Two kinds of astrobiology stories are in the wind this morning. One of them has to do with the weekend eruption of stories concerning evidence of fossilized life inside a meteorite. The other deals with scientific investigation off-planet, and although sparsely covered, it’s the one with the greater significance for finding life elsewhere. But first, let’s get Richard Hoover’s paper about meteorite life out of the way, for the growing consensus this morning is that there are serious problems with his analysis, especially as regards contamination of the sample here on Earth.
I have no problems with the panspermia idea — the notion that life just may be ubiquitous, and that planetary systems may be seeded with life not just from other planets within the system but from other stellar systems entirely. It’s an appealing and elegant concept, but thus far we have no proof, and despite what Dr. Hoover is seeing in samples from three meteorites, we still can’t definitively say that we’ve found fossilized microbes from any biosphere but our own. The skeptics are weighing in loud and clear, and Alan Boyle has collected many of their thoughts.
I want to send you to Alan’s Cosmic Log for the bulk of these comments, but let me lift one from Dale Andersen (SETI Institute) to give you the gist. Andersen acknowledges the excitement of the story if it could be proven true, but he’s worried about the fact that Hoover’s work is not playing well within the scientific community, and he is waiting for serious peer review:
“Peer review will include the examination of his and other scientists’ data and logic, and not until that has occurred will we see how the story unfolds. Occam’s razor will eventually be used to slice and dice the carbonaceous chondrites used by Richard to present his evidence. Is it more likely that upon looking into the interior of a meteorite collected on Earth and finding photosynthetic cyanobacteria, which on Earth are usually found in water or wet sediments, their presence is due to contamination from terrestrial sources or that it formed inside the parent body of comet or asteroid in deep space? There will be many other possibilities to rule out before one arrives at the extraterrestrial answer.”
You can find Hoover’s “Fossils of Cyanobacteria in CI1 Carbonaceous Meteorites,” in the Journal of Cosmology, Vol. 13 (March, 2011), which is available online.
Addendum: Be sure to check out Philip Ball’s “The Aliens Haven’t Landed,” published online in Nature today.
Meanwhile, Around Europa
Hoover’s ideas will be considered in a much longer process of investigation as other scientists analyze his evidence, but as is always the case, let’s be cautious indeed about claiming life from elsewhere in the universe. None of which is to argue that we slow down our investigations into such life, but alas, in the other astrobiology story of the morning, our chances of getting to a prime site for a closer look are taking a hit. Tightened budgets are threatening to push the Jupiter-Europa Orbiter further into the future, joining other big missions on indefinite hold.
The Jupiter-Europa Orbiter was conceived as part of a collaborative campaign between NASA and ESA called the Europa Jupiter System Mission, a two-spacecraft investigation originally planned for a launch around 2022. The troublesome news about the NASA contribution comes from a new report from the National Research Council, which while recommending a suite of flagship missions over the decade 2013-2022, also takes pains to note that budgetary constraints could limit NASA to small scale missions of the New Frontiers and Discovery class in that period. Steven Squyres (Cornell University) chaired the committee that wrote the report:
“Our recommendations are science-driven, and they offer a balanced mix of missions — large, medium, and small — that have the potential to greatly expand our knowledge of the solar system. However, in these tough economic times, some difficult choices may have to be made. With that in mind, our priority missions were carefully selected based on their potential to yield the most scientific benefit per dollar spent.”
The priorities are there, but throw in the fact that these recommendations were informed by NASA’s 2011 projected budget scenario — and that the 2012 projections are less favorable — and you can see that the Jupiter Europa Orbiter (JEO) is in trouble. The JEO comes in as the second priority for NASA’s large-scale planetary science missions, after the Mars Astrobiology Explorer Cacher (MAX-C), and while the report notes how promising Europa and its subsurface ocean may be for astrobiological studies, it says that JEO should fly only if NASA’s budget for planetary science is increased and the $4.7 billion mission is made more affordable.
Expect the launch of a Europa orbiter between 2013-2022, then, only if both the mission and the budget outlook change significantly. The same holds for the Uranus Orbiter and Probe, an exciting mission concept that would deploy an atmospheric probe to study this interesting world while the primary spacecraft orbits Uranus and makes planetary measurements as well as flybys of the larger moons. Space News notes the threat to all three missions, citing remarks by Jim Green, director of NASA’s Planetary Science Division on March 3:
…because the decadal survey’s findings are based on NASA’s more generous 2011 out-year funding projections, rather than the declining profile laid out in the 2012 budget request, it is unlikely the agency can afford to embark on any new large projects in the coming decade. Green, speaking March 1 at a meeting of the NASA Advisory Council’s planetary science subcommittee, warned members not to expect the funding outlook to improve.
