Now narrated by Dagoth Ur!
Mars.
A name that instills both fear and fascination in the human mind, as the red
star travels across the night sky in ever stranger courses. The first record of
humans taking notice of our red neighbour comes from the Ancient Egyptians, who
gave it the name Har Deshur or Her Deshur (hieroglyphics rarely recorded
vocals), which means “Horus the Red”. This denoted the planet as being one of many
aspects of the sky god Horus, one of the most revered deities in their pantheon. For millennia this
would be the most benevolent association Mars had ever received, as the red
coloration made most of humanity think of more sinister things. The
Mesopotamians knew it under the name Nergal, the god of burning desert heat,
fire and plagues. In Hindu texts it was called Mangala, the god of anger. The
Greeks knew it either as Pyroeis, the fiery one, or Ares, the god of war and destruction. From
Ares the Romans would derive the god Mars, by whose name the planet is now widely
called. People in those times rarely thought of the planets as material
objects, but rather abstract things beyond human comprehension, likely put into
the roof-like firmament as signs from the gods. Thus the idea that someone - or
something - could be walking on their surface was rarely thought of. Do not
misunderstand me, such speculations did indeed exist at times in the writings
of ancient authors, such as Aristarchos, Plutarch or Lucian, but most of
humanity preferred cosmologies that stroked their own ego, such as Ptolemy’s
geocentric model, in which, with its aetherial planets, there was no place for
biology beyond the orbit of the Moon.
"Who shall dwell in these worlds, if they be inhabited? Are we or
they lords of the world?"
The
Copernican Revolution arrived in the Early Modern Period and came to the
momentous conclusion that Earth was not the centre of the universe, but was
in fact one of many planets circling the Sun. But if Earth is like the other
planets, does this also mean that those planets are like Earth? We find
early examples of such speculations in the writings of Giordano Bruno, who
wrote about life on the Moon and the Sun (then still thought to be a solid
object) and, already in the 16th century, noted that extraterrestrial life must
not necessarily resemble terrestrial variants, as even on Earth organisms have
found multiple solutions for the same functions (Heuser 2008). The same
century, German astronomer Johannes Kepler used Mars to solve one of the
greatest problems in cosmology. When viewed over a certain period of
time, the motion of the red planet across the night sky seems to go into the same
direction as the other planets, until the planet suddenly moves backwards in a
loop-like fashion to then resume its previous course (which may have led
ancient people to believe that the planet was steered by an intelligent force).
Kepler figured out why the planet went through such paradoxical motions: The
planets did not revolve around the Sun in perfect circles, as previously
assumed, but in ellipses, a realization which is today known as Kepler's First
Law of planetary motion. Kepler (1619) was also the first to mention the
possibility of inhabitants on Mars specifically, providing us with the above
quote. The astronomer would go on to write (and posthumously publish) possibly
one of the first science fiction novels, the Somnium, though it would be
about life on the Moon, instead of Mars, a notion that, as fanciful as it may
seem, might actually still have some merits, given recent findings about lunar
habitability in the deep past (Schulze-Makuch & Crawford 2018).
Fig. 1: One of the earliest detailed maps of
Mars by Giovanni Schiaparelli, showing (natural) channels of water. Note that the South Pole is here shown at the top, not the bottom.
As
telescopes improved, so grew the interest in Mars. In 1659, the very first
attempt at a map of the planet was drawn by Christiaan Huygens, showing what
would later be known as Syrtis Major Planum. 1666 Giovanni Cassini would be the
first to note the existence of a large ice cap on Mars’ southern pole, one of
the first signs that water of some form existed on the planet. In 1777, William
Herschel would discover that this polar cap would grow immensely during Martian
winter, proving that Mars had seasons. In around 1800, Honoré Flaugergues made
first mention of ochre-colored veils travelling across Mars’ surface, this
possibly being the first discovery of dust storms and therefore an atmosphere on
the planet. Catholic priest and astronomer Angelo Secchi made some of the first
detailed colour illustrations of Mars. In 1869 he reported two dark and linear
streaks across the surface, which he interpreted as channels, possibly bearing
liquid water. Two years before, Pierre Janssen and William Huggins had first
used spectroscopes to view Mars and came to the, albeit controversial,
conclusion that water vapor was present in its atmosphere. During the 1877
opposition, Asaph Hall discovered the two tiny moons of Mars. The same year,
Giovanni Schiaparelli produced the first detailed Mars maps, which showed
multiple features of the same type as seen by Secchi, which were again called
canali. Schiaparelli interpreted these as being natural, water-bearing
features, the Italian canali meaning channel (such as the one between Britain
and France). However, many foreign publications mistranslated these maps as
showing canals, a term which denotes an explicitly artificial structure.
