Is Astrobiology a Legitimate Scientific Field?

[Lacy33]

Was wondering if "Astrobiology", as in SETI and The movie Contact is a "real" science?

 

[lisab]

Nope, it's no joke. You can study Astrobiology at my alma mater, the University of Washington.

http://depts.washington.edu/astrobio/

 

[Lacy33]

Nope, it's no joke. You can study Astrobiology at my alma mater, the University of Washington.

http://depts.washington.edu/astrobio/

Just fringe so I am told.
Thank you Lisab,
I was taking some time to look about their website and at some of the work they are doing. It is too bad and very sad that not only that group but SETI are also struggling with funding harshly. I remember when SETI lost some of the funding from NASA. Was it NASA? It's been a long time.
Well since this is not 100% crank, is there anyone else who is interested in this study if only a bit curious.
I don't go much for aliens, but I guess there has to be something living or evidence of something once living on other rocks besides this one my family and neighbors crash landed on.
s

 

[tribdog]

Oh I believe in aliens. I believe i alien civilizations more advanced than ours even. I don't believe any have ever visited us, but I believe they are out there. I don't even think that is considered fringe anymore. Fringe would be believing that we are the only ones in the Universe.

 

[Lacy33]

Oh I believe in aliens. I believe i alien civilizations more advanced than ours even. I don't believe any have ever visited us, but I believe they are out there. I don't even think that is considered fringe anymore. Fringe would be believing that we are the only ones in the Universe.

Ya, that's interesting and probably right on lotsa points. But until we take the "believe" out of that, the study will be in mad time war for the few pennies still available for research. And unless these aliens are going to have any military value to US government, the research is going to slip further down.
May... I give you one of my favorite hats and join you on the intersection?
You take one side I can take the other.
S

 

[Chronos]

Evidence of life on exoplanets is not as difficult to find as it might appear. Detection of significant amounts of oxygen in the atmosphere of an exoplanet would be a compelling indicator.

 

[Ulagatin]

I am a student (only in Year Ten) and I personally believe in aliens - I'll state why in a moment (it is important first to take a broader view, which may initially appear tangential) and I hope this post doesn't appear too philosophical, as I've tried for a scientific approach.

Look at the probability of life on Earth (one tiny planet, in an average solar system, in just one average galaxy in the [finite/infinite?] universe). The laws of physics allow for consciousness, and for human intelligence (through biochemical processes) and for "science" itself to exist (physics being the most fundamental branch of all of science). Ultimately, what is the chance that we are the only intelligent beings in the galaxy, let alone the universe? It's very likely small.

And why is there such order and beauty amidst the chaos that we call the universe? Mathematicians and physicists study non-linear dynamics (commonly referred to as chaos theory) and this theory/tool is so effective in modern society - it allows us to predict weather for example (although only for a few days at a time) and various other natural phenomena. Is it possible that non-linear dynamics (in reality, and not the mathematical construct in this case) plays a role in the creation of life itself?

All of the elements of the periodic table are obviously formed from protons and neutrons which then bond to create the other elements further on the table. This lead to the production of Earth's atmosphere which is crucial to life, many many many years ago.

Perhaps this is where astronomy/astrophysics/cosmology/chemistry comes in, to answer an astro-biology question.

What "systems" evolved from the big bang, and from the formation of stars etc, to lead to life? Perhaps there is some probability function that defines the chance of "non-linear dynamics" of the universe leading to the creation of intelligent life on Earth.

You may be thinking at the moment about evolutionary theory and genetics - this itself is a very chaotic system. But what gave rise to evolutionary theory? Evolution, while being a difficult theory to fully understand, particularly from a mathematical perspective, suggests that there is such a thing as genetic intelligence (it may not state this, but logically it follows that evolution suggests this hypothesis).

Referring back to the idea of a probability function defining the chance of non-linear dynamics to lead to the creation of intelligent life, what is this truly stating [in short, what is the probability function based on, or indicating to us]? Position in the universe is possible (how much sense does this make - can we truly define a location if the universe is infinite? If it's finite, we certainly can). But if this is the case, then why do we not find intelligent life on planets close to Earth (i.e. in the solar system)? The issue here however is the chemical structure of the atmosphere, and the fact that these other planets lie outside the so-called "habitable zone".

Does life occur in some "quantum-entangled" state, where with life here on Earth, there may be intelligent life (perhaps searching for us) somewhere entirely distant from Earth in the universe that somehow has a quantum-entangled relationship with intelligent life on Earth? (This is only a metaphor, please don't take it too literally!).

