29 May 2015

Tidal Locking, Flares and Photosynthesis on planets of Red Dwarfs (Geek Alert)

A short article about how the physics of atmospheres may act to prevent planets close to Red Dwarf stars from becoming tidally locked with respect to the star (like the Moon to the Earth or most of the giant planets' satellites with respect to them) (From Physicsworld):
(Link below)

This is all well and good, and I understand, of course, the interest in discovering any circumstances which may make the existence of habitable planets in M-V class (Red Dwarf) systems (which, after all, are something like 80% of all star systems) more likely.
 
However, an even bigger problem for the existence of habitable planets in such systems is the prevalence of powerful, and really quite lethal, stellar flares. You see, the following facts are well established:
 
  • 1. Solar (stellar) flares occur in approximately the same dimensions and power on medium to large size main sequence stars or on small, dwarf type stars. Proportionally, a flare may only be 1% increase in brightness on a G type main sequence star, but a similar flare, on a red dwarf, might briefly increase the star's brightness (and lethal hard radiation output) by 50% or even more.
  • 2. The habitable zone of such small stars is very close; within a few million miles, where proximity to such flares would be almost certainly lethal to any complex life residing on the surface of any planets located in those zones.
  • 3. Class M stars tend to have such flares quite frequently; in fact one group of such stars is referred to as "flare stars" because they occur as frequently as every few days.
The inescapable conclusion is that 80% of the stars, those in the M class, are probably ruled out for habitable worlds because of flares alone, regardless of whether they are rotationally stopped.
 
HOWEVER, one thing is sure. Humanity's investigations into these issues is in its infancy, and we really do not know the answers to these questions yet.

I will leave the issue that the frequency of light from such stars is believed to be insufficient, on pure physics principles, for effective photosynthesis by organisms to one side, although of course that is hardly encouraging either. The authors of the book «Revolutions that Made the Earth» (Lenton & Watson, Oxford 2010) were quite adamant that the photosynthesis reactions simply do not occur in the presence of light with peak radiation in the far red, which is what you have there, and that even tweaking the chemistry would probably not work because the problem is the amount of energy it takes to break a water molecule bond, which is higher than the energy of the red photons at issue. No substitution of alternative chemistry can change that fundamental physical parameter. I also leave to the side the tacit assumption that oxygenating (water-molecule breaking) photosynthesis, which evolved on Earth only the one time (the evolution of cyanobacteria, the common ancestor of ALL terrestrial oxygenic photosynthesizers), is absolutely necessary for the evolution of complex life, although I do believe that case is very, very strong.

28 May 2015

Bird Breath

Here's something every schoolkid should know, but very few people have ever even heard about. Despite centuries of study of bird anatomy, it was only really figured out in 2005! Read about it in Ward and Kirshvink, A New History of Life, which I recommend.

Birds (which are, for all practical purposes, saurischian dinosaurs whose last common ancestor with us was about 200 million plus years back) have a vastly superior respiratory system to that of mammals or other (extinct) dinosaurs. Their tracheas go all the way to where our diaphragm would be, and they have air sacs, which fill with air when they breathe. They then push the air through their lungs and out. So it's all one way flow, far more efficient than our in and out breathing. It was originally an adaptation to the low oxygen world of the early Triassic. It enables them to breathe independently of locomotion (mammals have to coordinate their breathing), use less energy to breathe, and survive on lower oxygen levels.

The homologous bones in saurischian dinosaur fossils prove that they too had this system.

Now, I bet you didn't know THAT.

23 May 2015

Interesting developments in thinking about the History of Life on Earth



I should like to briefly share with you some very interesting points made by Ward and Kirshvink in New History of Life.

First, they take very seriously the idea that life might have originated on Mars and been transported to Earth sometime after the Late Bombardment, roughly -3.8 GA. They explain that one of the most “difficult” processes in the origin of life was the synthesis of ribose (essential to the theory of the origin of life), and consequently of RNA and later DNA. These substances cannot have spontaneously arisen in water. But the Earth of that era was almost certainly entirely covered by oceans, with no land surface at all (there was about four times as much water as at present, and the continental crusts had not yet differentiated). So where did the ribose come from?

Experiments have shown that ribose can self-synthesize in the presence of boron salts, which can arise as a result of chained evaporitic lakes in severe deserts. The best contemporary example is the deserts of the Amargosa watershed (Death Valley). But it is quite plausible that something like this could have occurred in impact crater lakes on Mars of this era. Mars is believed to have had a thicker atmosphere and seas and lakes at that time, but to have been mostly rather severe desert.

Couple this with the fact that over 1 billion tonnes of material has found its way as the result of meteorite strikes to Mars, from Mars to Earth, since that era. It thus becomes quite plausible to suppose that ribose and “RNA world” life could have evolved on Mars, where, as a result of desiccation and UV sterilization it subsequently became extinct. But not before “contaminating” the previously sterile Earth with … life!

