Introduction to Life in the Solar System: Humans and our associated skin on the third rock out from the Sun are lords of life forms in the solar system. But, we’re not unique lords, just lords. Other abodes in the solar system, most probably Mars, Jupiter, Europa, and Saturn are anywhere from possible to probably habitable abodes to simple microbial life forms; perhaps something slightly above and beyond that. Taking each abode in turn…
But first a pat on the back for those terrestrial microbes; all those germs, bacteria, unicellular critters and even viruses (though viruses, depending on your perception of their actually being alive as we normally define ‘alive’ might exclude them from this discussion). They are tough, I mean they boldly go, survive, and even thrive where even angels fear to tread, far fewer humans.
Microbes can live in environments where other multicellular critters also fear to tread and often can’t: from the coldest terrestrial environments, up to the near boiling temperatures, from deep underground to the heights of the atmosphere, from inside water-cooled nuclear reactors and the interior of rocks, to intensely saline, acidic and alkaline environments, to ecosystems where the sun never shines, like the abyssal depths.
They can even survive outer space. Bacteria survived on the surface of the Moon – on Surveyor Three. This was possibly the most significant discovery of the entire Apollo Moon program and it hardly even rated a mention. Astronauts from the Apollo 12 mission brought back to Earth parts of the unmanned Surveyor Three Lunar Lander. Terrestrial bacteria on those parts survived the lunar vacuum, solar radiations (UV, etc.), the massive temperature extremes, and lack of water and nutrients. Experiments since done in low earth orbit have confirmed that given just minimal shielding, bacteria can indeed boldly go!
You’d be aware of how difficult it is to totally sterilize something, be it hospital equipment or a spacecraft bound for a Martian landing. They’re tough – have you ever read about a mass extinction event where a bacterial species, unlike say the multicellular dinosaurs, went poof? Microbes are easy to transport. They can be blasted off the surface of the Earth, shielded from radiation by the debris, and survive to land on another world and be fruitful and multiply. There’s little doubt that somewhere way out there, terrestrial bacteria have hitched a ride to the stars, bolding going where lots of microbes have gone before! Translated, I firmly expect that the universe (including our solar system) is teaming with microbial life in all sorts of places. The less than glamorous catch is that LGM is not going to stand for Little Green Men, but Little Green Microbes.
On Earth, microbes rule, OK? The biomass of all those bacteria, etc. put together easily equals the biomass of every other multicellular plant and animal added together. One could easily argue that microbes, not humans, are the jewels in God’s crown – He made so many of them, and talk about being fruitful and multiplying. The number and mass of micro-organisms are many orders of magnitude greater than the numbers and collective mass of humanity. If fact, there are millions of microbes living inside you – most beneficial. In fact, it could also be argued that you are nothing more than an elaborate colony of billions of unicellular organisms – your individual cells that make you, you.
Microbes have another decided advantage over more complex life forms, like plants. Solar energy (photosynthesis) isn’t the only kind of energy available to organisms. Even on Earth, there are lots of organisms, mainly unicellular ones, that use chemosynthesis as the means by which they directly derive their energy needs, directly as in from the chemicals in their environment. Today we know a great deal about chemosynthesis and the organisms that can produce organics from inorganic substances and derive energy from the process.
When I was a high school biology student (1962-63), it was absolute gospel (and no correspondence would be entered into contrary) that our Sun was the be all and end all of the existence of terrestrial life. No sun; no life. All life ultimately depended on photosynthetic plants which in turn couldn’t exist without sunlight. And while we don’t get our energy directly via photosynthesis, we’re still dependent on solar energy since we eat the plants or the animals that eat the plants.
A well known, if the little understood example of chemosynthesis is the colonies of microbes (dubbed ‘rusticles’) that are eating the iron structure of the RMS Titanic, resting some four kilometers below the surface of the North Atlantic. Within another generation or two, the famous shipwreck will have been basically consumed by microbes, without any benefit bestowed by our sun. Also from the marine environment, the entire ecological communities’ part and parcel of hydrothermal vent systems is ultimately based on chemosynthesis.
From terrestrial environments to those of outer space and our solar system is but perhaps a small step for microbes. Even back in those high school days, however, I seem to recall speculation by no less a scientist than the late Carl Sagan about the possibility of a non-photosynthetic based ecology in the atmosphere of Jupiter which gladdened my heart no end – however, it wasn’t Jupiter that broke the photosynthetic mould, but good old Mother Earth herself as it hydrothermal vent ecosystems among many others now known. So gospel ain’t gospel any longer!