Many of these ideas are difficult to communicate effectively, and only remain ideas, unless rigorously proven.

Ultimately, we may find answers to all of these questions if physicists are truly able to develop a theory of everything (my own area of fascination is string theory/cosmology/high-energy physics), in which there is much philosophical debate currently, regarding the mathematical aspects - questions such as "will we ever be able to describe a "theory of everything" using only mathematics and physics?" arise. Fascinating stuff.

Perhaps further research in non-linear dynamics may be able to try and answer some of these questions too.

Cheers,
Ulagatin

 

[LURCH]

The researchers who study the infamous "Mars rock," as well as the robotic missions on the surface of Mars (and hopefully Europa), are examples of Xenobiology (or Astrobiology), so it's not all that "fringe."

 

[Arch2008]

http://en.wikipedia.org/wiki/Drake's_Equation
People always spend a lot of effort explaining how there must be intelligent life elsewhere for a lot of intelligent reasons, and then sort of gloss over Fermi’s point.
http://en.wikipedia.org/wiki/Fermi_paradox
I believe there is no reason to doubt in simple life elsewhere in the universe. Chemistry and physics should conspire to create exobiology in lots of places. However, intelligent life eventually leads to advanced civilizations. That’s the whole point, right?
http://en.wikipedia.org/wiki/Kardashev_scale
A type III civilization would be here in our solar system right now. Their Dyson sphere around the Sun would be a little hard to miss. There are about a hundred million galaxies within a billion light years of us and not one single type IV/V civilization anywhere in that sphere has spread here. Either interstellar/intergalactic transportation is impossible for some unknown reason, or there simply are none.
Of course, one could argue that the universe is larger than a sphere with a diameter of a mere two billion light years or that only a few septillion planets in such a sphere are too small a sample, or that a few billion years was too short a time for them to get here. Maybe intelligent life/advanced civilizations are everywhere in the parts of the universe that are beyond our Hubble sphere, i.e., where we will never find each other. Their existence would remain forever a belief.

 

[Lacy33]

Thank you members who have responded to this thread.
I am very interested in the subject personally and would like very much to see people continue to do professional research in these hard times.
A few of the subjects I have invested in through the years have a direct science basis, but suffer distance from the solid center topics that get most of the attention and funding.
Perhaps some of the posters in the thread are already invested in this field, wonderful.
In addition I have contacted some people who currently are to have a look at the thread and hopefully share what is new in astrobiology.

 

[Chronos]

Apologies, Shoshana, I forgot to include links:
http://www.markelowitz.com/exobiology.htm
http://biocab.org/Exobiology.html

 

[Lacy33]

Apologies, Shoshana, I forgot to include links:
http://www.markelowitz.com/exobiology.htm
http://biocab.org/Exobiology.html

These links are wonderful. Thank you.
So is the study called astrobiology or better referred to as exobiology?
Do you know where R. Mark Elowitz is located?

 

[Arch2008]

Well at least NASA treats Astrobiology as the overarching head of all research for life in the universe:

“Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. This multidisciplinary field encompasses the search for habitable environments in our Solar System and habitable planets outside our Solar System, the search for evidence of prebiotic chemistry and life on Mars and other bodies in our Solar System, laboratory and field research into the origins and early evolution of life on Earth, and studies of the potential for life to adapt to challenges on Earth and in space.”

http://astrobiology.nasa.gov/about-astrobiology/

Exobiology is a component of astrobiology attuned to the actual search for this life.

“The Exobiology Branch conducts research in Exobiology seeking to increase our knowledge of the origin, evolution, and distribution of life in the universe. Answers are sought to questions such as: To what extent did chemical evolution occur in the primitive bodies of the solar system? How did life originate on the Earth, and what role did minerals play? What evidence exists regarding the early interplay between biological and environmental evolution?”

http://exobiology.nasa.gov/

I hope that helps.

 

[Lacy33]

Thank you very much.

