Category: SETI

Limits of Life

Life – but not as we know it – space – 06 June 2007 – New Scientist.

The Limits of Organic Life in Planetary Systems

These two, a “New Scientist” article and a short National Academies Press book, contain some of the more interesting concepts to hit astrobiology in a very long time. They discuss, in understandably limited terms, the limits of what Life might be like without the usual requirements of water, oxygen and even organic compounds – for example, gas-phase enzyme activity, life-in-sulfuric-acid, life using methylforamide, life in super-critical CO2 and life based on silicon in cryogenic conditions.

Let’s step back a bit. A certain Ph.D in Chemistry, one I. Asimov, also had a modest career as an SF writer. A short essay by him appeared in a SETI related magazine, in which he discusses possibilities for life-as-we-don’t-know-it…

Not as We Know it

…to which we can add some more possibilities now.

All this speculation, about which I will elaborate later, does make the Fermi Paradox seem a tad lame.

Intelligence helps Universes Reproduce…

K-State math professor looking to mathematical theories for clues on origins and future of life in the universe; Suggests artificial black holes play a big part.

Louis Crane has been working on an extension of Lee Smolin’s suggestion that black-holes are the birthing events of Universes, and Universes are “fine-tuned” to enhance that reproduction via a Cosmic Selection process. The above news-bite and link is a more recent piece, but here’s two more…

Building a Better Black-Hole (October 2007)

Ad Astra Kansas News (Fall 2006)

…all saying much of the same thing: Intelligent Life will learn to make black-holes for interstellar travel, and thus ‘mid-wife’ the creation of Universes (if Smolin & Crane are right.)

But what good is a black-hole? Actually it’s an incredibly efficient way of turning mass into energy. So long as you do it the right way. So what’s the wrong way? Accretion, but for an interesting reason.

Arthur C. Clarke used a black-hole as the heart of his fictional “Asymptotic Drive” in his 1975 novel, Imperial Earth. An accreting black-hole can convert 5% of the rest-mass energy of infalling matter. Thus it’s capable of an exhaust velocity of 0.33 c, which it uses to good effect doing a continuous boost between Saturn and Earth at 0.2 gee. Black-holes have also appeared as fictional power-sources in all sorts of odd places, including “Star Trek”. Black-holes can also store energy as rotational energy, controlled by giving the hole a large electromagnetic charge and subjecting them to a rotating field. Charles Sheffield uses them like so in his “McAndrews Chronicles”.

A question is: just how mass-efficient is energy release via black-hole accretion? A limiting process for all accreting systems is the Eddington Limit, which is when radiation pressure is sufficient to blow away infalling matter from an object. Plugging in the usual numbers that means the energy output is about 6.4 W/kg of black-hole… worse than an RTG! Not a starship drive. But some accreting systems can be above that Limit via ‘dirty tricks’ i.e. opacity effects.

Consider the Sun. It puts out about 4E+26 joules per second. And it masses 2E+30 kilograms. That’s a power-to-mass ratio of just 0.0002 watts per kilogram – far less than a battery. A bit higher if we factor in the fraction of the Sun that actually produces power (0.08) thus 0.0025 watts per kilogram. Not much better. We’re used to hearing about quasars, which are powered by black-hole accretion, having power outputs of a 100 galaxies or so. How much is that? A galaxy, like our Milky Way, shines with a light of about 30 billion Suns. Thus a quasar putting out 100 Galaxies of power is shining with about 3 trillion Suns of light. How big a black-hole is needed? At 6.4 watts per kilogram at the Eddington limit it’s massing ~100 million Suns. That’s big, but consider the monster at the heart of M87, recently measured at 6.4 BILLION solar masses.

Yet all with a piddling 6.4 watt per kilogram power-output.