Now back to taking each abode in turn…
Mercury: The planet Mercury, the closest planet to our Sun, unfortunately, lacks any atmosphere to speak of, and broils on the side facing the Sun and freezes on the side facing away – much like our Moon, and is, in fact, is similarly heavily cratered. There’s no liquid water on the surface, and overall, like our Moon seems totally inhospitable. Bacteria might be able to exist in a dormant sort of way in 100% sheltered niches, but actively survive and thrive they do not.
Venus: The planet Venus had long been thought of as Earth’s twin sister. It’s the second planet out from the Sun and has a size and density very close to terrestrial values. It also has an atmosphere. Being closer to the Sun than Earth, Venus was, pre-space age, thought to be warm and moist, a totally tropical environment of lush vegetation where maybe dinosaur-like creatures or dragons roamed and chased scantily clad maidens! Alas, once space probes crossed paths with, and landed on, Venus, such dreams of a tropical paradise was dashed. Well its tropical alright, if ‘tropical’ means a surface temperature of 900 degrees Fahrenheit. But the atmosphere, mainly carbon dioxide – a greenhouse gas – is so thick and dense that the atmospheric pressure is way massive relative to Earth’s. So, Venus turned out to be more akin to Hell than a tropical Heaven. No life here!
But wait, perhaps such a judgment is premature. What happens here on Earth as you climb up a high mountain? Well, the temperature drops and the air gets more rarefied – and so too on Venus. In the upper atmosphere, the temperature and pressure of Venus drop to more terrestrial surface conditions. There can’t be surface life-as-we-know-it on that planet, but what about simple, say microbial life existing in the upper atmosphere?
Earth (Terra): Home! Nothing further need be said.
The Moon (Luna): Like Mercury, our Moon is airless and subject to extremes in temperature depending on whether the Moon is facing towards or away from the Sun. While the first couple of crews of Apollo Moon landing astronauts were quarantined after their missions, just in case, no extraterrestrial life forms of any kind were ever discovered. But that’s not quite the end of the story. The Apollo 12 astronauts brought back with them a few bits and pieces of the unmanned Lunar Surveyor that had landed about three years previously. Terrestrial bacteria within those bits and pieces were found to still be viable after exposure to the lunar vacuum, intense radiation exposure, and temperature extremes. While hardly indigenous Lunar life forms, they give credibility (as if any were needed) that microbes are composed of the right stuff to survive the rigors of outer space.
The Moon (Luna 2): The Hollow and Inhabited Moon Theory: Once upon a time, not all that long ago, in fact, there really wasn’t a satisfactory scientific explanation for the natural origin of our satellite, Luna. All the three major theories had fatal flaws. And thus, the possibility that the Moon wasn’t natural at all, but some sort of hollow world, perhaps a UFO base and colony ship, wasn’t all that implausible to some. However, some scientific genius came up with a fourth natural explanation that satisfied all the previous scientific stumbling blocks. Thus, using Ockham’s (or Occam’s) Razor as a guide, methinks the hollow moon theory has proved to be a bit, well, hollow. Pity! However, if UFOs should prove to be space vehicles, the products of alien intelligence, then it logically follows that ET will have explored our moon, maybe even have a base of operations there. That might account for part of the various transient lunar phenomena (TLP) witnessed over many centuries.
Mars (The Red Planet): Microbial life on the red planet Mars is just about a sure fire a thing as death and taxes, albeit it’s probably spread very thinly.
The concept of there being not only life but intelligent life on the red planet Mars has been a part of the imagination of astronomers and the general public for the better part of a century. Science fiction novels and short stories, films and TV episodes, and popular astronomical books all speculated on Martians and the dying Martian civilization with its system of canals warding off the inevitable global drought. Even the two tiny Martian moons were seriously suspected of being artificial.
Then Mariner IV flew past Mars in July 1965 taking the first ever close-up pictures of the Martian surface. Alas, no canals, no cities, no signs of intelligent Martians. But hope dies hard, and when the Viking orbital spacecraft photographed the ‘Face on Mars’ there was a massive outpouring of the popular literature at least that even if there wasn’t a current civilization on Mars, there at least once was. Alas, however wonderful this would have proved to have been, an obvious intelligently designed alien artifact sitting on the Martian surface, it really was just a mirage – a trick of light and shadow – as later photographs showed. It proved to be a case of wishful thinking, like the canals and moons of Mars were, and a case of a mountain made unnecessarily out of a molehill.
But that doesn’t mean there aren’t Martians existing on Mars right now. It’s just that our Martians are microbial, and the evidence, while not yet conclusive, is very suggestive. Several independent discoveries have all but proved that life, albeit simple life, probably exists, currently exists, on Mars.
Firstly, the two Viking Landers, equipped with three separate life detection experimental techniques, all scored positive hits. It was only because the detection of organic molecules proved negative that it was thought prudent to look at exotic inorganic soil chemistry as an alternative explanation for the positive life detection results. There are those scientists involved with those Viking experiments who still maintain that microbial life was detected on Mars in 1976. Certainly, it’s not 100% proof, but it’s certainly a pro-life run on the scoreboard.
