with Christmas and New Year, and with it being in the middle of summer, a lot of journalists take holidays, and the media, with a skeleton staff, have to find almost anything to fill in the spaces that the media makes available. So, in the spirit of getting off to an early start, I noticed an image from Mars that looks as if someone left a cannon ball lying around. (The image is easily found on the web, but details are not, so I am not sure where it was found.) So what is it?

Mars_Ball

Needless to say there were some loopy suggestions from “the fringe”, but while it is easy to scoff, it is not so easy to try to guess what it is. The idea of a cannon ball and nothing else borders on the totally bizarre. So what can we see from the image? The remarkable point about this object is it seems to be lying on the surface, which suggest it did not strike it, as otherwise there would be indentations, or, if it were a meteorite, there would be a crater. There clearly isn’t. Equally, however, it looks smooth, which suggests it has been fused, which means it did not arise there. Some have suggested it is a haematite spherule, but that, to me is not that likely, in part because it is so large (the so-called “blueberries” were quite small) and also because there seems to be only one of it, while what created the “blueberries” created a lot of them. In my opinion, it is probably an iron meteorite, and the reason there is no impact crater is that it landed somewhere else, and rolled to this spot.

So maybe time to get a little more serious, and think about iron meteorites. What can we say about them? The Curiosity rover has also found “Egg rock”, which is an iron meteorite about the size of a golf ball. The Rover found iron, nickel and phosphorus as significant constituents, and the phosphorus is present as iron phosphide. There are two important issues here: how did the iron/nickel ball form separately from everything else, and equally important, how did iron phosphide form? That last question may need explanation, because phosphorus does not normally occur as a phosphide, and phosphides only form under highly reducing conditions. (Reducing conditions are usually in the presence of hydrogen and or an active metal at higher temperatures. The opposite, oxidising conditions, occurs when there is oxygen or water present, but not enough hydrogen or metal to scavenge the oxygen.)

Iron phosphide is known to occur in certain iron meteorites, but such meteorites can always be attributed to having formed at a little more than 1 A.U. from, or closer to the star. Chondrites that formed further out, such as in the asteroid belt, always have their phosphorus in the form of phosphate, which is a very stable, oxidised, phosphorus compound. The point about 1 A.U. (the distance of Earth from the sun) is that was where the temperatures were hot enough to melt iron, and the phosphide would form by the molten iron reacting with phosphate to form the phosphide and iron oxide.

Now for the reason for going on about this. One of the JPL team explained that iron meteorites originated from the cores of asteroids. The premise here is that during initial accretion, the dust assembled into an asteroid-sized object, the object got sufficiently hot and the iron and nickel melted and sunk to the core. Later, there was a massive collision and the asteroid’s core shattered, and the meteorites we see are the fragments from the shattering. (Note, the same people argue planets formed by asteroid sized bodies, and bigger, colliding and everything stick together. Here is having your cake and eating it in action.) The first question is, why did the rock melt? One possibility is radioactive isotopes, so it is possible, nevertheless for the explanation to work the asteroid had to melt hot enough to melt iron, and to hold those temperatures for long enough for the iron to work its way to the centre through the very viscous silicates in a very weak gravitational field. A further problem is that the phosphate would dissolve in the silicates, in which case it would not form iron phosphide because the iron would not get there. Calcium phosphate has a density of about 3, very similar to many of the silicates, so it might be difficult for iron phosphide to form in such an asteroid. Only a very few asteroids, and Vesta is one, have iron cores, and there are some reasons to believe Vesta formed somewhere else and moved.

The reason for my interest is that in my ebook, “Planetary Formation and Biogenesis” I argue that the first way accretion started was for the dust in the accretion disk to get hot enough to get sticky, or to form something that could later act like a cement. When the temperatures got up to about 1550 degrees Centigrade, iron melts and in the disk would form globules that would grow to a certain degree. Many of these would also find molten silicates to coat them, so the separation occurred through the temperature generated by the accreting star. Provided these could separate themselves from the gas flow (and there is at least a plausible mechanism) then these would become the raw materials for rocky planets to form. That is why (at least in my opinion) Earth, Venus and Mercury have large iron cores, but Mars does not.

That, of course, has got a little away from the “Martian cannonball” but part of forming a scientific theory is to let the mind wander, to check that a number of other aspects of the problem are consistent with the propositions. In my view, the presence of iron phosphide in an iron meteorite is most unlikely to have come from the core of an asteroid that got smashed up. I still like my theory, but then again, I suppose I am biased.

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Origin of life, and a challenge!

