by Duncan Lunan

When the phrase ‘life as we know it’ was first coined it was as a cautionary note, a reminder that life might take different forms elsewhere.  For a time in the mid-20th century it came to be used as a blanket denial that there could be life elsewhere, at least in the Solar System, and in reply, in 1962 Isaac Asimov wrote a survey of other possible life-forms for the planets as we then thought them to be  (‘Not As We Know It’, reprinted in his essay collection The View from a Height, Dobson, 1964).  Carl Sagan also criticised refusal to consider other possibilities as ‘carbon-chauvinism’, ‘oxygen-chauvinism’ and ‘water-chauvinism’, and I headed Chapter 3 of my 1974 book Man and the Stars ‘Life, as We Know It’, with the comma intended to express due caution, though the chapter was based on a lecture by the late John W. Macvey of Saltcoats, and he had stressed that while other forms might exist, as a chemical engineer he thought they were all relatively unlikely.  When NASA announced that its mantra in the search for life would be ‘Follow the Water’, some of us felt disappointed that the search would be so limited. 

Hollywood was and still is hung up on the idea that water is scarce in the Universe, so much so that it would be worth the trouble for aliens to come here and steal it, as in films and series like V.  But even then (1983)  it was known that the polar caps of Mars were water ice, while several of Jupiter’s moons were covered in it, and most of Saturn’s appeared to be made of it.  Continuing exploration of the Solar System has revealed that in one form or another water is almost everywhere.  Even in the Moon, which was thought to have lost all its volatile compounds in the collision which formed the Earth-Moon system, it turns out that there are water-bearing layers which gave rise to the Apollo 14 ‘rusty rock’, the Apollo 15 ‘green beads’ and the Apollo 17 ‘orange soil’, all of them formed and excavated by impacts which penetrated far below the surface.  Even liquid water is common in the Solar System, and the special conditions for other forms of life are so rare that if ‘life as we know it’ exists out there, it’s liable to have grabbed all the resources available.

Mercury

In the early 17^th century, in the astronomy section of his Anatomy of Melancholy, Robert Burton hailed the discovery that the planets were actual worlds, orbiting the Sun like the Earth.  He assumed that conditions on all of them would be like Earth’s, and they’d all be inhabited, because why else would God have created them?  For the inner planets at least, such beliefs lasted well into the 20^th century.  Mercury, for instance, was believed to have a thick atmosphere, and in 1962 the Arran-based astronomer V.A. Firsoff calculated that despite its nearness to the Sun, Mercury could have ice-caps. 

Although there’s actually no atmosphere at all, except for a very thin bow-shock in the Solar Wind, the ice-caps were detected by the giant Arecibo radar in the early 1990s, and confirmed more recently by the Messenger orbiter.  The deposits are confined to shadowed crater floors, and it’s not known whether they come from cometary impacts, volcanic outbursts or both.  But the planet has a molten core, and there are other features which appear to be volcanic, so there may be more surprises to come.

Venus

On the face of it, Venus, with the highest surface temperatures in the Solar System and the atmospheric pressure of carbon dioxide equivalent to a kilometre underwater on Earth, and with clouds of sulphuric acid droplets extending up to 90 miles, would seem the last place to find liquid water.  Yet there is some, particularly at the base of the clouds in the permanent storm over the south pole, and if it was all collected there would be enough to cover the flat surface to a depth of a foot or so.  Around 30 miles up, temperatures and pressures are in the range found at Earth’s surface, and the clouds are dominated by huge features, almost invisible to the naked eye, but standing out starkly in the ultraviolet because they absorb up to 90% of the u-v radiation which falls on them – and, closer to the Sun than we are, that’s a lot of energy being taken up by something.  The idea that there might be life in the clouds got a major boost in 2020 when it seemed that phosphene had been detected there, in such quantities that it might be generated by life.  The readings and their interpretation were challenged, but looking back to the results obtained by the Pioneer 12 entry probes in 1978, it turned out that there was some phosphene there, at least , along with ammonia and other possible indicators of life – ‘not known, because not looked for’, to quote T.S. Eliot.  I have a major article in preparation about this, so there will be more to say later.