Another issue they discuss in some depth is the near-complete abandonment of the former paradigm of uniformitarianism. It is now pretty well established that there were at least two, and possibly as many as four, planetwide “Snowball Earth” glaciations, each of which lasted tens of millions of years, before the eventual emergence of “complex life” in the latest Proterozoic, setting the stage for the “Cambrian explosion.”

As they explain (the reasoning based on chemistry is complex), it is highly likely that the first, and in fact only  time oxygen producing photosynthesis ever evolved (the emergence of cyanobacteria) occurred just prior to, and in fact triggered, the first of these Snowball Earth episodes (water-breaking photosynthesis drew down previously high levels of greenhouse gases methane and CO-2). The emergence of an oxygen rich atmosphere depended on these “catastrophes,” as did the emergence of complex life and probably even of nucleated cells (Eukarya). This is completely contrary to former views that evolution of photosynthesizing life, then nucleated cells, then complex, multicellular life, was all a smooth process of increasing complexification.

Previously, it was always assumed that “Snowballs” could not have happened, because if the Earth ever had surface ice all the way to the equator, because of ice albedo effect, that condition would be permanent ---at least until the ever-brightening sun finally melted the ice, in around 1 GA from now, although by then it would be too late for complex life to evolve, since the melting of the ice would be followed almost immediately by the complete loss of the oceans to space; which is what will happen around that time in any case. If a “Snowball” were to occur in the future, in fact, that is what would happen, because volcanism has declined over time and there would be no effective mechanism for melting the ice. (Indeed, until the 1970s even the fact that, as a typical main sequence star, the Sun has been brightening continuously throughout its life and will continue to do so was scarcely appreciated, and the effects it had on the evolution of habitability on our planet was not thoroughly analyszed or understood. This fact, and its implications, are not well known to most educated people even today).

It is now understood that the ancient Snowballs eventually came to an end because there was nothing to remove CO-2 from the atmosphere, so it built up, from volcanic release, primarily. Indeed, CO-2 levels may well have reached 15% or even higher, before finally reaching a high enough level to increase air temperatures sufficiently to cause the ice to melt. But with such high levels of CO-2, the Earth’s climate shifted suddenly to ultra-Greenhouse, and temperatures reached extreme levels. Only when the deposition of carbonate rock (which reached maxima in these post-Snowball eras) finally brought CO-2 levels down, did the climate stabilize. The whole process was a close call, however. Had the Earth been only a little further from the Sun, it’s believed that the temperature at the poles during Snowball episodes would have been below the freezing point for dry ice. Had that occurred, the CO-2 from the volcanism would have precipitated at the poles, and the Earth would indeed have remained ice-covered until the brightening sun finally melted the ice, some 1 GA from now.

All of this shows that previous uniformitarian models are clearly wrong, and a form of pre-Lyellian catastrophism is actually closer to the truth. It also supports a growing consensus that the complex life of our planet is the result of quite a series of relatively unlikely evolutionary and geo-system events, some of which life barely managed to squeak past, including the origin of RNA (which may have depended on importation of life itself from Mars!), the evolution of oxygen-producing photosynthesis, which happened only one time and in the context of global environmental catastrophe, and the origin of endosymbiotic nucleated cells, which also apparently only happened one time.

Heady stuff.

21 May 2015

Fast Track Rammed through Elitist Senate

I deplore what the old boy's club known as the Senate has done... ramming through Fast Track Approval for the TPP and other trade deals negotiated in secret and without adequate safeguards to ensure that they are compatible with our representative form of government. This is a very, very bad development for the interests of ordinary working people, and a very good one for the interests of the largest corporations and richest Americans. Why Democratic senators or representatives or this president, think this is going to be acceptable to rank and file members of the Democratic Party, I cannot say. Because it is not acceptable to me and I will not be voting for any of them when they present for re-election, nor for any candidate seeking the presidential nomination who supports this travesty of democracy.

20 May 2015

Reid opposing Fast Track Authority

I'm glad to see Harry Reid standing against the very unwise cession of Congressional authority over trade treaties (no matter what they're called, Rand Paul is right (for once) that these are actually treaties and should be subject to Constitutional constraints on treaty powers). On this issue, as I've been saying, I'm convinced that President Obama is just wrong, and I note that he has completely failed to make a substantive and coherent case for his position to the American people.

See this 

24 April 2015

Countering Intentional Right Wing Disinformation on Social Security, Medicare, and the Affordable Care Act

If you ever believed, or still believe, the Oligarchic Propaganda that raising the retirement age for Medicare and/or Social Security; and/or cutting those benefits, is fiscally necessary, or even sensible; OR if you have been convinced, against all evidence, that the Affordable Care Act is costing more than expected, or has caused health insurance rates to rise (it's caused the rate of increase to go down); please read Krugman's piece today in the New York Times: "Zombies of 2016".