Not the way to power a starship. However black-holes also turn their mass into energy via Hawking Radiation and that’s a bit more power dense – with a caveat. Old school semi-Classical quantum mechanics applied to General Relativity means a black-hole drags virtual particles out of the vacuum, making them real. In doing so it loses energy/mass. A black-hole is a very simple astrophysical object, defined by its mass, spin and charge and its ‘size’ is really the size of the Event Horizon it wraps around itself. The size of the horizon determines the temperature of the Hawking Radiation that it radiates as well as how much energy it then radiates over its whole Event Horizon area – but the two are at odds i.e. the inverse of the radius determines temperature, while the radius to the 2nd power is the area. Thus once you do the sums the black-hole gets brighter to the inverse of the 2nd power of the radius, and thus the mass (Rs = 2GM/c2.) The energy is being radiated away so quickly for lower masses that the last 228 tons of mass-energy is radiated away in the last second! That’s an explosion of 4.9 million megatons of TNT equivalent energy as very hard gamma-rays.

A heftier hole, about 1 million tons, radiates at about 356,200 W/kg – better than any current power-plant. But for a really high performance starship we want megawatts per kg. A hole massing 100,000 tons is putting out 356.2 MW/kg… except it’s mostly as hard gamma-rays. A gamma-ray reflecting material would be a big plus.

Back to Louis Crane’s work. He’s using a quantum gravity theory to analyse the behaviour of black holes at such extremes, so the figures I quote will be different to what his analysis derives. Hopefully he’ll figure out the gamma-ray problem for us too :)

Greater Longevity for Planets with Life?

Caltech Scientists Predict Greater Longevity for Planets with Life – Caltech.

The paper is:

“Atmospheric pressure as a natural regulator of the climate of a terrestrial planet with biosphere,” by King-Fai Li, Kaveh Pahlevan, Joseph L. Kirschvink, and Yuk L. Yung

…and its abtract is here at the “Proceedings of the National Academy of Sciences.” The chief researcher is King-Fai Li, whose publications page is here and well worth a LOOK…

Basically the pressure of an atmosphere enhances the IR-absorbing spread of carbon dioxide molecules, which is how they trap heat so efficiently on Venus, and apparently how a miniscule 380 ppm of CO2 on Earth plays such a key-role on Earth, but 950,000 ppm on Mars isn’t so effective. If the total nitrogen – some 78% of the atmosphere – declined with the rise in solar insolation, then Earth’s average temperature would remain more stable over the next 2.3 billion years compared with previous models (eg. Lovelock or Caldeira & Kasting.) By the end of that period, when a runaway greenhouse finally kicks in, the surface pressure has dropped to ~0.1 of today. So just how ‘habitable’ Earth remains is a different question. Over such a long-span then life would no doubt adapt, but a luckless Time-Traveller might be a tad short of breath.

Another atmospheric processes guru involved in the paper is Joe Kirschvink, whose webpage is a rewarding visit for all who want to dig deeper. He has papers online about Enceladus and the ALH84001 Martian meteorite which maybe had microfossils.

Brightside of Meteorite bombardment & Junk DNA

Meteorite bombardment may have made Earth more habitable, says study.

Saved by Junk DNA.

The Origin of Life on Earth is a puzzle that biologists, biochemists, physicists and geologists – to name a few – have chewed on over the past 150 years since Darwin opened up the conceptual doors and let in the refreshing light of natural selection. One related question is just when was Earth first inhabited and habitable. The first study above seems to indicate that both Mars and Earth were made more clement by that last gasp of accretion, the Late Heavy Bombardment, which pounded the Inner Planets some 4.0-3.9 billion years ago.

How so? The infalling meteorites released both water and carbon dioxide, thus wetting & warming both planets, perhaps sufficiently for liquid water to remain stable on the open surface. Prior to that event, water may well have been mostly frozen. There’s good reason to think that the process of making long-chains of biomolecules, an important step before ‘Life’ itself, was via concentration of smaller sub-units within ice. Tiny channels of unfrozen liquid become increasingly concentrated in solutes as watery solutions freeze, providing an accelerated reaction environment for polymerisation. In such conditions even quite short pieces of RNA become capable of ‘ligation’, the fusing of RNA sub-units into longer chains.

Once RNA Life has given way to DNA Life what drives the evolution of ever longer strings of DNA and thus ever more complex Life? The second news piece is about evidence that so-called ‘Junk DNA’ – mostly repetitive segments of DNA with no obvious function – actually promotes faster evolution of organisms by altering the rate of gene mutation and gene expression. It seems the ‘Junk’ can make a gene’s DNA sequence more exposed and liable to change when the ‘Junk’ situated next to it has changed in length.