 

[LURCH]

http://en.wikipedia.org/wiki/Drake's_Equation
People always spend a lot of effort explaining how there must be intelligent life elsewhere for a lot of intelligent reasons, and then sort of gloss over Fermi’s point.
http://en.wikipedia.org/wiki/Fermi_paradox
I believe there is no reason to doubt in simple life elsewhere in the universe. Chemistry and physics should conspire to create exobiology in lots of places. However, intelligent life eventually leads to advanced civilizations. That’s the whole point, right?
http://en.wikipedia.org/wiki/Kardashev_scale
A type III civilization would be here in our solar system right now. Their Dyson sphere around the Sun would be a little hard to miss. There are about a hundred million galaxies within a billion light years of us and not one single type IV/V civilization anywhere in that sphere has spread here. Either interstellar/intergalactic transportation is impossible for some unknown reason, or there simply are none.
Of course, one could argue that the universe is larger than a sphere with a diameter of a mere two billion light years or that only a few septillion planets in such a sphere are too small a sample, or that a few billion years was too short a time for them to get here. Maybe intelligent life/advanced civilizations are everywhere in the parts of the universe that are beyond our Hubble sphere, i.e., where we will never find each other. Their existence would remain forever a belief.

I have always felt that the Fermi paradox could be resolved within the confines of the Drake equation (given the typically generous variables), if and only if some mitigating factor could be shown to restrict the genesis of life to a relatively late period in cosmological history. Perhaps waiting on the existence of second or third generation stars (main sequence, of course) or a sufficient abundance of heavy metals.

 

[Arch2008]

My pleasure Shoshana. I have only found a homepage for Elowitz, which does not have an e-mail or explain much about the AAS he is a member of. Maybe LURCH has the scoop.

Interesting post LURCH. As you may know, the giant stars that create all of the heavier elements form rapidly in stellar nurseries and then go nova after only a few million years. The shock wave from their explosion may actually contribute to the formation of smaller stars like our Sun by causing the collapse of hydrogen clouds. So after a few million years, many solar masses of these elements should already be there when longer lived stars like ours form. Something like half the stars in the Milky Way are ancient red dwarf stars that are billions of years older than the Sun, yet there is no evidence of one civilization billions of years older than ours around. Intelligence is not necessary for an organism to survive. With perhaps 10^12 species that have existed on Earth, less than 10 could build a campfire.

 

[Chronos]

I have more dire solutions to the Fermi paradox - 1: Advanced civilizations learn how to destroy themselves before learning how to resist the urge. 2: Interstellar travel is as difficult as it appears to be. I agree with Arch2008 that intelligence is not necessary for survival, but, it does appear to be an evolutionary trend - e.g.,
http://www.space.com/searchforlife/seti_intelligence_030821.html

 

[Lacy33]

I have more dire solutions to the Fermi paradox - 1: Advanced civilizations learn how to destroy themselves before learning how to resist the urge. 2: Interstellar travel is as difficult as it appears to be. I agree with Arch2008 that intelligence is not necessary for survival, but, it does appear to be an evolutionary trend - e.g.,
http://www.space.com/searchforlife/seti_intelligence_030821.html

1. the point was brought up in the movie Contact.
2. one fish might say, "I don't know how happy I am in this water, but when force of the water moves me, I notice I am further away from my favorite rock. How did I get here?"
I realize science has to be based in fact but that remains as we understand it.
The physical structure of other planets is well documented and that's exciting. Basic life forms are exciting to wait for. A higher intelligence, I suspect may be very different from what we have ever imagined. On our very own planet we have we have so many species. Does anyone know how many recorded?
I don't expect to be able to communicate with an alien any more than I might have a conversation with a puffer fish, but I would hope that if the alien finds us first I hope they can just read my mind without getting their version of a headache. Could be a brane ache.

 

[Widdekind]

...
http://en.wikipedia.org/wiki/Kardashev_scale
A type III civilization would be here in our solar system right now. Their Dyson sphere around the Sun would be a little hard to miss. ...

Why ?

A rigid Dyson-Sphere would completely block (& absorb) their stars' Luminosity. A Dyson-Sphere would be completely black.

For, such advanced Type II+ civilizations would have the capacity to turn Energy into Matter. The Sun's Luminosity amounts to ~ 4 megatons of matter per second.

By comparison, a typical Imperator class Star Destroyer (allegedly) masses 27 megatons*, while Captian Kirk's Enterprise massed ~0.2 megatons**. So, by the time a race gets to be Type II, they could convert their star's Luminosity straight into matter, to be used in assembling huge fleets of massive star ships (for example).

* http://www.kitsune.addr.com/SF-Conversions/Rifts-SW-Vehicles/Imperator_SD.htm
** http://www.comicvine.com/uss-enterprise/18-54366/​

More seriously, there is no fundamental reason why a Dyson-Sphere would have to radiate in the IR. A Type II civilization could make better use of the energy, w/o wasting it as IR radiation.