Comcast Time Warner Cable Merger Dead!

So the NYT is reporting the Comcast Time Warner Cable merger is dead, dead, dead (Comcast having pulled out in anticipation of regulatory disapproval). To which I sing: ♫ ♪  Ding Dong! The Witch is Dead! ♫ ♪ The Witch is Dead! ♫ The Witch is Dead!

Only natural for Jeb to distance himself from James Baker

Of course Wannabe Bush III is "distancing himself" from former Sec. of State James Baker. Baker dared to criticize Netanyahu at a J Street event recently. He, along with Brent Scowcroft and Lawrence Wilkerson, are just about the only sane Republicans still in existence in public life.
  
And, as sane Americans, all three of these recognize that the views of the current Right Wing government of Israel do not necessarily coincide with the interests of the American people. And that is putting it mildly indeed.

23 April 2015

Earth Day 2015

On Earth Day, perhaps everyone ought to reflect on the emerging reality that our world, now 4.6 billion years old, more than 1/3 the age of the universe, and about 550 million years into the epoch of Complex life, is something REALLY rare in the Universe. Star Wars and Star Trek are nothing like reality, which is that the conditions that make for a world like Earth, with its robust and incredibly dynamic world-ecosystem, require a host of relatively unlikely conditions. In combination, the likelihood of a truly Earthlike world becomes very small; so small Earth may be one of only a handful of such planets in our entire Galaxy of 100 billion or more stars. If these facts don't impel us to develop a sense of responsibility for the stewardship of our precious planet, I don't know what could.
(Posted on Earthday to Facebook)

22 April 2015

Carly Fiorina, a Joke



Carly Fiorina

Unkind comment alert. This picture of Carly Fiorina -- who announced she's a candidate for president today, not content with running HP into the ground and losing to another Corporate CEO type in the Republican Primary for California governor (at least in Nixon's case it was the general election) --  brings three phrases to mind.

1.  Delusions of Grandeur
2.  Someone's idea of a token woman in the GOP field
3.  Reanimated skull

OK, that last one was pretty mean. 

20 April 2015

No Supercivilizations in 100,000 nearest galaxies?

Link

I am of the view that the apparent absence of "supercivilizations" within a pretty great distance from the Milky Way says virtually nothing about the possibility that spacefaring civilizations, which may have reached a steady state or slow growth but remain essentially undetectable at great distance, could well exist in some numbers. This is one more nail in the coffin, though, for Carl Sagan's prediction that millions of civilizations exist out there in our Galaxy, with its 100-300 billion stars. It's now pretty clear that intelligent life is at best rare.

19 April 2015

David Lauter's Lazy, Lazy Political Analysis in L. A. Times

I was more than a little dismayed to read this sentence in an article in the Los Angeles Times: "Who wins [in 2016] will almost certainly depend on which proves more powerful -- the hunger for change or the inexorable demgraphic wave." The thesis of the article is that changes in demographics favor the Democrats in the presidential election, but that some undefined "desire for change" favors the Republicans, as if the nature and direction of that change, in terms of actual policy that affects real peoples' lives, made no difference. The entire thesis is unsupported by any reference to specific evidence, in the form of polling designed to isolate voter attitudes towards actual Right and Left policy change, and it's frankly insulting to the intelligence of readers and voters, with its implicit presumption that people unthinkingly slaver for "change, any change. " Detailed polling published over the last several years in fact shows is that the majority of Americans want very specific changes, which, as it turns out, have not been put in place during the present Democratic administration largely because of obstruction by Republicans in Congress. Perhaps it is actually true that the majority of people are unthinking reactionaries, literally, and will just vote to "throw the bums" out without a care for who or what replaces them, but if that is Mr. Lauter's conclusion, the polling he cites referencing "tolerance" and "preference for a strong leader" is not convincing.

17 April 2015

The Fix is in on TPP

Pretty obvious the FIX is IN. Congress is in the process of waiving its Constitutional authority to regulate international trade ... again... and signaling that the very pro-Corporate, undemocratic and anti-working people Trans Pacific Partnership is likely to be rammed down our throats, after having been negotiated in secret and quite deliberately kept away from public scrutiny. I am quite certain that if the majority of the public knew what was in this agreement, they'd say hell no! and want the SCALP of their Congress member voting for fast tracking this travesty.

This is one issue where I vehemently disagree with the Obama administration. Obama voted against CAFTA in 2006, but he's drunk the kool aid on corporatist trade deals. 