Secondly, there is the evidence from the Martian meteorite found in the Antarctic (ALH 84001). Recall there were four separate and independent reasons for coming to the conclusion that the meteorite contained fossil microbial life forms from Mars. While each taken apart could have a non-biological explanation, the four together were highly suggestive of microbial life on Mars. Make that two runs on the scoreboard.
Thirdly, spacecraft orbiting Mars have detected methane in the Martian atmosphere. Methane is chemically reactive, and would disappear in short time frames were it not replenished by some source. A major source of methane on Earth is from micro-organisms. While there are non-living sources of methane (volcanic activity), lack of such activity on Mars suggests that one chalk up yet another run on the board for life.
Fourthly, there’s no longer any question about Mars once having had extensive water. The Spirit and Opportunity spacecraft rovers have settled that chestnut, and that’s quite apart from the visual evidence of what look like water channels, etc. on the Martian surface. Where’s there water, there’s the probability of life.
Lastly, it’s obvious that Earth and Mars would have exchanged materials via rocks being impacted off one planet and arriving on the other (so-called ‘ballistic panspermia’). Since microbial life exists on Earth, some of it would have been transported throughout geological history to Mars. It’s quite possible that Mars seeded Earth as well, maybe even initially. Perhaps we are the Martians! And of course, it’s probable that both Mars and Earth were seeded from another outside source. That, IMHO, settles that.
However, the environmental conditions on Mars are very harsh. The temperatures drop well below freezing, only rarely getting above the freezing point. The atmosphere is so thin that liquid water can’t exist on the surface because the atmospheric pressure is so low. There’s hardly any oxygen, so no source to provide for a real ozone layer to block the Sun’s ultraviolet rays from impacting the surface full strength. For life to exist there, it’s either below surface (where it’s warmer, wetter and less exposed) and/or in tiny pockets like an oasis where conditions are every so slightly better. Regardless, relative to microbes on Earth, the microbial population on Mars, assuming it really exists, will be spread rather thinly.
Jupiter (The Giant Planet): Jupiter is the largest planet in our solar system, but composed mainly of gas. In fact, Jupiter has been insulted by being compared to our Sun, but a failed Sun. If Jupiter had been a fair few masses larger, it would have ignited in a ball of thermonuclear fusion and become a second stellar object in our solar system, turning it into a binary star system. Jupiter, however, is still solar enough such that it emits more energy than it receives from the Sun. Jupiter, because of the intense gravity, compresses its own stuff, and compression produces heat. Important point number one: Jupiter has its own internal energy source.
Important point’s number two, three and four: Secondly, Jupiter’s atmosphere is composed of the right sorts of chemicals that one identifies with an origin of life events – hydrogen, methane, ammonia, water vapor, etc. Thirdly, Jupiter’s atmosphere is turbulent such that there is a lot of mixing of those elements and compounds. Fourthly, the atmospheric bands of Jupiter are highly colored, an indication that there’s lots of complex chemistry, including organic chemistry going on within.
The upshot of all of this is that it is not implausible that within the upper reaches of Jupiter’s atmosphere, as per the case of Venus, simple life forms couldn’t exist, survive and thrive. You have the chemistry – you have the energy. And maybe Carl Sagan was right and that something more complex than just a unicellular ecosystem could exist in Jupiter’s atmosphere. But it would have to be an atmospheric ecology.
Europa (A Satellite of Jupiter): Europa is, apart from Mars, the current darling of the exobiology (astrobiology) set. There is evidence that Europa has a liquid water ocean underneath a thick ice cap that is kept from freezing solid by the flexing action imposed on the moon by its parent planet, Jupiter. If you have liquid water, an energy source, you, therefore, have possible life, or so goes the thinking. I’m not quite as optimistic. The ice cap is thick enough so that any energy source available for life won’t be solar. The ocean will be in eternal darkness. That is however not a death blow as not all critters on Earth rely on solar energy. There could be hydrothermal vents, with associated living communities on Europa as there is on Earth. But, with the ice cap, there would be little in the way of resources added to the ocean from outside; that’s not the case on Earth. All chemicals that would sustain such life would have to be efficiently recycled. Life on Europa – possible, but it’s going to prove to be very difficult to explore that ocean, so I’m not expecting a definitive answer anytime real soon.
However, there remains the possibility that materials contained within that hypothetical ocean may, due to tidal stresses, may be squeezed through cracks in the ice and find their way to rest on the surface. It’s, therefore, possible that a robotic craft that lands on the icy surface might detect organics and/or fossil or frozen solid microbes or even dead multi-cellular life forms resting on the surface.