 

[Arch2008]

You answered your own question. As you quoted me, my statement was about a type III civilization's Dyson Sphere "in our solar system". A type III controls the energy of a galaxy, and so would control the energy of our Sun. Since obviously no black Dyson Sphere, or any other kind, encircles our Sun, then there is no type III in the Milky Way and by extrapolation no type IV in a billion light years, or they would be here too.

 

[Widdekind]

I should have read your post more carefully, sorry.

Would, however, it be fair to say, that Alien Dyson Spheres need not be visible? Wikipedia shows a DS re-radiating in the IR. But, if they could actually convert energy --> mass, there's no reason why a distant T-II civ.'s DS would be visible at all.

 

[Arch2008]

http://www.daviddarling.info/encyclopedia/D/Dysonsp.html

Who’s to say what a technology a thousand centuries (or millennia) ahead of ours might do? Perhaps a portion of the mass we ascribe to dark matter is actually a type III/IV civilization that is somehow hidden from our view? One can imagine a lot, however, imagination should not be our only guide. There must be a plausible reason that no type III civilization exists in our galaxy and no type IV anywhere nearby, because by definition, they would be right here using the energy of the Sun in some obvious way.

Let’s say we build a DS around the Sun by 3001 and use some of the energy to power a massive laser that propels a colony ship with a light sail to a nearby star at a fraction of the speed of light. (Pretty basic stuff, really) This colony grows and after one millennium we and our colony both send out updated colony ships. This modest exponential growth continues and every ten millennia our colonies’ number grows by a factor of a thousand (2-4-8-16-32-64-128-256-512-1024), i.e., a million colonies after twenty thousand years, a billion colonies after thirty thousand years and so on. Even at this slow rate, in much less than a thousand centuries, we would be a type III civilization. Thus, a type II civilization would relatively quickly become a type III. Whether or not we could see a distant type II, or type IV, is rather irrelevant. They should be here.

I see no reason that there shouldn’t be life in lots of places, but apparently no advanced civilizations have sprung from that life in our part of the universe.

 

[Widdekind]

That seems like a valid point. According to archaeologists & geneticists, early modern humans took ~100,000 years to spread from Africa to Australia & the Americas. They never took time out to question the time-scales involved. We, their sons, should probably prepare for similar epic journeys, to the stars.

A journey of a thousand miles begins with the first step -- Chinese Proverb​

It could be, that Earth Life is about as "precocious" as Life gets in our galaxy. For, primitive microbes on Habitable Planets must first produce enough Molecular Oxygen (O₂) to make a protective Ozone Layer (O₃). But, Ozone is manufactured from Oxygen by UV from the central star, and bigger, brighter, hotter stars, having "harder" radiation, generate more UV. Thus, more Ozone is formed from the same amount of Oxygen, so an Ozone Layer w/ Optical Depth ~1 appears sooner. And so, Complex Lifeforms can evolve earlier*.

* https://www.physicsforums.com/showthread.php?t=278815​

The most "precocious" Habitable stars are F-Class, but the Sun (G-Class) is not far behind. Conversely, K-Class stars, having such "soft" radiation, require billions more years to manufacture sufficient Ozone.

Moreover, w/in 1000 light years, there are:

• ~300,000 F-Class stars*
• ~500,000 G-Class stars**
• ~1,000,000 K-Class stars***

Thus, most Habitable star systems will probably "incubate" for billions more years, before they evolve Complex Life. At present, those worlds look like the early Earth, ~1 billion years ago, with primitive microbes, confined to the oceans, steadily producing Oxygen from under the protection of the water, as their (orange) suns blaze overhead. But, above the water-line, they are barren, desolate, and look like Tatooine (as it were).

* http://www.solstation.com/stars3/100-fs.htm
** http://www.solstation.com/stars3/100-gs.htm
*** http://www.solstation.com/stars3/100-ks.htm​

PANSPERMIA:

Life, while common across our galaxy, is apparently absent from others*.

* https://www.physicsforums.com/showthread.php?t=176698&page=2&highlight=wickramasinghe [post #18]​

 

[Arch2008]