Looking to Smaller Missions
The upshot of all this is that the flagship missions with the most interesting astrobiological profile are on hold in an environment favoring only small- to medium-class probes. We still have one flagship mission, the $2.5 billion Mars Science Laboratory rover, set to launch later this year, but the grim truth is that the MSL will probably be the last planetary flagship mission for quite some time. What had appeared a promising funding prospect in the 2011 NASA budget request has reversed itself with the administration’s 2012 request (released in mid-February), which shows a decline in planetary spending after 2012 that will not support these larger mission plans.
So while we have support for both Mars sample collection and the Europa orbiter in the report, the reality is that the money is just not there. For the time being, then, realistic outer planet prospects focus on Juno, a mission slated for launch this summer that will spend a year orbiting Jupiter. The solar-powered vehicle will orbit Jupiter’s poles 33 times to study the planet’s atmosphere, structure and magnetosphere. Juno is currently undergoing environmental testing at Lockheed Martin Space Systems near Denver.
We’ll wait, too, to see what medium-sized missions NASA will select for the New Frontiers program, which would benefit from the trimming of the flagship missions. The agency is selecting one New Frontiers mission now and the committee recommends two more missions be chosen for the 2013-2023 period. Possibilities include a Jupiter mission to Io and a trojan asteroid rendezvous. The Discovery program of low-cost, highly focused planetary science missions also receives the report’s blessing as hopes for the big missions are deferred.
Pre-publication copies of Vision and Voyages for Planetary Science in the Decade 2013-2022 are available from the National Academies Press.
I apologize if I am missing something fundamental here, but what ever happened to all that money previously used to fund manned spaceflight? Was the reason for cutting the Shuttle program and transfering responsibility of manned flight to private companies not to free NASA’s budget for hard science missions? Yes, the new budget is not as promising as they expected, but it does not incorporate massive cuts. Instead of getting a little more, they are getting a little less. “Little” may be a relative term, but the cost of the shuttle program was comparatively “Huge.” Within a year or two, shuttle launches will end, freeing up the roughly $0.5 Billion per launch cost. Where is all that going?
I’m afraid that when I read these findings are from a meteorite that has been on Earth since at least 1864, I stopped reading. It’s disappointing but this work will never be accepted as definitive proof of ET life because of this one fact.
Just a thought. . . . .
Compared to “on comets or inside asteroids”, Mars, Europa and probably quite a few other places are much more hospitable to life, cyanobacteria or otherwise. IF one had to pick a single salient, and irrefutable, feature of life I would propose that the view of a teeming petri dish through a microscope might tell it all. When you see all of those little squigly things flopping around and the green of chloroplasts, etc, you will know it. IF a principal objective was to look for life, make your probe the simplest possible. An imaging system, a petri dish, and a landing system. Make it the same for the southern reaches of the Moon, for Europa, Enceladus or Comet Halley (no atmosphere), and a modified one with a parachute for Titan and perhaps Mars. Make a whole flock of them and shoot them out there (made small enough perhaps even as part of one single booster launch!) with two or more for each “target”. Look for little squigleys! For Europa and Encladus PERHAPS allow the complication of an acoustic sensor to see if the extraterrestrial equivalent of “whale song” comes up through the ice, but keep each of these PAINFULLY simple. Keep the cost down, share the cost across a bunch of science missions, add multiple probes for each target, keep the “technocrats” at bay, and ROLL THE DICE.
Debate is nice but debate is just debate, and how many angels will fit through the eye of a needle. Turn up some little squigleys under the microscope and no matter how grainy the picture, you have something!
Just a thought of course. . . . :-)
Panspermia is fascinating idea, but many years ago (sorry I don’t remember any details) I heard a report of an analysis that concluded that we’d see the end of the universe before there was any likelihood of a piece of a terrestrial planet that was blown into space landing on a planet in another star system. The author doubted that such an event had ever happened anywhere. Space is too big, planets too small.
Well, we already know of a bunch of meteorites from Mars on Earth, ALH84001 being one of the most famous. What the astronomer probably did not take into account is that Mars had some massive impacts ages ago, which sent lots of surface debris into space, thus improving the odds of at least some of them finding their way to our planet. We also know of some lunar meteorites that also got here by the same method. Now if we could just find a meteorite from Venus. And it has been shown not too long ago that it is at least possible for simple life forms to survive this process inside rocks.
ljk, the analysis was only questioning the likelihood of interstellar panspermia, rather than panspermia between planets within the same stellar system.
It seems to be little remembered that meteorites are mostly fragments of ancient protoplanets, and protoplanets near the solar systems beginning were very warm and active places compared to similarly sized bodies today. This was mainly due IIRC to left over accretion heat and a much greater abundance of short lived radioactive isotopes. From studies of meteroites we have found that many of these worlds, despite being possibly only a few hundred km across, were differentiated bodies like true planets, and there is abundant evidence that many of these ancient bodies saw subsurface liquid water ineracting with complex carbon chemistry from the pre-solar and solar nebula.