Here is a chance to test yourself as a theoretician. But do not worry if you cannot solve this. Most people will not, and I predict nobody will, but prove me wrong! And as a hint, while nobody actually knows the answer, as I shall show eventually, getting a very reasonable answer is actually relatively simple, although you need a little background knowledge for the first question.

Just before Christmas, I posted with the title Biogenesis: how did life get started?” (http://wp.me/p2IwTC-6e ) but as some may have noticed, I did not get very far along the track indicated by the title. The issue is, of course, somewhat complicated, and it is easier to discuss it in small pieces I also mentioned I was about to give a talk on this early this year. Well, the talk will come on March 4, so it is approaching quickly. Accordingly, I have put out an abstract, and am including two challenges, which readers here may or may not wish to contemplate. Specifically,
1. Why did nature choose ribose for nucleic acids?
2. How did homochirality arise?
Put your guesses or inspired knowledgeable comments at the end of this post. The answers are not that difficult, but they are subtle. In my opinion, they are also excellent examples of how to go about forming a theory. I shall post my answers in due course.

The question of, why ribose, is a little complicated and cannot be answered without some chemical knowledge, so most readers probably won’t be able to answer that. Notwithstanding that, it is a very interesting question because I believe it gives a clue as to how life got underway. RNA is a polymer in which each mer is made up of three entities: one of four nucleobases, ribose and a phosphate ester. The nucleobase is attached to C-1 of ribose (if you opened it up, at the aldehyde end) and the phosphate is at C-5 (the other end, ribose being a five carbon sugar. The nucleobases are, in general, easy to make. If you leave ammonium cyanide lying around, they make themselves, but that is the only thing that appears to be easy about this entity. Sugars can be made in solution by having formaldehyde, which is easily made, react in water with lime, and a number of other solids. That seems easy, except that when you do this, you do not get much, if any, ribose. The reason is, ribose is a high-energy pentose (five carbon sugar) because all the hydroxyl groups are eclipsing each other in the closest orientation (axial, for those who know some chemistry). In the laboratory, double helix nucleobases (duplexes) have been made from xylose and arabinose, and in many ways these have superior properties to ribose, but nature chose ribose, so the question is, why? Not only did it do it for RNA, but the unit adenine – ribose – phosphate turns up very frequently.

Adenine combined with ribose is usually called adenosine, and the adenosine phosphate linkage turns up in the energy transfer chemical ATP (adenosine tripolyphosphate), the reduction oxidation catalysts NAD and FAD, where the AD stands for adenosine diphosphate, and in a number of enzyme cofactors, to give solubility in water. Giving solubility in water is an obvious benefit, but putting a sugar unit on the group would also do that. Giving an electric charge would also be of benefit, because it helps keep the entity in the cell, nevertheless there are also other ways of doing that. You may say, well, it had to choose something, but recall, ribose is hard to make, so why was it selected for so many entities?

The phosphate ester also causes something of a problem. In the laboratory, phosphate esters are usually made with highly reactive phosphorus-based chemicals, but life could not have started that way. Another way to form phosphate esters is to heat a phosphate and an alcohol (including the hydroxyl groups on a sugar) to about 180 oC, when water is driven off. Note that if water is around, as in the undersea thermal vents that are often considered to be the source of life, the superheated water converts phosphate esters to phosphate and alcohol groups. Life did not start at the so-called black smokers, although with sophisticated protection mechanisms, it has evolved to tolerate such environments. Another problem with phosphate is that phosphates are insoluble in neutral or alkaline water, and phosphate esters hydrolyse in acidic water.
However, notwithstanding the difficulty with using phosphate, there is no real choice if you want a linking agent with three functions (two used up to join two groups, one to be ionic to enhance water solubility). Boron is rare, and has unusual chemistry, while elements such as arsenic, besides being much less common, do not give bonds with as much strength.

Homochirality is different matter. (Chirality can be though of like handedness. If you have gloves, your left hand has its glove and the right hand its, even though they are identical in features, such as four fingers and a thumb. The handedness comes from the fact you cannot put those fingers and thumb on a hand where the top differs from the bottom without making the right hand different from the left.) The sugars your body uses are D sugars (think of this as right handed) while all your amino acids are L, or left handed. The problem is, when you synthesis any of these through any conceivable route given the nature of the starting materials, which have no chirality, you get an equal mix of D and L. How did nature select one lot and neglect the others?
Put your guesses below! In the meantime, my ebook, “Planetary formation and biogenesis”, which summarizes what we knew up to about 2012, is going to be discounted on Amazon for a short period following March 6. This is to favour those going to my talk, but you too can take advantage. It has a significant scientific content (including an analysis of over 600 scientific papers) so if your scientific knowledge is slight, it may be too difficult.