At least statistically, some species has to be the first type I/II/III, so maybe we are going to be the ones initiating “First Contact” with some distant exobiology. In a way, our species has reached a defining point in our evolution. A thousand centuries ago we were basically herd humans and arguably, a thousand centuries from now we could colonize the entire galaxy. I don’t think that we need worry about destroying ourselves in the meantime, although we should endeavor to avoid this. This isn’t optimism. I believe that if we were so totally incompetent as to possibly destroy our environment or civilization through miscalculation, then we should logically be too incompetent to actually accomplish such total destruction. However, the galaxy is more dangerous than any primitive human instinct. We already suspect what an asteroid or comet impact might do to life on Earth. The radiation from a supernova could prove disastrous to all life within forty light years. A merger of two neutron stars would be even more cataclysmic and a Gamma Ray Burst may sterilize anything in its path out to a thousand light years. Our world is a graveyard for countless extinct species and a bull’s eye floating in space. It is a fact that as long as we live only on the Earth and only around this star then we are doomed. Other species on other planets orbiting other stars are also subject to this fate. For this reason, we or they would not remain invisible. We need to find each other. Ultimately, a type III should be a union of our species and all exobiology in a galactic civilization that would perhaps be beyond extinction.

 

[buffordboy23]

On our very own planet we have we have so many species. Does anyone know how many recorded?

I believe that there are close to 2 million recorded species. Predictions suggest that there are many millions more (5-30 million in all) that we have yet to discover.

 

[Widdekind]

Prof. Chandra Wickramasinghe says, that Life began in the "gooey" cores of comets. There, radioactive decay could have heated up the interior, enough to form a sludge of liquid water & organic compounds, which could have reacted to make (primitive) microbes. Indeed, the Deep Impact mission found evidence of clay on comet Temple 1. And, the only known mechanism for making clay is w/ liquid water*.

* National Geographic Naked Science -- Birth of Life (TV)​

Now, the Triple Point of water is (613 Pa , 273.01 K)*. Below pressures of 613 Pa, ice sublimates straight to a gas. Thus, comets must be big enough, to produce enough core pressure, to allow liquid water to form from melting ice.

* Iain Gilmour & Mark A. Sephton. An Introduction to Astrobiology, pg. 92.​

We use a simple 1-Cell model of a (relatively small) comet. For such comets, of (approximately) constant density, we have (from Hydrostatic Equilibrium):

$$\frac{\partial P}{\partial r} = - \, g \, \rho$$​

Thus (1-Cell Approximation):

$$\frac{\Delta P}{R} \approx - \, g_{R/2} \, \rho$$​

For constant density, g ~ r, so:

$$P_{center} \approx R \times \frac{1}{2} \, g_{surface} \, \rho$$​

But:

$$g_{surface} = \frac{4 \, \pi}{3} \, G \, \rho \, R$$​

So:

$$P_{center} \approx \frac{2 \, \pi}{3} \, G \, \left( \rho \, R \right)^{2}$$​

Now, typical comet densities are around 1.6 g cm^-3*. Thus, assuming a central pressure of 613 Pa, we have that:

$$R_{min} \approx \rho^{-1} \, \sqrt{\frac{3 \, P_{c}}{2 \, \pi \, G}}$$
$$\approx 1300 \, m$$​

Thus, potentially "microbe-habitable" comets would need to be about 3 km across, to create sufficient core pressures, to support liquid water. For comparison, comet Temple 1, containing clay, is ~6 km across**. This is consistent w/ our (simple) analysis.

* https://www.physicsforums.com/showthread.php?t=278022 [post #7]
** http://en.wikipedia.org/wiki/Comet_Tempel_1​

 

[LURCH]

...Now, the Triple Point of water is (613 Pa , 273.01 K)*. Below pressures of 613 Pa, ice sublimates straight to a gas. Thus, comets must be big enough, to produce enough core pressure, to allow liquid water to form from melting ice.

I don't know if I would make that assumption. If ice is trapped in a pocket within a rocky comet, and the temperature of the rock begins to warm, the ice will indeed sublimate to gas. However, if this gas cannot escape the pocket, the gas will form a trapped "atmosphere," with pressure increasnig as more ice sublimates. At sufficient pressures, ice begins to melt to form liquid water.

Just saying; gravity is not the only way to create the required pressure.

 

[Vanadium 50]

Pity the thread got hijacked.

Lurch, you're right. Gravity is not the only way to create that pressure. In fact, comets are too small to be held together by gravity; it's largely chemical bonds that keep them together.

Also, the calculation done by Mr. Widdekind uses his own...um...highly speculative model for densities. Observations, such as those made on 19P/Borrelly, 9P/Tempel 1 and Shoemaker-Levy 9 indicate that the densities of these comets are lower than Mr. Widdekind predicts, by possibly an order of magnitude or more.

 

[Widdekind]

I don't know if I would make that assumption. If ice is trapped in a pocket within a rocky comet, and the temperature of the rock begins to warm, the ice will indeed sublimate to gas. However, if this gas cannot escape the pocket, the gas will form a trapped "atmosphere," with pressure increasnig as more ice sublimates. At sufficient pressures, ice begins to melt to form liquid water.