Or, less verbosely: the meteorites weren’t habitable but the objects they came from had heat, liquid water (at least intermittently) and carbon chemistry, the three big prerequisites (we think ) for life.
None of that makes Hoover right, explain how these supposed life forms arose so quickly, or elimates any of the huge problems with contamination. But it is not true to think that IF these were microfossils they must have come from some far distant star system, or that the bodies they came from were nescearily less hospitable to life than Enceladus, Europa etc.
@bob keeter
“IF one had to pick a single salient, and irrefutable, feature of life I would propose that the view of a teeming petri dish through a microscope might tell it all. ”
The problem with this is the neat view of life you had, staring down a microscope at algae from a rich soup of pond scum, doesn’t work too well under other circumstances.
bacteria are hard to see that way. Under many conditions, they adhere to surfaces, making them even hard to see. Under cold conditions, they may be dormant, or just move extremely slowly. On earth, we usually need to culture the critters, as a result of which, we barely know the vast numbers of species that we have unsuccessfully cultured. That was part of teh reason Venter just went sampling waters around the world and did shot gun DNA sequencing of everything, to look for different life forms.
The simple approach is therefore just too risky for the costs of a mission. The usual approaches use proxies for life instead. But this also has the flaw that the proxies are for earth based life (as we know it).
John Freeman makes the vital point that the supposed inadequacy of lithopanspermia in spreading life throughout our galaxy rests on the assumption than biogenesis and/or ecosystems develops late (in relation to planet formation), and only on the surface of large planets.
Arrhenius’ radiopanspermia is an idea of genius but suffers from similar problems. Hoyle has come up with the only working idea to date that looks like it could salvage it, and that involves cycling bacteria through growth cycles in nascent comets on a vast scale. His proposal should not be as disparaged as it has been because it can only work with an all-or-nothing outlook – either Earth-like life is everywhere we look or he was wrong. And that brings us back to Hoover’s fossil.
There must be many ways to prove the voracity of such a fossil – such as if it were trapped in an old crystal, or that there were clear signs that it had a unique biochemistry. Until such time as he has published all criticism should be aimed at premature evaluation of the results, not the unpublished results themselves! Can’t we just acknowledge that it can occasionally be justified to call for help if the issue is important enough?
Just a thought to thoss out there for you all to think about. I envision a cheep mission which would not need any new technology, I simply call it MARS, which stands for Martian Atmospheric Recovery Sample. An orbiter would be sent to Mars, it would gingerly dip into the upper atmosphere (which has been before for atmospheric breaking) and after it did that several times a rocket booster would fire to return a reentery projectile to Earth. We and the Japanese have both recovered such space reentries before. The MARS satellite would contain and expose some of astro jell to the Martian atmosphere, and if we timed it just right during one of the planets major dust storms…perhaps..maybe we could recover dust samples from the Mars without actually landing. It would be a start for sample returns from Mars…………..Tom
Tom, your idea reminds me of the Europa Ice Clipper concept, which would fire a Deep Impact style ball into the moon, kicking up some of the icy surface which the probe would collect ala Stardust and return the samples to Earth.
http://www.astrobiology.com/europa/ice.clipper.html
This proposal was not picked up. Probably cheaper than the other plans for Europa, but it’s too late now.
I was reading Alex Tolley’s reply to Bob Keeters proposal when I realised that my one salient property for life would be the lowering of local entropy that was only indirectly coupled to enthalpy changes. This property is better than reproduction, since that is too common, and would need to be augmented by either the minimum requirements for evolution, or by use of van Neumann’s definition for nontriviality.
This in turn made me think of Viking’s life experiments, and in particular how complex organic matter could have been synthesized when unsterilised Martian soil was exposed to light from a xenon lamp. How was it possible that ordinary minerals could boost the energy from each photon enough to fix carbon?
Let’s start afresh and prioritise one mission above all others. We must return to Mars and repeat the labeled release experiment with varied optical isomers, and we must repeat the pyrolytic experiment in a way that can give us a better idea of what type of carbon compounds were being photosynthesized.
I concurr with the comment of Rob Henry. There appears to be a rather premature rush to dismiss these findings without due account of the measures described in Hoover’s paper to assess the risk of contamination and to test for the possibility of non-biological origins for the forms under discussion. I would be interested indeed in seeing a peer reviewed paper that can account for these data through non-biological processes rather than the dismissal by soundbite that is an unfortunate byproduct of the current media interest in the story. Whilst there is a range of data from peer reviewed sources that suggests these results are not beyond what could be considered theoretically possible (e.g. the work of Hoyle and Wickramasinghe and the 2004 papers by Napier on lithopanspermia mechanismisms and Wallis and Wickramasinge on interstellar panspermia mechanisms. See also the 2010 paper by Paul Wesson on this topic, which if memory serves is available on the Xarciv), we need to defer judgement until there has been time for a possible alternative mechanism to be proposed, peer reviewed and published (or not!).