15 April 2015

The principle of Mediocrity and the rarity of intelligent life

The "Principle of Mediocrity", which suggests that the default assumption should be that conditions here on Earth are broadly typical of what you would find anywhere in the universe, is being challenged to its core. Current thinking, based on reading of the evidence from Earth history, seems to indicate that, to the contrary, quite a series of each in itself rather difficult and unlikely transformations had to have occurred for the current planetary environment to have emerged as a stable state. But the broader principle, that the laws of physics and the history of matter and energy, (including the formation and evolution of galaxies), in its broadest view, is much the same throughout the universe (including the great majority of it that is beyond the horizon where light could ever reach us from there).... remains. So, if complex life, including intelligent life, even if the product of a series of necessary but unlikely developments, the combination of which becomes increasingly unlikely, it remains a fact that the sample size is so enormous that we can scarcely imagine it. Rather than giving support to the Religion minded who are biased in favor of finding that life is the unique creation of their Crazy Sky-War God, all it really does is make it likely that complex biospheres like Earth's, capable of giving rise to intelligent beings like ourselves, appear likely to be quite rare in the universe. But rare does not mean unique, necessarily, and from what I understand the combined probabilities amount to "maybe unique in the Galaxy," but not "probably unique in the universe."

Apologies to those who hold such religious views. I am sometimes irremediably snarky in my secularism.

13 April 2015

No to Democratic Party Clinton Juggernaut

Have to say I strongly object to the way the DSCC, the House Majority PAC, the DNC, and other supposedly non-candidate related Democratic organizations all sent out e-mails urging Democrats to "stand with Hillary," etc. I DON'T stand with Hillary. I want there to be a primary process, and for real Progressives, including on foreign policy, to step up and present their alternative agendas. We do not have anointments of heirs apparent from ruling dynasties in this country.  
 
I have requested to be unsubscribed from every one of these, and will support the Sanders campaign, ActBLUE and the Progressive Campaign Committee instead.

11 April 2015

Sunshades in Solar Orbit and other seeming pie in the sky ideas to mitigate Climate Change

It seems pretty obvious to me that we are not going to be able to prevent enough of the Climate Change taking place as a result of the near doubling of the CO-2 in our atmosphere since the start of the Industrial Age (or Antrhopocene, if you prefer). The process is too far along, and there is no way human civilization is going to immediately shut down all fossil carbon use.  
  
So, we will have to take various global-scale macro-engineering steps, sooner or later. This idea, of building sun shades at the Solar LaGrange point (see link below) is one that will probably engender a reaction of (almost literally) "pie in the sky" from most people, but, apparently, space engineering folks have looked at it pretty carefully and it actually is likely to be feasible.

http://en.wikipedia.org/wiki/Space_sunshade 
  
One issue this kind of macroengineering, direct redress of solar heating, does not deal with is the acidification of the oceans. So it would have to be coupled with other measures, especially, the rapid move away from fossil carbon fuels, in order to be effective. We all know our long addiction to fossil carbon simply has to come to an end, and the sooner the better. We can't just keep on spending the millions of years long sequestration of carbon that our planet has accomplished (for good reason), without dire consequences. 
  
But combined measures likely actually could stop major further warming of the Earth, and, gradually, at least, the oceans could be protected. (We will also have to stop expecting them to supply unlimited amounts of wild fish; that era, too, is all but over). And we humans need to get busy working on, and funding the work on, all plausible efforts. 
  
One of which may be helped by this recent discovery: possibly far superior method of producing hydrogen for engine fuels (hydrogen can also be used as a jet fuel).  Of course the combustion of hydrogen just yields water. And there is no danger of using up all the oxygen; even if all the fossil carbon on Earth were burned, it would have less than a few tenths of a percent effect on oxygen content of the atmosphere, and even that would be temporary; the Earth's oxygen surplus is built into its bioregulatory systems very robustly). Using hydrogen as a manufactured fuel, as long as fossil carbon isn't involved in making it, is climate-friendly. It's also the case that oxygenic photosynthesis converts water into oxygen, so artificial processes that directly convert hydrogen and oxygen into water are fairly readily offset by the natural systems of the Earth.

https://richarddawkins.net/2015/04/new-discovery-may-be-breakthrough-for-hydrogen-cars/

Another possible temperature mitigation is aerosolization of sea water by ocean going vessels, which could greatly increase, well, fog, over the ocean surface. This is already a major regulatory mechanism by which the Global Feedback System (Gaia) keeps the Earth very much cooler (about 40° C cooler) than it would be in the absence of life: much of the surface mist over the ocean results from bacteria in the water. Anyway, the technology to do this is already in development. But this, too, does nothing about the encroaching acidification of the oceans, which cannot be ignored. 
  