But there’s always a trade off. ‘Junk’ DNA is reduced in some organisms, very noticeably in birds, while it has immensely expanded in some organisms, like certain plants and slow-living creatures like amphibians and lungfish. One’s pace of life style has a distinct selective role on ‘Junk’ DNA’s quantity – fast-living reduces its presence, and perhaps its selective advantage. Birds need to rapidly churn out proteins from their DNA genes and operate at a higher blood temperature too. This might make the DNA more liable to change – birds are immensely speciose – without any ‘Junk’ DNA putting pressure on genes at all. Lungfish, and their kin, live ‘cold-blooded’ rather sedate lives, and carry around a large load of ‘Junk’ that ensures their DNA remains healthy, making the invasion of ‘DNA’ parasites, like viruses, much harder because the host DNA is already full of virus-like ‘Junk’.

Catalytic Nuclear Ramjet

Catalytic Nuclear Ramjet(application/pdf Object).

Robert Bussard first proposed the Nuclear-Fusion Interstellar Ramjet in 1960 and it caught the imagination of researchers (like Carl Sagan) and fiction writers (like Larry Niven & Poul Anderson) alike. Basically Bussard proposed to scoop up the interstellar medium and fuse it for propulsion, thus allowing a rocket to refuel for its entire journey. A 1,000 ton rocket could theoretically scoop propellant and fly at 1 g ‘forever’ – at least until drag became equal to its thrust.

A problem arose – hydrogen is very hard to fuse all by itself. The reaction rate of proton-proton fusion at “low” (i.e. an achieveable 100 million degrees) temperatures is essentially negligible and only powers the stars because they’re so gigantic. The Sun’s energy production rate is a bit more than 10 Watts per cubic metre of the fusion part of its core, which is far less than the power packed into a battery, for example. Unlike a battery, of course, that energy can trickle out for billions of years – but that’s no good for propelling a starship.

How do we make the reaction go faster? Physicist Daniel Whitmire proposed we burn the hydrogen via the well-known CNO Bi-Cycle. Basically a hydrogen fuses to a carbon-12, then another is fused to it to make nitrogen-14, then two more to make oxygen-16, which is then highly ‘excited’ and it spits out a helium nucleus (He-4) to return the nitrogen-14 back to carbon-12. Since the carbon-12 isn’t consumed it’s called a “catalytic” cycle, but it’s not chemical catalysis as we know it. Call it “nuclear chemistry”.

The CNO-cycle was first proposed by Hans Bethe as the means by which the Sun makes heat & light. It’s one means by which the Sun does so, but proton-proton fusion is dominant at the lower temperatures in the Sun and lower mass stars. Slightly bigger stars are predominantly powered by the CNO cycle and as the Sun evolves and its core contracts as helium builds up, it too will burn mostly via the CNO cycle.

Whitmire’s paper gives a rough guide to how well the CNO cycle improves a ramjet’s power-levels and shows, reasonably, that a Catalytic Nuclear Ramjet could propel a ship to the stars…

…the rest is an exercise for the student.

Rydberg Matter… Invisible Hazard?

Rydberg Matter in Space.

Rydberg atoms are atoms orbitted by electrons in an excited state which leaves them barely attached to their nucleii, often for surprisingly long time periods. Rydberg Matter is basically clusters of Rydberg atoms attracted to each other and forming stable small n clusters. These small clusters typically form “atomic snowflakes”, but these in turn link up in long chains. Collected together in space Rydberg matter composed of interstellar hydrogen is largely transparent to radio, visible and IR light, thus it’s almost impossible to see… and possibly a billion times denser than the 106 protons per m3 that makes up most of the ISM. That’s bad news for interstellar travel as running into the stuff at high-speed (~0.1 c or more) would cause incredible meteoric levels of heating ~25,000 K.

An explanation for Fermi’s Paradox? Unsure. The chief researcher on Rydberg Matter seems kind of solitary in his research, perhaps indicating it’s dubious nature. More data from other researchers would be helpful.

Proterozoic Metazoans?

Complex life pushed back in time (ABC News in Science).