Further to Andrew W and John Freeman, on panspermia and the distance between planetary systems:
if we talk about true universal panspremia, i.e. not just one limited to one solar system (comets, etc.), then we must take the vast distance between different stellar/planetary systems into consideration, and then I think Andrew’s comment is rather on the mark.
Let me elaborate a bit: it is reasonable to assume that ejecta from planetary impacts are scattered non-directed and in this respect comparable to (diffuse) light or any other (diffuse) radiation. Therefore, as with light power (insolation), we can then say that the chance of an ejected rock (meteorite) hitting a particular target diminishes with the square power of distance.
Since the distance to the nearest star system (guess…) is roughly 100,000 times the average distance to Mars, the chance of an ejected rock from our neighboring star system reaching and hitting us is about 10^10 times as small as a so-called Martian meteorite. And we know how rare those are.
Add to this the fact that the chance of an ejected rock escaping the gravity of an earthsized planet is also much smaller than one from much smaller Mars, and we then have to come to the conclusion that Andrew is probably right in indicating that the chance of a ‘Alpha Centaurian meteorite’ hitting earth is indeed infinitesimally small.
Ronald, I recall a discussion about long-range panspermia in a comment thread here a couple or so years ago. I was in on it as well, and there were some calculations discussed. Unfortunately I don’t believe we can search comments on this blog, just the articles, so it would be a bother to find the reference.
My recollection was that it was possible but (in my view) unlikely to be competitive (in the Bayesian sense) with abiogenesis. But at best that’s still a guess since data is (obviously) lacking.
Some objects seen on the Martian surface by the two Mars rovers Spirit and Opportunity struck me as being rather like fossils, but you hear very little about them in the professional circles:
http://www.msnbc.msn.com/id/4480097/ns/technology_and_science-space/
Of course if fossils or living creatures are ever found on Mars, they will be the first ones screaming from the rooftops that they knew it all along and discovered them first! Just like the pros will do with SETI, where I am sure both artificial alien signals and celestial scale artifacts have gone unrecognized, ignored, or hidden away. Humanity has a lot of maturing to do when it comes to being part of the Cosmos.
ljk. I was not aware of the Europa Ice Clipper, you are correct in that it is the same concept. I do thank you for the link, I appreciate it. The MARS sample return would, I believe, be easier to accomplish. It would not need a larger rocket to get to Jupiter, nor would it require an impact device. On return it would not be required to escape from Jupiter larger gravity nor need a larger return rocket for the longer trip to earth. Smaller, cheaper and faster. Thanks for the imput………Tom
This misuse of occams razor.
There is the famous, if you hear hoofs clapping, its horses.
That is, as long as your not on the serengeti…
Using occams razor is only possible if the parameters are known. We dont know if panspermia is common therefor its not possible to say its more likely its earthly contaminations. Occams razor can ony be used when you know the relative likelyhood of something.
All this badmouth of his research is “earthistic”, a almost religious belief that life must have orginated on earth since, well maybe since Darvin suggested it so ?
I dont see any reason to think what he found was contaminations, that he has ensured to minimize. The question is otoh if its biological or not, a question that is harder to answer.
This question should be further researched, for example, sample comets and catching meteorites in space.
Tom, I think there maybe a major practical problem to your Mars sample return, Mars circular orbital velocity is about 3.5km/s, so the kinetic energy difference between the sampler and the atmospheric sample is in the thousands of degrees C, The sampling system you propose would also be restricted to sampling at such a high altitude that I’m doubtful that anything originating from near the surface could be collected. Mars dust storms aren’t THAT big.
In theory an alternative would be to use a rotating tether, a tether a few hundred km long made from carbon nanotubes could be spun fast enough so while the main body of the probe remained in circular orbit, at its closest point to the surface the end of tether would briefly be stationary relative to the surface, in theory a sample could be taken at a fairly low altitude, and a new sample could be taken from a different location with each rotation of the tether, with carbon nanotubes the weight of the tether would need to be only about the weight of the sampling device it was supporting.
All up though, it would still be a very challenging mission.
One important point brought up that I myself have asked for years is what is the dark material seen in the lines across Europa? Is it “simply” dirt and rocks dredged up from the moon’s ocean floor? Or is it organic?
A lander probe could answer this question. We may not have to drill under the ice and send in a submersile probe to know what is in Europa’s global ocean. It may be sitting right on the surface in plain sight.