Some people reflexively balk at "geoengineering," but I think we absolutely must consider all options. We have done a LOT of geoengineering since we started burning coal, oil, and natural gas on a massive scale, and it promises to wreck our world as cozy and comfortable habitat for humanity. Either we get smart, and think these things through carefully, control for all effects, and proceed cautiously to do what has to be done, or we fail, and billions will likely die in the catastrophe that ensues. That's the choice. Reflexive rejection of "geoengineering" is not an option.

08 April 2015

Lenton & Watson: predictions regarding life elsewhere


I found the following, slightly edited excerpt from Lenton & Watson's Revolutions that Shaped the Earth (Oxford, 2010), so interesting that I post it despite its length in hope that some few others might also find it compelling. I added a couple of notes in italics inside square brackets.


6.7 Predictions regarding life elsewhere

In this section we make some predictions from the assumptions of the anthropic model, concerning unknowns about life elsewhere. These are interesting in themselves, and since they are open to testing in the future as our knowledge increases, they could help establish or disprove the anthropic view of Earth history which we have been exploring.

So let us now review those fundamental assumptions and what they imply. The main one is that the pace of evolution on Earth, to ourselves, as complex, intelligent observers, has been constrained by the necessity to pass through a small number of intrinsically very unlikely events. These are sufficiently improbable that a priori, they would not have been expected to all occur during the limited time that the Earth will be inhabitable. However, on Earth, by lucky chance, they occurred considerably more quickly than was to be expected, which is how we came to be here and are able to ask these questions.

This idea has implications for how much life, of what kind, and around what kind of stars, we ought to expect to find in our part of the universe. These implications have some hope of being tested, because over the next few decades, astronomers and the space agencies will be putting huge effort into discovering and characterizing extra-solar planets, using telescopes both at the Earth's surface and in space. Ultimately, the goal will be to spectroscopically analyze the atmospheres of any planets that are found, looking for the raw materials we know to be necessary for life, such as water and carbon dioxide. But the investigators will also be hoping to detect ‘bio-signatures’ — in particular, ozone. This is thought to be diagnostic for oxygen, which itself is difficult to detect in a planetary atmospheres, since it does not exhibit visible or infrared absorption lines. Ozone however, is comparatively easy to detect and is expected to be present only when there is also significant oxygen.

The planet-finding missions of this century will build on ideas going back to the middle of the last one. Jim Lovelock was the first to suggest that analysis of planetary atmospheres could be used to diagnose the presence of life, an idea he developed with the philosopher Dian Hitchcock. They pointed out that the best bio-signature is not just one gas, but the presence of two that are in strong chemical disequilibrium with one another. They suggested that if you trained an infrared telescope on the Earth, you would be able to detect the simultaneous presence of ozone (hence oxygen) and methane in the atmosphere. Since methane and oxygen react with one another rapidly as carbon dioxide and water, you would be able to deduce that something must be producing them from their reaction products with equal rapidity, and this something must be life. (Actually, it's not just life, but [oxygenating] photosynthesis, that you would have diagnosed from the measurement. Some twenty-five years later, in one of his last papers, Carl Sagan and colleagues demonstrated that the technique works for the Earth, using the instruments on the Galileo spacecraft looking back to Earth while on its way to Jupiter.

One way or another, in the first half of the twenty-first century, we are going to get lots of evidence that bears on the habitability of nearby solar systems. There are some 250 star systems within thirty light years of Earth. Let's be optimistic and assume that we find many of them have planets in their habitable zones. What does the anthropic theory suggest we should find when we examine them with these planet-finder missions?

We suspect prokaryote life is not so common as to always arise on a planet within a habitable zone, but it involves at most one really difficult step (you'll recall, we can't really be sure it is critically difficult), and we consider that there is a reasonable chance that it will have arisen on some of the systems we will be able to observe. Pre-oxygenic photosynthesis is not apparently a critical step, and it arose relatively quickly on Earth after prokaryotes were established. This would ensure an energetic biosphere to exist, with a characteristic atmosphere full of exotic trace gases such as hydrogen sulfide and methane as well as carbon dioxide and water. We have some hope, therefore, of being able to detect a few planets that have this kind of biosphere, during the next few decades.

But oxygenic photosynthesis was a difficult step — it may be genuinely critical in the sense of the model, in which case it would occur on, at most, one in ten of the planets that took the first step, and more probably many fewer. In a sample of a few hundred planets we would be lucky to observe any that took this second step. We conclude that most likely we will not find any evidence for abundant oxygen on any of these target planetary systems.

There are some interesting predictions that also come from the anthropic model. For example, if complex life is rare, it is likely that Earth will be found to be one of the most favorable possible spots for it to have evolved, a cosmic Garden of Eden. This leads to an interesting question: Is there anything about our Solar System that marks it out as unusual compared to most others, and might make it particularly conducive to hosting complex life? Of course it has Earth, ideally situated in the habitable zone, and it may be that few other solar systems have such planets — but we don't know as yet what the distribution of planetary systems is, so we must put that aside for the moment. Is there anything else unusual about the Solar System?

