Exobiology: Life In The Inner Solar System

Exobiology was the original term given to the sciences central to the question of life-in-the-Universe. It’s now been largely replaced by Astrobiology, but I’ll stick with the original. To investigate life-in-the-Universe perhaps we should start a bit closer to home and our own inner solar system – Mercury, Venus, Earth, the Moon and Mars. Apart from Planet Earth, we have no absolute proof positive to date that any other life, albeit relatively simple forms compared to terrestrial life forms such as ourselves and associated companion animals and plants, exists within the confines of our solar system. However, the odds are fairly high, almost certain in fact; those other, extraterrestrial life forms do exist in our immediate cosmic neighbourhood. 

Humans and our associated kin 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 inner solar system, most probably Mars, are possible even probable habitable abodes to simple microbial life forms; perhaps something slightly above and beyond that.

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 less humans.

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 absolutely 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 little understood example of chemosynthesis are the colonies of microbes (dubbed ‘rusticles’) that are eating the iron structure of the RMS Titanic, resting some four kilometres 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 taking each abode in turn…

Mercury: The planet Mercury, 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 scantly 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 drops 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?

The question therefore is, if life couldn’t have arose on the surface of Venus because it is just too damn hot (and no water), where hence did the atmospheric life (microbial most likely) come from? Well, the surface of Venus, way back when, may have been far more hospitable than it is now. However, there’s also Panspermia – life forms adrift in space that eventually impact other worlds. The upper atmosphere of Venus may have been seeded with microbial life forms at some time in the past via such a mechanism, the same mechanism that may have kick-started life on the early Earth, and perhaps seeded other planetary abodes in our solar system. 

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 rigours of outer space.