While we keep getting excited about finding ancient traces of water on Mars, the next planet over has several moons the size of small planets containing whole oceans of briny liquid water, more than all of Earth’s oceans combined! Europa’s global ocean may be up to sixty miles deep! And this is not worth a few dollars to explore? Humanity’s priorities are out of whack.
Just develop the conversation on some of the earlier discussion around the probability of organisms ejected from earth reaching a nearby star system. There appears to be some confusion between the lithopanspermia mechanism in which the organisms are within rocks / metoerites etc and mechanisms proposed for possible interstellar panspermia. Some of the previous comments are absolutely correct in that lithopanspermia appears suitable for interplanetary transfer within the solar system, but would be extremely inefficient at interstellar distances, as noted by earlier contributors. If I understand it correctly this is now generally accepted. The inclusion of organisms into cometary bodies via collision, as suggested by Hoyle and Wickramasinghe has been shown to plausibly occurr (a relevant paper is Wallis and Wickramasinge, 2004, ‘Interstellar Transfer of Planetary Microbiota’, Mon. Not. R. Astron Soc, 348, pp 52-61). A major area of controversy is the time organisms could remain viable for in such environments. Wesson (2010) argues that all would become unviable over the timescales required, but that the delightfully termed necropanspermia would lead to the arrival of signifcant volumes of dead organisms in varying states of disintegration – leading to a significant head start for abiogenesis in terms of the available information content (e.g. DNA strands etc etc). Others contend (e.g. Wickramasinge et al 2010, ‘Comets and the Origin of Life’) that a small fraction would retain viability for up to around one million years, just sufficient to allow interstellar panspermia to operate. The evidence for the long term viability of micro-organisms in such environments is problematic, as it requires extrapolation from much shorter term experiments. It is curious to note that the limits of viability appear to be close to the required value, rather than any random value from a few seconds upwards. In terms of natural selection this would imply some form of selection pressure for these rather surprising characteristics of micro-organisms.
It is reasonable to conclude that lithopanspermia (interplanetary panspermia) is established fact, along with necropanspermia. The transfer of viable organisms accross interstellar distances is uncertain but may be possible based on current data. The a priori rejection of Hoover’s findings (and other similar results in the past by Hoover and other researchers) is unjustified, in my view, although clearly the conclusions will need to be scrutinised for possible flaws.
Lots of things have been seen in meteorites, and in images and data from probes and even radio telescopes (eg the ‘wow’ signal) that have made people go : ” What the frack, that looks like XXXX”. The reason why no one is swinging from the rafters over these things is that because it looks like XXXX, because its even enough to convince a few people (even specialists in astrobiology) it is or at least could be XXXX, does not mean there is enough there to build a scientificly iron clad case that it is XXXX. And without an iron clad case, which is why the ‘extraordinary’evidence is needed, anyone writing a paper concludeing that we are not alone is gonna get torn down by the more conservative members of the scientific community. This is a mistake that keeps getting made by people who have seen enough to convonvce themselves and a few others, and think this means they have an iron clad case.
For the case of life in comets it will, IMHO, take a sample return mission to return a chunk of comet and viable organisms to be found inside that chunk. And this will have to happen several times to quell doubts about contamination.
The conventional view is that the abiogenesis of ribosomal-type life occurred on Earth soon after life could take hold here. In fact so soon after the late heavy bombardment, that the first rocks that could show signs of life do so. The conventional view is that evolution operated so quickly thereafter that a modern and advanced group of bacteria, the cyanobacteria, appeared and started building stromatolites well within their first three hundred million years here. The conventional view is that evolution operated so slowly that nothing much more happened till around a billion years ago.
The conventional view is backed by an annoyance. Panspermia does not solve the problem of abiogenesis: unless we also believe in the steady state theory and that intergalactic transfers are possible (as in the directed panspermia of Crick) it just transfers it. It can thus be seen as a flawed device to bypass work on another important problem.
Since we have shown how easily lithopanspermia can transfer life on the local scale we should begin to question the conventional view, and even start to ask if orthodoxy here constitutes an extraordinary claim.
Actually, in my view, the paradigm approach to science is not scientific itself – textbooks are just there to help neophytes come to grips. Science is about the wonder of the journey into uncertainty, not an appeal to authority.
From: http://www.panspermia.org/bacteria.htm
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Amber is tree-sap that hardens and persists as a fossil. This amber had entrapped some bees and then hardened between 25 and 40 million years ago. Bacteria living in the bees’ digestive tracts had recognized a problem and turned themselves into spores. When placed in a suitable culture, the spores came right back to life.
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If we have such examples is it really so unbelievable that life could spread from one star system to another.
With 25 million years to get there even a velocity of 5 km/s would allow it to travel 3.9 * 10^15 km, or 416 light years.
There are approximately 800,000 star systems within this distance that it could intersect with.