As a matter of fact, there is: the Sun  is an unusually bright star, brighter than more than 90% of its neighbors. [A histogram of the 250 local stars within 10 parsecs (about thirty-three light years) of the Sun shows that]: the Sun is well out on the bright limb of the distribution.  This is a quite strong indication that most of the stars in our neighborhood, which are cool and dim type-M red dwarfs, with on average only about half the mass of the Sun, are really not as suitable for hosting complex life as our bright yellow type-G star. As this is all the more surprising because type M stars do have one factor that ought to make them more hospitable for complex life, and that is their long life span. Smaller stars burn their nuclear fuel more slowly, and the lifetime on the main sequence of type-M stars is typically (more than) twice as long as the Sun’s. According to our thinking and the evidence of our own planet, the shortness of the habitable period is a major factor limiting the chances that complex life develops. And yet we have awoken to find ourselves orbiting a bright, showy, short-lived firework of a type-G star, rather than a long-burning but dim and dull type-M.

We predict, therefore, that there is another factor or factors which make it difficult for life to develop to complexity around fainter stars, and explains why we don't find ourselves orbiting one. In fact there are several possible disadvantages to living around a type M star, but it's not clear as yet that any of them are so severe that they can tip the balance against them as good nurseries for life. Astrobiologists have recently been looking harder at type-M stars, as possible targets for SETI searches, for example, and the discussion below owes much to some recent papers from a symposium on the subject.

The habitable zone of a red dwarf is much closer to the parent star than is the Sun's — stars rapidly becoming much less bright as we go towards smaller size, and a star half the mass of the Sun emits only a few percent of its energy. A habitable planet around a M star would therefore have to huddle close to it for warmth, orbiting closer than Mercury to the Sun. Here, it will become tidally locked; its rotation slowed by internal energy dissipation until it equals the orbital, and the same face is always to the star. (The Moon of course is tidally locked to the earth, the reason why we always see its same face). Tidal locking will mean that there will be permanent huge temperature extremes on the planet, between the boiling daytime and the freezing, nighttime faces. If the atmosphere and ocean are not very efficient at transporting heat, this will mean that only a narrow strip around the terminator will actually be inhabitable, and we would expect that all the water on the planet would end up frozen out on the cold side by a 'cold trap' effect. However, with a sufficiently thick and mobile atmosphere, this fate could be avoided. [Note: it may not, according to some calculations, necessarily be the case that planets orbiting Class M dwarfs at the habitable zone distance will be tidally locked, but their rotational periods will be longer compared to their periods of revolution than is the case with Earth, possibly even longer than their 'year'].

Another problem may simply be that small stars tend to have small planets. The mass of the central star will certainly be related to the mass of the nebula that accretes around it, hence to the size of any planets that eventually form. As we have seen, it is critically important that it planet is big enough to hold onto a sufficiently thick atmosphere, and also to generating internal geothermal he to power plate tectonics (tidal dissipation would help there by adding a source of heat to the interior). We don't know enough about the planetary formation process to make very firm predictions, however. Current ideas tend to favor a picture of planetary formation as quite stochastic in nature, so while we certainly would expect a tendency for smaller stars to have smaller planets, perhaps there is nothing against a star half the size of the Sun having a rocky planet as big as the Earth.

There is a third problem for life around a faint star, as first pointed out by Ray Wolstencroft and John Raven. This one seems to be a really serious barrier to the development of complex life. Red dwarfs are the color they are because they are cooler than the Sun, typically only reaching about half its surface temperature. This means that the photons they emit are lower in energy. Chlorophyll makes full use of the high-energy photons emitted by the sun, absorbing strongly in the high-energy blue region of the spectrum, (as well as in the red, leaving the central, green wavelengths reflected — hence its characteristic color). But as we've discussed, for photosynthesis to split water, a high-voltage must be generated by the photo systems in plants, and even on Earth. This has required that two photo systems be coupled together. Under a much cooler star, very few high-energy photons would be available, and it is likely that three or even four such systems would have to be coupled together to accomplish the task of splitting water. But the evidence is that it was no simple task for evolution to arrive at water splitting photosynthesis even on Earth, so it may be much more difficult still to accomplish this under the light of a cooler star. What this might mean is that though type-M biospheres may evolve photosynthesis, they would find it nearly impossible to evolve the water-splitting variety, and, as we'll discuss in future chapters, without oxygen, there is very unlikely to be animals, let alone intelligent animals.

[At this point the authors might well have also mentioned the additional issue that red dwarf stars typically emit powerful flares, which, with the otherwise potentially habitable planets being as close as they are, could be highly disruptive to any life that might exist on the surface of those planets].