Mars

I wrote about the historical and current ideas concerning life on Mars in July-August last year, so I don’t need to say too much more here.  There’s possible evidence for seasonal outbreaks of water from crater walls, and that remains controversial:  it would need to be very salty to flow for even short distances in the thin Martian air – like Earth’s 20 miles up, but almost all carbon dioxide.  But there is lots of water below the crust, as well as in the polar caps – the most recent discovery is of near-surface deposits in the great equatorial rift valley, Valles Marineris.  (Matt Williams, ‘The Bottom of Valles Marineris Seems to Have Water Mixed in With the Regolith’, Universe Today, 22^nd December 2021.)  It was thought that liquid water lakes had been found below the south polar cap, but that finding has been challenged and they may ‘only’ be hydrated clays, which are formed elsewhere on the planet, especially north of Valles Marineris. 

Europe’s Trace Gas Orbiter has not yet found any traces of methane in the Martian atmosphere, though previous observations from Earth, from orbit and from the surface seemed to suggest that there were seasonal outbursts which could be signs of volcanic activity, or of life.  But both the Curiosity and Perseverance rovers have recently found organic compounds in the soil, which at least could mean that there was life there in the past.  The search continues.

In the Asteroid Belt between Mars and Jupiter, there has turned out to be lots of water, at least in the larger asteroids.  Studied by the Dawn probe, Vesta proved to have large concentrations of water-bearing rock, with evidence that it had flowed as liquid at some time in the past. 

The other layers of the largest asteroid or dwarf planet, Ceres, proved to be rock-and-water clathrate, while bright spots seen by the Hubble Space Telescope proved to be salt deposits left by outbreaks of water, coming from a reservoir of brine below and seemingly frequent.  A bright and evidently recently formed mountain, Ahuna Mons, may be the mud plume of a ‘cryovolcano’, with water flowing like lava, and it too is coated in salt.  There’s no evidence for life in any form, but organic compounds are plentiful in the outer regions of the Belt, and as we have learned that the asteroids exchange surface rocks more often than anyone suppose, one wonders what else they might be carrying.

Jupiter

In his Cosmos TV series, Carl Sagan made much of the possibility that life could exist in the clouds of Jupiter, in the layers where liquid water exists and can be spotted through ‘blue holes’ in the cloud cover.  Asimov had assumed that Jupiter’s interior would be ‘cryo-abyssal’, with intense cold and pressure inside.  Instead it has turned out to be ‘pyro-abyssal’, with temperatures of 50,000 degrees at the core, and vertical currents in the atmosphere are so strong that life could not remain in layers of constant temperature.  After the Pioneer 11 flyby it was thought that the polar regions might provide stable oases, but the recent observations by the Juno orbiter have shown them to highly turbulent, filled with huge cyclones.  Nevertheless, it looks as if those storms may be long-lasting, perhaps semi-permanent like the Great Red Spot, and since they’re the size of the Earth, maybe their ‘eyes’ could be stable enough to support life – different kinds in each, perhaps?

Jupiter’s four giant moons are like a solar system in themselves, and in its early history Jupiter put out so much heat that it was really like a sun to them.  With resonant orbital periods, they exert strong tidal forces on one another, generating internal heat and much more varied conditions than anyone expected before the Voyager flybys of 1979.  Io is covered with active volcanoes and blasted both by radiation belts and by multimegaton lightning bolts from the planet, so it bids fair to be the most dangerous place in the Solar System – yet some models suggest there could be liquid water inside, with a great deal of geological and chemical energy available, if there’s anything to make use of it.

Europa, the next moon out, is covered with tidally heated water, to unknown depth.  We don’t know how thick the ice is, but it behaves like pack-ice on Earth, and geysers of water vapour erupt through it from time to time, so daylight must penetrate into the depths – and on the sea-floor there may very probably be hot-water vents, like the ‘deep smokers’ in which life may have originated on Earth.  Of all places in the Solar System, this is currently the most likely to search for life, and ESA’s Jupiter Icy Moons Explorer will be going there to advance our knowledge, launching in 2022. though its principal target is Ganymede.  It will be followed in 2024 by NASA’s Europa Clipper, originally intended for the Space Launch System, but now for Elon Musk’s Falcon Heavy – and as that is already flying, while the SLS is subject to multiple postponements, there’s a good chance it will go on time.