At about 5 km/s a star the size of the sun could capture it within 7 billion km. Since most stars are not the size of sun, say 2 billion km is the range at which a star could capture our interstellar voyager, on average.
That would be a circle around the star with a radius of 2 billion km, or a surface area of 1.2 * 10^19 square km. With 800,000 such surface areas within 400 LY there would be 1 * 10^25 square km where capture could take place.
The surface area of a sphere with a radius of 416 LY would be: 2 * 10^32. So our voyager would have a probability of [1 * 10^25]/[2 * 10^32] = 0.5 * 10^-7, or 1 chance in 20 million of getting captured.
Once it had been captured by the star system what are the odds of it being captured by a planet with suitable conditions? As a guess 1 in 100,000? So one chance in 2 trillion for successful colonization?
That is low. Maybe. For a single voyager. Though that estimate itself is on the low side and would increase if one changed a few of the parameters. Such as allowing 45 million years instead of the 25 million or a greater velocity.
When Theia impacted the earth and formed the moon how much material was thrown off of the earth. How much of that, given the violence of the collision might have escaped not just the the earth-moon system but the solar system itself? If life had already started on the earth then it could have not only have been our first interstellar voyagers but might have reseeded the earth after the impact. (or, maybe the life started on theia in the first place and not the earth)
Maybe the earth, and theia, were barren at the time. But such impacts show the possibility for immense collisions that could emit massive amounts of material from a planetary body into space. Potentially hundreds of trillions of projectiles carrying fragments from a once living world to seed dozens of other worlds.
We cannot yet say that such a process isn’t taking place. Or, for that matter, that it is. We know so little of the origins of life. I think the numbers show it to be possible, but maybe not too probable. “Though I like the idea and want it to be probable.”
The above is of course just a rough guess by someone who’s math skills are sadly lacking compared to most who post here.
When our Sol system was forming, the Sun became a natural nuclear fusion reactor in what is known as a T-Tauri phase. A *lot* of dust and debris was blasted right out of our young system into interstellar space. Most stars went through a similar phase.
And when they are dying, most stars blow off their outer layers into deep space, taking a lot of their lighter solar system particles with them. Those are often the colorful planetary nebulae which astronomers like to display for pretty celestial picture time.
How many organics or even living organisms went along on these dusty journeys is hard to determine at present, but we do know that a LOT of dust particles have been spread around the galaxy and beyond for billions of years.
Did some of it “seed” our worlds, and did we seed other systems? Maybe in the far future when we examine the DNA of alien life (assuming they have DNA or its equivalent), we will finally know the answer.
David, I can’t claim much in the way of math skills either, but simply because an object comes within 7 or 2 billion km of a star doesn’t mean the star will alter its trajectory enough to capture it, just as a meteoroid coming within a million km or so of Earth will almost certainly just swing past and carry on its way, it really requires the object be slowed by atmospheric drag, or the combined gravitational interaction with two bodies (planet and moon, or star and planet) for the velocity of the voyager to be slowed enough for capture.
ljk, I was rather interested to see you mention the T-Tauri phase. The nature of this important stage in a star’s life has great importance to many topic we discuss, yet every reference I look up or paper I read on it seems to give very different parameters for it. I have this horrible thought that the seeming failure of observation and/or theory to fit a single pattern has resulted in it being put in the to hard basket, and thence its most important implications being neglected. I mean of course the potential detail, not the gross overall effect thereof.
Andrew is right, David’s concept of a gravitational capture cross section does not work. Any body coming from outside the system will be on a hyperbolic trajectory, and leave just as fast as it arrives. Unless it hits something, or undergoes a very fortuitous planetary flyby braking it down. The cross section for either of these, unfortunately, is many, many orders of magnitude smaller than what David is working from.
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David, I can’t claim much in the way of math skills either, but simply because an object comes within 7 or 2 billion km of a star doesn’t mean the star will alter its trajectory enough to capture it
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Agreed. However the escape velocity of the sun at 7 billion km is around 6 km/s. So at that distance there is certainly going to be some interaction. The object will be drawn towards the star to some degree that will increase its chances of hitting another object in the inner part of that star system.
The initial velocity of the object combined with the velocity added by acceleration due to gravity might very well slingshot the object out of the star system again, after it has passed through the inner part.