6.8  Summary

Our speculations in this chapter lead us to predict that simple (prokaryote) life might be moderately abundant in the universe at large — or possibly not, depending on just how difficult evolution to this first critical step [turns out to be]. Whether prokaryotes are rare or common, however, complex life will be rare. This idea has been named the Rare Earth Hypothesis, since it was put forward in a book of that name by Peter Ward and Donald Brownlee. Our analysis agrees very much with their thesis, though we find their arguments frustratingly qualitative. Ward and Brownlee argue their case based on the fortunate position of the Earth, our possession of a good-size moon, a friendly big brother planet in the shape of Jupiter, etc. The difficulty with their argument is precisely the 'self-selection bias' problem that we've tried to tackle in this and the previous chapter. We see that the earth has these attributes and (perhaps) that they have contributed to the evolution of complex life on her, but with only a single example of an inhabited planet, we don't see how to decide which of these properties are really necessary for us to be here, and which are not.

Maybe other solar systems have even more favorable circumstances? Anyway, we are subscribers to the Rare Earth Hypothesis, and expect to be born out when we eventually start to get data from other star systems on the atmospheres of Earth-like extrasolar planets. However, while waiting a decade or two that it is going to take before this data begins to come in, we hope that the more formal approach that we have taken here can provide some theoretical support for the 'Rare Earth’ view.

07 April 2015

Oxygen-producing photosynthesis as a Critical Step in the evolution of life on Earth

Below is a slightly edited excerpt from The Revolutions that Made the Earth by Watson & Lenton (pp. 88-89). They are making the case here that the evolution of oxygen-producing photosynthesis is one of what they refer to as critical steps in the evolution of life on Earth, i.e., events so unlikely that if you were to 'play the tape over' as Stephen Jay Gould described in his 1989 book Wonderful Life, they would likely not happen again. (If you think about it, that implies that most planets where life might have evolved based on initial conditions will not have experienced the critical steps described). 

[W]e think the case for [oxygenic photosynthesis’] being a critical step is very strong. Photosynthesis powers the planet, and we’ll describe [later in the book] the evolution of this remarkable mechanism, and just how profoundly it has transformed the Earth system. Here we simply want to establish reasons for believing it may have been one of our critical steps, using the two criteria of uniqueness and timing by which we hope to recognize such transitions.
 
The bacterium that first produced oxygen by splitting water with sunlight coupled together to earlier-evolved photosynthetic pathways, called photosystems I and II. In addition it possessed a unique enzyme containing four manganese atoms, called the water-splitting complex. The biochemistry of oxygenic photosynthesis is staggeringly beautiful and complex, involving the absorption of a total of eight protons in the process of splitting two water molecules to release one molecule of oxygen. Among all the prokaryotes, only the cyanobacteria evolved this ability, and later, they became the ancestors of all the chloroplasts in all the eukaryote algae and plants [on Earth].
   
The evidence is consistent with the original invention having occurred just once, with no sign that it evolved separately ever again, and the water-splitting complex has remained essentially unchanged over billions of years. This is significant, because oxygenic photosynthesis is an amazingly useful trick for any organism to possess, since it enables the uptake of carbon and the production of energy using just light, carbon dioxide and water — three of the most abundant resources on the surface of the planet. An organism that can do it is well equipped to make a living in most habitats on Earth, and, therefore, you might suppose that if it were easy in evolutionary terms, it might have happened more than once. Since it did not, we can assume that it was no everyday event: this was a red-letter day for life on Earth.

I just love this stuff. What's really amazing is that they make a pretty good case, based on statistical reasoning pioneered by Brandon Carter in the 1980s, that the critical steps they argue occurred on Earth (evolution of the genetic code, this one, emergence of endosymbiotic eukaryotes, and the evolution of language-using observer sophonts (i.e., us)), are all so unlikely that they would not be expected to occur in the 4 billion years or so that Earth has been habitable (even if stretched to add in the 100 to 500 million years its habitability has yet to go). In fact, all four are far more unlikely even than the origin of life itself, which actually occurred just about as quickly as it was physically possible for it to have done. (Setting aside the possibility of panspermia, i.e., extraterrestrial origin of life on Earth, or miracles; the first being difficult to assess but generally considered unlikely and the second being beyond the realm of science). Similarly, the evolution of macrobiota, meaning visible multicellular plants and animals, which is usually front and center in any history of life on Earth, is almost certainly not a critical step, as there is good evidence that it happened independently several times. But the upshot is that at least four entirely unique, and very, very unlikely evolutionary developments, each of which required the ones before to have happened, and in that sequence, all happened in the evolution of life on Earth. (Think about that: without the genetic code, photosynthesizing bacteria could not have evolved; without photosynthesis, there would be no oxygen atmosphere, and eukaryotes could not have evolved; without eukaryotes, intelligent animals, which are necessarily eukaryotic, could not have evolved; so the sequential order is necessary). The assessment of improbability is based on timing and uniqueness. The chance of all four occurring on any given potentially habitable planet, in order, and during the time the planet remains habitable (that is, before its star gets too hot to allow life there, which is the universal fate of habitable planets)... is truly miniscule. Of course, what that doesn't address is the likelihood that other, unknown and potentially equally remarkable, critical steps could be occurring or have occurred elsewhere, leading to unimaginably different biospheres. I suspect if and when humanity encounters life that did not originate on Earth, we will be simply amazed by how different the initial potentials can turn out.