The Vindyan sedimentary basins in central India have been contentious for quite some time, with claims of animal body and trace fossils going back into the Meso-Proterozoic (1.6-1.0 Gya.) That claim has been given a shot-in-the-arm by new work reported in the Proceedings of the National Academy of Sciences which describes fossils found in the Vindyan from 1.65 Gya (late Paleoproterozoic) akin to those usually found in the Cambrian and Ediacaran.

So what’s going on? Is complex life really so old? Did it experience a global setback and took a billion years to recover?

There’s merit to that idea. A current puzzle of astrobiology, that I’ve blogged on time and time again, is the Fermi Paradox – in sum, if Life is similar in age to the Galaxy then why isn’t the Galaxy full of obvious signs of Life? As in alien space-junk in our Solar System, or stars shrouded in Dyson Swarms or whatever…

But what if there’s a faulty assumption? What if Life only gets so old, then gets knocked back to microbes? Perhaps we’re now in an epoch of recently removed restrictions on Life – like frequent gamma-ray bursts – and soon, in cosmic terms, the Galaxy will be colonized. Or perhaps what we’re seeing in the fossils is a reminder that the Galaxy may still be a dangerous place.

Giant trilobites had (complex?) social lives

Giant trilobites had complex social lives (ABC News in Science).

Trilobites ganged together to molt, just like modern day horseshoe crabs – themselves relicts of a bygone age. Such “social” behaviour is surprisingly old, as molting groups have been found dating back 520 million years, making me wonder why Arthropods never did develop intelligence as we know it in larger animals. Possibly it’s because they usually don’t reach huge sizes, though multi-metre long Eurypterids (“Sea Scorpions”) are known. On land they seem to be limited by their circulatory systems and the weakness of their chitin-based exoskeletons. Perhaps on a different world that might not be the liability it is here on Earth… imagine a low-gee moon with a dense atmosphere.

Maybe we will know one day.

Where Are They?

It’s said that Enrico Fermi wondered why we didn’t see signs of aliens since nuclear energy made interstellar travel possible. It’ s a worthwhile question and one which has been answered in a dizzying number of ways. Here’s my $0.02 worth…

Personally I think there’s two likely equilibrium states that answer the Fermi Paradox. Either we’re in the pre-Colonization era, before anyone ventures forth and colonizes the lot, due to Life starting late in the Universe’s life because of some recently changed astrophysical process.
Or the Galaxy has been colonized, in which case two sub-divisions seem reasonable:

(1) We’re an undeveloped patch missed by the last few waves of colonization as per Geoff Landis’s Percolation Theory.
(2) Or we haven’t been missed and we’re colonized, but just not how we usually imagine.

Consider: There’s so much room in the Edgeworth-Kuiper Belt, or the Opik-Oort Cloud, and if ETIs exist as star-travellers then that’s where they are. While the inner system seems warm and inviting, perhaps they avoid it because of coronal mass ejections, UV or solar flares or some such. Remember they’re used to living in space in their own habitats, not on wild, native planets.

Perhaps there’s a whole lot of deuterium lying around ready to be scooped up from the surfaces of cold Plutoes or Marses that might be Out There. Imagine a Mars mass planet that has chilled to ambient temperatures and all the hydrogen has frozen out – its atmosphere would be helium! Makes getting at the He3 easier. Assuming it formed with a primordial atmosphere that is, which is a fair assumption for the Outer Solar System.

Even if they only use fusion for propulsion and they use solar for life-processes, there’s sufficient materials for constructing huge solar collecting mirrors (“sollectors”) in most average sized cometoids. A sufficiently efficient closed-loop ecology can be sustained on a lot less sunshine than we experienced here on Earth.

Just how efficient is the Earth’s biosphere? About 100 billion tons of carbon is fixed by primary production in the land and sea biospheres – about 250 billion tons of carbohydrates – per year. If the energy content is ~480 kJ/mole and a mole is about 0.03 kg, then the total energy stored by the biosphere is ~4,000 exajoules per year. Earth absorbs from the Sun about 3,850,000 exajoules per year, so the biosphere captures and stores just ~0.11% of that energy torrent. So, in theory, we could get by on less energy than what drenches the Earth currently.