Everyone is pointing out that David Lewis has totally forgotten that gravity is a conservative force, yet other than that detail his figures look reasonable to me. Let’s take his term ‘capture’ to mean close approach. Now an object approaching the sun from infinity at 5km/s tangential to a sun centred sphere of 7billion km radius, should have a perihelion of 3.47 billion km/s whereupon it should be moving at 10km/s. We must also here assume that he was taking a typical system of planets to also have this diameter. Thus it ‘just’ needs to loose about 1300m/s at that perihelion in an encounter with a planet, or from the drag of gas (if the system had just started forming) to get the right delta V squared for true capture. Putting the frequency of such interactions at one in 100,000 also seems fairly reasonable if it is passing within the radius of a typical solar system (after all the cross-sectional area of the Hill spheres of the planets in our solar system compared to the area of the solar system has a lesser ratio). Once such a perturbation has occurred its orbit would be unstable, and if we perchance forget that it could be expelled from the system, crash into the sun or giant planet, the probability that it would go on to hit a terrestrial planet or icy moon looks close to one. Even if we factor these in properly we would only lower our estimate by a couple of orders of magnitude.
Of course, if I do a lot more work and flesh out the details David’s whole scenario could come a cropper, but it seems okay for a first glance ballpark model.
Rob: A body is not captured by the Hill sphere of a planet. Rather, the capture cross section of a planet is only a small factor larger than its physical cross section. With planet radii in the 36,ooo km range, compared with the 3.6 billion you give for the system, that makes a factor of 1 in 10 billion for the probability of capture, far from your 1:100,000.
Now, gas drag in a young stellar disk is a different matter. I feel that it would be negligible for larger bodies, but a very small grain of dust just might be stopped. And as others have pointed out, dust certainly does make the interstellar journey, from the insides of stars all around the galaxy. If we imagine some of that dust to be spores, we are getting closer.
Thanks Eniac for your correction. I admit that I haven’t done celestial mechanics since my varsity days. Actually I knew that Hill spheres do not capture planets per se and had meant to write that the cross-section of their area was much much smaller than the 1:100,000 ratio, but I did not want to do the further work of constructing a Monte Carlo of suitable orbits and planets to find that true ratio for capture. I had no idea that the concept of capture area within the three body problem had been solved. The concept of this sort of defined area greatly eases such problems and I will look it up. Thanks again.
Eniac, not only can I not find your concept of capture area around planets for bodies entering our solar system (a lesser problem as a will eventually find it if it exists), but my further calculations raise the spectre that you might have confused the potential of planets with moons capturing stray objects into their own orbit, with their potential to entrap extraterrestrial asteroids to our solar system. Please check the following simple calculation.
In the simplest case, an extraterrestrial asteroid enters our own solar system and encounters Jupiter (the most useful planet in our system for such purposes) moving parallel to its orbit. If its hyperbolic excess is 5km/s its velocity wrt the Sun must be 19km/s from the conservation of energy as it crosses a vacant Jovian orbit. At this velocity it just needs to loose 670m/s for capture. Now Jupiter’s gross gravitational effect can be seen as an elastic collision with a moving object. Thus the velocity of that asteroid can be seen as the velocity of Jupiter (13km/s) + an extra 6km/s. Its velocity after encounter wrt the sun is thus the vector addition of 13 + 6 with the angle between these vectors being 180 – s where s is the turning angle. From basic trigonometry, the square of its new speed is thus
6^2 + 13^2 – 2*6*13*cos(180-s) working in km/s and degrees
Taking this as equalling the square of the capture velocity of 18.33km/s and solving for s, we find we need at turning angle of 33 degrees.
Now looking at the same encounter wrt Jupiter we have an object arriving from infinity with a hyperbolic excess of 6km/s. Jupiter has a mu that is just 1000 times less than the sun and in this special case the hyperbolic excesses are similar, so already your low estimate of the capture area of planets looks dubious. But now work out the turning angle around Jupiter’s mass by an object with a hyperbolic excess of 6km/s and an asymptote 11.9 million km from the centre of Jupiter’s mass. I’m sure that you will also find that it is 33 degrees (though I admit this is a difficult calculation – yet the only difficult calculation here).
The conditions for capture are absolutely ideal here yet they still give a ballpark figure for what that of the planets should be, and it seems vastly bigger than your estimate. You clearly know more than me about celestial mechanics but you must be wrong in this case.
I think the maths of all this has actually been done – see the references in my earlier comment. The key mechanism re interstellar distances would be organic material ejected from planets but then incorporated into comets. The trasfer of rocky debris alone is far too improbable when the maths is done, beyond interplanetary distances. It is the cometary process which comes out as marginally possible or impossible depending on the probability of micro-organisms remaining viable for up to 1my in these conditions (cosmic ray exposure is the key factor in these conditions, UV for more exposed organisms). At the moment there seems little doubt that considerable volumes of material would transfer between systems over time. The debate is over its viability and to what extent non-viable remains might themselves be an important factor in the origin of life re: their information content. The experimental data isn’t precise enough to confirm it one way or the other at the moment, but it it is certainly close to the required value (remembering that the result would stand if only a very small fraction remained viable). Can I just conclude by saying how refreshingly logical and balanced this discussion has been compared to the curious wider reaction, which looks like a classic example of normative science. The idea that the organised elements Hoover reports could be modern contaminants simply doesn’t stand up to scutiny. I’m much less confident about non-biological mechanisms for producing similar forms (I just don’t know much about that side of it), but so far I haven’t seen any serious proposals as to mechanisms that might apply – mostly soundbite rejection, which makes me rather curious!