Just saying; gravity is not the only way to create the required pressure.

Prof. Wickramasinghe et al have calculated that Comets must be 2 km across to generate sufficient core pressures, and 8 km across to generate sufficient heat for "millions of years*".

* International Journal of Astrobiology (2007). The Origin of Life in Comets.​

If Biogenesis happened in comet cores, then those primary microbes were probably Chemosynthesizers (??). This is consistent w/ claims, that the earliest Earth Life formed around volcanic vents (??).

 

[LURCH]

Prof. Wickramasinghe et al have calculated that Comets must be 2 km across to generate sufficient core pressures, and 8 km across to generate sufficient heat for "millions of years*".

* International Journal of Astrobiology (2007). The Origin of Life in Comets.​

If Biogenesis happened in comet cores, then those primary microbes were probably Chemosynthesizers (??). This is consistent w/ claims, that the earliest Earth Life formed around volcanic vents (??).

I'll follow the link after I get through the forums today. However, it sounds as though we're counting on gravitational pressure to generate heat and core of the comet. Is that the model used in that article? I was assuming an external heat source, like the sun (or other star, in the case of solar comets).

 

[Lacy33]

Thank you for the continued interest in this thread. I am not qualified to contribute but so enjoy learning as it goes on.

 

[Widdekind]

I'll follow the link after I get through the forums today. However, it sounds as though we're counting on gravitational pressure to generate heat and core of the comet. Is that the model used in that article? I was assuming an external heat source, like the sun (or other star, in the case of solar comets).

I understand, that Prof. Wickramasinghe considers Nuclear Fission, from the heavy elements w/in the rocky parts of the comet, as the primary heat source in the comet's core.

 

[Chronos]

I seriously doubt there is enough fissile material in your typical comet to be of any consequence.

 

[Widdekind]

I seriously doubt there is enough fissile material in your typical comet to be of any consequence.

Prof. Wickramasinghe et al (cited above) conclude an ~8 km comet can keep its core warm "for millions of years", which they say is sufficient for Life to have developed amidst the organic-rich goo in that comet core.

That primordial microbes were comet-carried Chemosynthesizers, adapted to the dark, is consistent with claims that Earth's earliest Lifeforms were Thermophiles found around volcanic vents. Indeed, all Earth Lifeforms can be depicted on "the universal tree of life based on ribosomal RNA data". This Phylogenetic Tree groups Earth Life into three (3) clusters: Bacteria, Archaea, and Eukarya. And:

The organisms closest to the center of the tree, those that populate the deepest & shortest branches, are the Thermophiles & Hyperthermophiles. These are heat loving microscopic organisms found near hot springs and deep-sea hydrothermal vents... One interpretation of the ribosomal RNA tree is that the course of evolution has generally moved from high to low temperatures.

Another important feature of the ribosomal RNA tree is that the majority of the deepest branching organisms do not use light as an energy source. This suggests that Photosynthesis may be a later development than processes utilizing geochemical energy sources... The phylogenetic tree seems to be telling us that our Last Common Ancestor may have been similar to heat-loving Chemosynthetic organisms that populate hydrothermal vents today*.​

Microbes buried in comets' cores could, also, be most likely to survive impacts. Perhaps comets gave the early Earth heat-loving microbes, which first flourished around volcanic vents, which were (somewhat) similar to the hot hearts of their parent comets.

* Iain Gilmore & Mark A. Sephton. An Introduction to Astrobiology, pg. 39.​

 

[Widdekind]

Pity the thread got hijacked.

Lurch, you're right. Gravity is not the only way to create that pressure. In fact, comets are too small to be held together by gravity; it's largely chemical bonds that keep them together.

Also, the calculation done by Mr. Widdekind uses his own...um...highly speculative model for densities. Observations, such as those made on 19P/Borrelly, 9P/Tempel 1 and Shoemaker-Levy 9 indicate that the densities of these comets are lower than Mr. Widdekind predicts, by possibly an order of magnitude or more.

They have Specific Gravities of 16 or more ? That's denser than Lead & Mercury. Even Osmium & Iridium only have densities of ~22 g cm³.

Carroll & Ostlie (Intro. to Mod. Astrophys., pp. A3-A4) cite densities, for Icy Bodies, of ~2 g cm³. And, Prof. Wickramasinghe et al corroborate, that comets must indeed be about 2+ km across.