Of course a biosphere would have to stay warm, but with sufficient insulation the heat-loss can be brought into line with the biosphere’s own waste-heat, so the habitat is not radiating as a black-body at the biosphere’s preferred temperature, but at some lower value in line with its actual heat use. But I’ve covered similar ground in a different post. The main thing is that ETIs can lay-low in the OOC, if they so wish. And we’re not the cosmic backwater that Fermi scepticism might lead us to think…

Direct contact

ScienceDirect – Planetary and Space Science : Direct contact among galactic civilizations by relativistic interstellar spaceflight.

Carl Sagan caused some academic excitement in 1963 [i.e. the academics blasted him] by modestly proposing that interstellar travel was a reasonable way for civilizations throughout the Galaxy to pay each other a visit every few thousand years, and that at some point in history (the last 10,000 years) we had actually been visited – or at least it was worth checking myths and legends for a sign of such a visit. Robert Bussard’s Ramjet paper inspired Sagan’s optimism – here was a seemingly reasonable design which could give a technological civilization the Keys to the Cosmos.

A lot of arguing over interstellar travel, alien life and the Fermi Paradox has happened since then. Can we conclude anything from all the arguments? One positive thing is that interstellar travel can be achieved at relativistic speeds even if interstellar ramjets can’t be made to work. All sorts of beamed-energy designs mean that it’s an unreasonable objection to visits by aliens to claim interstellar travel is impossible. It’s not.

But could it be made even easier than we imagine? One technology that would enable easy interstellar travel – in so far as packing a closed-loop environment or a lot of frozen meals is “easy” – would be total annihilation drives. Frank Tipler’s current formulation of the Omega Point Theory requires the invention of macroscopic sphaleron generating… somethings to annihilate matter and in one version he proposes the conversion of baryons into lots of neutrinos. This would allow the drive to be operated without melting down the local topography with terawatts of gamma-rays – in otherwords it could launch from the surface of a planet, even your own backyard.

Once you’re in space what else is liable to impede one’s progress? Interstellar matter. So turn a problem into a virtue and Bussard scoop the lot into one’s mass annihilator. Thus a ravening proton-storm becomes one’s neutrino-beam to the stars. The Galaxy is yours.

Except… well there is the travel-time issue. The basic equations are well known for constant acceleration flight:

let me introduce B & g (actually a beta and a gamma.) B is the ratio, v/c, where v is your velocity and c is lightspeed, then g = (1 – B²)-1/2.

The travel time, onboard ship, is then…

t = (2c/a)*arcosh(g)

…a is acceleration and arcosh(g) is the inverse hyperbolic cosine (“cosh”) of g, which is ln[g + (g²-1)1/2]. Thus, at large g values, it simplifies to good approximation to:

t = (2c/a)*ln(2g)

…but one variable is missing: distance, S. It’s a part of g i.e. g = (1 + aS/2c²) …notice I’m computing a trip starting and finishing at zero relative velocity to the starting point.

So what does that mean for the people at home? Well Planet-time – as measured by observers at rest with respect to one’s starting point – is computed pretty straightforwardly as T = 2v.g/a. Of course you can expand v and g out in terms of S and a to get…

T = [4S/a + S²]1/2 …or…

T = 2[(S/a)(1 + aS/4c²)]1/2

…or even purely in terms of g & a…

T = (2c/a)(g²-1)1/2

…which shows us that the ratio of the two, t/T, is…

ln[g + (g²-1)1/2]/[g²-1]1/2

which simplifies in the high g limit to…

ln(2g)/g

…so to get one’s experienced trip-time down for a given distance, then you have to hit very high g factors. Now at an acceleration of 1-gee (Earth gravity) the distance travelled to increase g by 1 is c²/a ~0.97 light-years.

To get to high g factors in a short time requires high acceleration. Some fictional examples…

  • Robert Forward illustrated this quite memorably in his book “Timemaster”, wherein the hero has to accelerate at 30 gees for 5 weeks to get to a g factor of 10. Naturally he’s floating in a gee-tank.
  • Stanislaw Lem propels his heros in “Fiasco” to the planet Quinta, some five light-years from the mothership, in 3 months of subjective flight-time at 20 gees.

    • Other examples, as meticulous, are harder to find. Alastair Reynolds is an SF-writing astrophysicist (like Greg Benford before him) and probably is as careful with the time-factors – I’m yet to finish one of his books, so I’ll let you know.