[Note: To avoid unnecessary repeats, as a general rule of thumb, any satellite around Jupiter, Saturn, Uranus or Neptune that has a substantial part of it’s crust made up of ice, and is subject to extreme tidal heating by it’s parent planetary body (i.e. – you get a liquid ocean underneath a thin covering of ice), you have the potential for, as in the case of Europa, a habitable water-rich environment.]
Saturn (The Ringed Planet): Saturn is a quasi-twin of Jupiter. Although slightly smaller and farther away from the Sun than Jupiter, the same general arguments that apply to Jupiter apply to Saturn. That is to say, Saturn has the right sorts of chemistry – and an internal energy supply. It’s however slightly less dense than the other gas giants, such that if you could find an ocean big enough, Saturn would float! That, however, has no bearing on the issue of finding an atmosphere-based ecosystem there.
Titan (A Satellite of Saturn): The satellite of Saturn, Titan, is one of the largest moons in the solar system, and in fact, if it existed all by its lonesome, could be considered a planet in its own right. Titan has, fairly unique among satellites, a dense atmosphere. It’s denser in fact than our own atmosphere. It also has the right sorts of chemicals that we identify as having a strong connection with organic and biochemistry. Were Titan the same distance from the Sun that Earth is, well, you could have a real twin of Earth, unlike our false twin, Venus.
Unfortunately, Titan is way, way, way – far away – from the solar energy source that makes Earth such a relative paradise. Thus, Titan is Earth, but an Earth in slow motion because Titan is so cold compared to Earth. If you think of Earth as liquid water at the equator, Titan is molasses at the poles!
Uranus: Uranus is a poor cousin compared to the likes of Jupiter and Saturn. It’s mainly a gas planet, albeit way smaller, but so far out from where it’s all happening that it isn’t too likely that even simple life could flourish in the atmospheric depths, although the chemistry isn’t dissimilar to that of the closer-in Jovian planets of Jupiter and Saturn.
Neptune: The same sorts of arguments apply in general to Neptune as to Uranus, with one slight exception. Although farther out, Neptune, like Jupiter and Saturn, radiates out excess energy. It’s solar independent, at least as far as any life forms might describe their environment and energy supply.
Pluto: The planetoid Pluto was recently officially demoted from strict planetary status, and is no longer acceptable to refer to it as the ninth planet. However, when I was growing up, it was the ninth planet, and so I say buggers to astronomical officialdom. That rant aside, Pluto is hardly top rock for vacation seekers. It’s cold. I mean it’s really cold. It makes Antarctica seem absolutely tropical in comparison. I mean polar bears would freeze to death on Pluto, not that there’s anything reasonably resembling an atmosphere as we know it for them to breathe. Again, Pluto is too cold to allow for the high-temperature chemistry we associate with life-as-we-know-it. If you’re looking for life in our solar system, Pluto wouldn’t be your first port of call.
Comets, Asteroids/Planetoids, Meteors: These relative tiny bodies can’t really qualify as habitable abodes to life, except, there’s evidence that not only can some of the above be rich in the sorts of chemicals associated with life (water, carbon compounds, and organic chemistry), they could indeed be environments that could house dormant life forms or fossil life forms of a unicellular kind. Meteorites which have been gathered up and analyzed on Earth (like ALH 84001) have yielded if not fossilized bacteria, then at least enough chemical evidence to suggest that they could have had a close connection with contributions towards an origin of life event.
Conclusions: Life is ultimately nothing more than an on-going series of complex biochemical reactions. These sort of reactions tend to be optimal at warmer (quasi Earth-like) temperatures. Too cold and the necessary chemical reactions are too sluggish if they proceed at all. Too hot, complex organic molecules rapidly break down. So, Titan is probably way too chilly, and the surface of Venus, way too hot. Some real estate is both too hot and too cold at the same time – Mercury and Luna (our Moon). My favorite solar system locations for (probably) tough-as-nails microbial life, Mars apart, are the upper atmospheres of the Jovian (gas giant) planets (Jupiter and Saturn; maybe Uranus and Neptune). Their atmospheres are rich in organics and no doubt water vapor. The gas giants, Uranus excepted, radiate more heat energy than they receive from the Sun. There will be regions in their upper atmospheres that have Earth-like temperatures; there will be a lot of atmospheric mixing (useful for bringing different chemicals together); and of course these planets will also have been seeded with organics and water from space via comets, meteors, cosmic dust, etc., if not in fact seeded directly with microbial life forms via panspermia. The problem in finding out for sure is going to revolve around sending appropriate instrumentation into those planetary atmospheres that will be able to detect actual life forms, as opposed to just measuring just purely physical parameters and chemical constituents. So, don’t expect definite answers anytime soon.
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