Ganymede, the JUICE mission’s target, is the largest moon in the Solar System.  At the time of the Pioneer 10 flyby in 1973, it was seriously thought at NASA’s Ames Research Centre that Ganymede might be a single drop of water, larger than the planet Mercury, encased in ice as Europa turned out to be!  While Ganymede is not that weird, its low density indicates that it is largely composed of water, and its magnetic field indicates that much of that is sub-surface ocean.  The rock and ice crust shows large surface movement, so much so that the British Interplanetary Society reported at the time that parts of it had moved between the various spacecraft flybys.  It clearly has happened in the past, but not that recently.

Finally there’s Callisto, the only one of the Galilean moons to lie outside Jupiter’s ‘supralethal’ radiation belts.  It has a surface of mixed rock and ice, heavily churned up by ancient bombardment and with little sign of more recent internal heating – though the larger impact features are subsiding into the landscape, over millions of years.  Of the four large moons it’s the least studied, simply because it’s the furthest out, so here too there may be surprises in future.

Saturn

The Dan Dare animation series of 2001 moved Carl Sagan’s ‘cloud whales’ out to Saturn, and indeed there’s no reason not to, because the internal heating is still present, and overall the atmosphere seems calmer, though its overall patterns are different from Jupiter’s, particularly because of the shadows of the rings.  The Cassini orbiter didn’t see any living creatures down there, but they weren’t supposed to be that big.

The moons of Saturn are mostly composed of ice, and since Kuiper discovered in 1948 that the largest moon, Titan, has an atmosphere, the idea that there could be life there has a long history. 

In 1955, Chesley Bonestell’s classic painting of Saturn over an icy Titan landscape was the cover for Life on Other Worlds by Sir Harold Spencer Jones, then the Astronomer Royal.  The Voyager flybys in 1980 and 1981 found that Titan had a much thicker atmosphere than expected, with a dense fog of hydrocarbon aerosols and twice Earth’s atmospheric pressure.  Temperatures were around 100 degrees Absolute, and the possibility that life on Titan might be methane-based, popular in the 1960s, enjoyed a brief resurgence.  Studies by the Cassini orbiter and the Huygens lander, starting in 2004, showed that methane is indeed at its ‘triple point’, existing as solid, liquid and gas, at least in the equatorial regions, as water does on Earth.  But the lakes which have been revealed by radar and infrared imaging are mostly of ethane, in the polar regions, while the mountains are water ice, behaving like rock at those temperatures and pressures.  Whether there’s liquid water inside, and whether there are cryovolcanoes on the surface, is still hotly argued over – if that’s the right word to  use.

When it came to Enceladus, the third moon inwards from Titan, there was no argument about it.  We might have learned a lot more, sooner, if Voyager 2’s camera platform hadn’t jammed temporarily during the close Enceladus pass, but there was clear evidence of water eruptions and resurfacing.  The long-distance Cassini images demonstrated that particles from Enceladus were feeding into the rings, and close-ups showed these were coming from ‘tiger stripes’, blue-coloured fissures near the south pole which had at one time been on the equator, before big surface movement over liquid below.  There definitely is a liquid subsurface ocean, and flying through the plumes, Cassini detected salts, carbon dioxide, methane, acetylene and propane mixed in with water – nothing explicitly organic in origin, but highly suggestive.  According to Saturn, Exploring the Ringed Planet, edited by Keith Cooper and published by Astronomy Now in 2015, Enceladus may be a better place than Europa in the search for life.