05 April 2015

Lenton & Watson, Revolutions that Made the Earth, and the prevalence of intelligent life in the universe

I'm reading Lenton & Watson, Revolutions that Made the Earth (Oxford, 2010). It's a whole evolutionary history, from an Earth-System ("Gaia") point of view. Very interesting. Although hardly central to their thesis, they agree in general with Brownlee & Ward (Rare Earth) that complex life may be very rare in the universe. The "Archean" revolution, (Genetic Code, origin of life, replicating organisms, some kind of sustainable autotrophy; the emergence of the enzyme Rubisco, or something very like it); and probably the second "revolution" that resulted in photosynthesis (not necessarily oxygen-producing, there are at least two other systems still extant on Earth).... may be relatively "easy." Thus living worlds that have accomplished these developments may be common elsewhere in the universe. Other "revolutions," however, including the endosymbiotic adaptation that resulted in eukaryotes, the remarkable combination of Photosystem I and Photosystem II to create a really powerful system of oxygenating photosynthesis (resulting in the evolution of cyanobacteria, which were subsequently endosymbiotically combined with eukaryotes to produce plants), may have relied on chance circumstances sufficiently unlikely that comparable events may not frequently occur in the history of life elsewhere, such that complex life may be quite rare in the universe. The evolution of macroscopic organization, i.e., the Cambrian Explosion, they seem to treat as more or less inevitable, but it couldn't have happened without these other, less likely, earlier revolutions. Then there's the Great Fourth Biological Revolution: the emergence of human culture. We are already processing 1/10 of the 100,000 gW/sec. of energy that the entire rest of the biosphere produces, and, as Lovelock discusses in his most recent book (A Rough Ride to the Future), our "rate of evolution" (transmitted as information outside our bodies, not just our genes), is about 1 million times faster than previous biological evolution. So our existence is a very big deal in the history of life on earth, objectively. (Many people are resistant to this idea, but if you really think about it, it's actually undeniable). These guys seem to think this development is also probably rather unlikely. In 600 million years, since the emergence of macroscopic animals, no other animal, including our close relatives the chimps and gorillas, even came close. Hard to say, but you could imagine, as Stephen Gould used to analogize, "replaying the tape," a number of times, even starting with, say, the Mesozoic, and not getting the equivalent of humans most of the time. 

Incidentally, I am not at all sure that the first of the "revolutions," which Lenton and Watson seem to treat as pretty likely, namely the origin of life at all (what they refer to as "Inception") isn't just possibly the most unlikely of all. We just don't know. Other than the fact that it seems to have occurred on Earth at just about the earliest physically possible date, I've not seen an explanation for why this should be considered an "easy" transition. From non-life to life? Seems to me quite conceivable, as old fashioned thinkers used to argue, that this one could turn out to have been spectacularly unlikely. We modern folks (including me) prefer to think that life is common in the universe, but there is no real hard evidence for that presumption. 
  
All of this has implication for our favorite topic, the prevalence, or non-prevalence, of human-equivalent civilized life elsewhere in the universe. Of course no one knows, for sure. But there is a pretty robust intellectual case for the idea that even planets as favorably situated at the outset for the emergence of life as Earth was at the outset, may only quite rarely result in the emergence of intelligent beings and technological civilizations. 

Lurking behind all of this ratiocination is one or other level of the anthropic principle. We cannot, of course, really say whether the combined probability of all of these "unlikely" revolutions adds up to the Earth being a nearly impossible miracle, or something much more likely to occur, in broad outlines. Because, it almost goes without saying at this point, but for all of this having occurred, just as it did, we would not be here to think about it. So we cannot assess, without more information about other instances of life, how likely rough alternatives may or may not have been, which might have led to our rough equivalents. Or not. 
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The authors are relying on two things. The complexity of the adaptations involved, which they plausibly translate into a measure of the "difficulty" for evolution to come up with a specific major adaptive change. And the other is that certain kinds of change, like the evolution of complex body plans from single celled organisms, apparently happened over and over again, which is more than a hint that it's an "easy" development. But the "revolutions" they consider to be "unlikely" occurred only once, and usually after long periods of time at any point during which they could have happened but did not.