Anthony Mugan, the maths has definitely been done for the transfer of interstellar ejecta, but they assume that these impacts all occurred post Late Heavy Bombardment. In that case they have been found to be an inadequate mechanism to drive interstellar panspermia, but the “David Lewis Model” assumes that life began much earlier than that and, presumably, became sterilised from the surface of planets many times, being re-infected after each bout. We are thus examining a new problem that has many orders of magnitude more material expelled from a system. This brings up other questions that I have not addressed, such as, is there a still a large “spall zone” (where rock can be ejected with little heating) in the largest impacts? Perhaps the confusion lies in my failure to address these issues.
I feel as if I should add this perspective to my previous comments. I actually believe that the comet model for interstellar panspermia, as outlined by Anthony, is almost a proven possibility (though this very different from believing that it has almost been proven to have happened). By comparison, the model I was investing is obviously very speculative, but I believe it warrants further investigation.
Rob – good point well made! (the ‘david Lewis’ model. I hadn’t thought of that – probably more than one mechanism at play here, you are right.
Perhaps I should finish my analysis of capture potential of a solar system, just in case anyone is interested for further modelling purposes.
The fairest exact solution to the above problem, is if the ET meteorite & Jovian orbits meet at right angles. Such an encounter could increase or decrease the velocity of the meteorite wrt our sun. I will now continue with using parameters taken from above.
A Jovian would see this encounter as having a hyperbolic excess of 23km/s (just the hypotenuse of a triangle with other sides of 19 and 13). To them the encounter angle is not 90deg but arctan(19/13) = 55.6. We would need this angle to reduce for capture.
Now, as above the new velocity wrt the Sun is a vector sum, this time of 23km/s and 13km/s, and, as before its speed must be < 18.33 km/s. Using the cosine rule we find that its new angle wrt the orbital path of Jupiter is <= 52.7 for capture. Thus a turning angle of 2.9 degrees is needed.
To get a turning angle of 2.9deg with a hyperbolic excess of 23km/s wrt Jupiter, we only need to approach Jupiter at just less than 10 million km. This is very close to the 11.9 million km found for the parallel approach example, and implies that the radius for capture is fairly uniform for all angles of approach. The PROPORTION of meteorites captured at each angle is, however, far from uniform. To adjust for this we need a new parameter that I will call p©.
Now p© = 1 for a parallel encounter, and 0 for encounters that approach within 2deg of an antiparallel encounter and 0 < p© .5, but here it must loose ALL that excess energy so it will be < .5. The hyperbolic excesses wrt the sun that we use in this model are low however, so it will not be very much lower. Lets put p© = 1/3
Thus for our system the chance of capture is
1/3 x the capture area of Jupiter / the area of “our” (= David Lewis) solar system
= 1/3 x (10 / 3500)^2 = about 1 : 400,000
I cannot help noticing how close to David’s original estimate this is.
Rob,
You make a good argument. I had counted as capture area of a planet the cross-section at which a body would be directly captured by the planet (through collision or aerobraking). I had further characterized the flyby effect (capture with respect to the sun caused by a planetary flyby) as “very fortuitous”.
Your math shows that the flyby is actually very much more likely to get an object captured than a direct hit, and I think you are right.
Of course, the reverse should be true, too. An object captured in this manner will be much more likely to be ejected from the system in a second encounter than have the direct collision required for panspermia. This could introduce a very substantial factor in disfavor of capture, perhaps undoing most of the “flyby cross-section”.
Yes Eniac, the way the object is captured would make its new orbit pass close to Jupiter. Because the object will typically still have a high orbital velocity compared to Jupiter, for straight thermodynamic reasons, I can say that every time it passes close enough to Jupiter for delta V considerations to allow the thought of expulsion, it is actually more likely to loose energy, thus its likely number of subsequent close encounters with Jupiter will be > 2. Because of the factor fc I used earlier I should probably make this number 3.
I thus need to calculate the ratio of likely ‘close’ (as defined by delta V conciderations) encounters with terrestrial planets (were our intruders orbit could be lead away from intersecting Jupiter’s) to those of Jupiter. Earlier I suggested a factor of 100 should be added for such process, so here a ratio of 300:1 for rocky planet: Jovian interactions would be needed. For our system that would be optimistic but it ‘sounds’ fair enough to be correct for the typical system.
Oops, above I wrote my ratio around the wrong way. 1:300 is optimistic but 300:1 is ridiculous (your would need a system with tens of thousands of large terrestrial planets for every Jovian!)