Uranus

In the early 1980s it was thought that an entire layer within Uranus might be liquid water.  The idea was mentioned in Pioneering the Space Frontier, the 1986 report of the US National Commission on Space, and featured in The Planets, a lavish anthology of fact, fiction and art edited by Byron Preiss in 1985.  A similar model had been suggested for Jupiter, and Asimov calculated that if it contained ‘life as we know it’ to the same extent as Earth’s oceans, the total organic mass would be about a quarter of Earth’s Moon’s!  After the Voyager flyby of 1986 the water ocean was discounted, but it’s worth remembering that almost all we know about Uranus was gained in the snapshot of that single 11-hour encounter.

More recently attention has been focused again on the moons of Uranus, and as I reported here in November, Ariel and Titania both have huge geological faults on their surface, unlike anything seen elsewhere in the Solar System, but possibly the result of subsurface movement of water ice.  There have recently been calls for a new mission to Uranus, preferably an orbiter, but neither NASA nor ESA has so far taken the bait.

Neptune

Again with Triton, the largest moon of Neptune, there’s no argument about sub-surface liquid  (correctly predicted by the late Ed Buckley, in a painting for my 1984 book Man and the Planets).  In this case the liquid is nitrogen, but there’s more to it than that:  the plumes and their deposits which were photographed by Voyager 2 are jet black, showing that the geysers contain more than pure nitrogen, which is colourless.  There’s definitely internal heating, because Triton’s orbit is retrograde and elliptical, evidence of some ancient catastrophe, and it’s even possible that the cryovolcanic activity is water-driven;  but again, almost all we know comes from a single flyby encounter.

Pluto

Pluto is no longer classed as a planet, partly because its orbit is within Neptune’s for part of its year.  When it was there last, in the 1990s, an atmosphere containing methane was formed, and that atmosphere is rapidly freezing out as Pluto moves away from the Sun.  The 2015 New Horizons flyby discovered extensive deposits of methane and ammonia ice on the surface, and mountains of nitrogen ice.  But one of the largest features visible was a huge plain, Sputnik Planum, which is a crust of nitrogen ice over a liquid water basin, still retaining heat from the impact which formed the huge impact crater unknown years ago.  The surface ice is divided up into moving plates, and water icebergs move along the cracks, powered by convective cells below.  Internal heat looked to be the driver, but a recent study suggests that the cooling effect of the surface nitrogen may be more significant.  (Nancy Atkinson, ‘Now We Know Why Pluto Has These Strange Features on Its Surface’, Universe Today, 21^st December 2021.)  There are organic compounds on the surface, too;  and who knows what else in the depths?

Pluto was demoted from planetary status by an initiative led by Prof. Mike Brown, whose team are now looking for a ‘Planet 9’, far out in Kuiper Belt, whose presence may be indicated by disturbances in the movements of the other Kuiper bodies.  If it exists, it might possibly be a stray planet, captured by the Solar System early in its history.  The existence of such unattached worlds was predicted by Harlow Shapley in 1962, and as his title ‘Crusted Stars and Self-Heating Planets; indicated, they could have strong internal heating due to ongoing gravitational contraction.  The first such discovery was made in 1998, and with 70 more added to the list just recently, the total is now at least 140.  There may be as many as two stray planets for every star in the Galaxy, and that could have major implications for the search for life;  but that’s a story for another time.

Even if many of the environments in the Solar System could sustain life, as we know it, could it originate or evolve in any of them?  It might seem not, or highly unlikely, but the work of Prof. Arjun Berera of Edinburgh University has reopened the old debate about panspermia, the possible spread of life from world to world.  Recent discussion has been centred on the idea that secondary meteorites, blown off planetary surfaces, could carry life from Earth to Mars, or vice versa.  But such events would be both intermittent and rare.  Instead, Prof. Berera calculates that micrometeorites grazing the atmosphere could carry off organic molecules and even microorganisms, in such numbers and in an ongoing process, that terrestrial life could be spread right across the Milky Way in only a billion years  (assuming it met no competition from elsewhere).  Filling all the potential niches in the Solar System could then have been comparatively easy, and rapid.  The longer-term implications on the interstellar scale are definitely to be considered later!

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This is part of an excellent series of astronomy articles by Duncan Lunan. To find more just type it into search.