1892, noted French astronomer Camille Flammarion reported seeing the same
features as Schiaparelli, but unlike him made an explicit connection to
extraterrestrial intelligence. Flammarion was the first to speculate that a
race of intelligent Martians, more advanced than humanity, used these grand
structures as an irrigation-system to redistribute polar meltwater into the
drier equatorial regions. Such ridiculously large construction projects were
thought possible and intuitive at the time for an advanced race, considering
that the Suez Canal was completed only a few decades prior and a few decades later the Panama Canal would begin construction.
Fig. 2. Mars’ vast system of canals, this time
imagined by Lowell to be of artificial origin by a dying Martian race.
Around
the same time, Pierre-Simon de Laplace’s nebular theory had become widely
accepted. The theory states that the planets farther from the sun formed out of
the primordial stellar nebula earlier than those closer to it. Thus, Mars was
an older planet than Earth, already past its prime and on the way to becoming
uninhabitable like the Moon (as a side-note, by the same logic, Venus was also
younger than Earth and thus imagined as quite prehistoric, sometimes even with dinosaurs). Based on this, American
astronomer Percival Lowell, one of the founders of planetology, speculated that
the canals were built by a Martian race that had not just become more advanced
than humanity by virtue of being older, but had also become quite
desperate, attempting to stave off extinction and planetwide desertification
with these monumental geoengineering projects. In 1894, he built the Lowell
observatory in Flagstaff, Arizona, with the purpose of observing this dying
Martian civilization as good as was possible with the instruments of the time.
Lowell produced a great many maps of various and extensive canal-networks and
also speculated that the capital of the Martians was in Solis Lacus due to how
many canals he thought were crossing through that region.
“At most terrestrial men fancied there might be other men upon Mars,
perhaps inferior to themselves and ready to welcome a missionary enterprise.
Yet across the gulf of space, minds that are to our minds as ours are to those
of the beasts that perish, intellects vast and cool and unsympathetic, regarded
this earth with envious eyes, and slowly and surely drew their plans against
us.”
Greatly
inspired by such writings, a man of the name Herbert George Wells, who along with Jules
Verne would go on to become the founder of modern science fiction, wrote in
1897 a short story by the name of The Crystal Egg. In it, an antiquarian
discovers that a crystalline orb from his collection, if viewed in the right
angle of sunlight, acts as a window to view through another such crystal egg on
the surface of Mars. Inside a lush valley with a straight canal bisecting it,
the observer sees lichenous trees, red weeds, dim and primate-like bipeds,
insect-like animals and a rather bizarre Martian race. Wells’ intelligent Martians
are basically all head adorned by tentacles and come in two variants: Winged
Martians, which fly about Mars, live in houses that have only windows and no
doors and use the crystal eggs to observe the surface of the Earth and possibly
other planets. Then there are the wingless Martians, possibly of the same species but a
different caste, which amble about on the ground with their tentacles like
spiders and which seem to feed on the bipeds.
Fig. 3: One of the early covers for The Crystal
Egg, showing a winged version of the later octopus-like Martians, observing
Earth through a crystal orb, perhaps planning to invade.
The
same year, Wells began a story of serialized articles in Pearson’s Magazine, in
which the Martian civilization gave up their desperate attempts of maintaining Mars’ biosphere and instead used their greatly superior intellects and
technology to invade and colonize Earth with the help of enormous bionic
machines. In 1898 the articles were all compiled into a novel titled The War
of the Worlds. Probably being one the most famous science fiction stories
of all time, one might think that reiterating its details to the reader would
be superfluous. However, much like other classics, such as Moby Dick and
Frankenstein, this story is known by the general public more by its many
adaptations than by its original iteration. And those adaptations often tend to
do a great disservice to Wells’ Martians. Many details give away that The
War of the Worlds takes place in the same continuity as Wells’ previous
short story. The Martians which invade Earth greatly resemble the flightless
Martians of The Crystal Egg. Their cephalopod-like body consists of just
a huge skull with an equally large brain. The face consists of two large eyes
and a v-shaped mouth that resembles a fleshy beak. The beak is surrounded by up
to sixteen tentacles. No nostrils are present and the closest to an ear is a
tympanum at the back of the skull. There were no internal organs tasked with
digestion, instead the Martians fed by directly injecting their arteries with
the blood of lower creatures (humans). On their native Mars they “fed” for this
purpose on vaguely humanoid creatures, very likely those dim primate-like ones described in The
Crystal Egg, which were grey, bipedal and possessed a siliceous skeleton similar
to that of a glass sponge. Due to our shape resembling their cattle, the
invading Martians developed a great taste for human blood. The Martians also
spread the same kind of red weed as seen in the short story, in an attempt to
xenoform Earth into a second red planet.
Fig. 4: The inhabitants of Mars, as imagined by
H.G. Wells. An attempt at rendering genuinely alien life or instead a parody of what
humanity might become one day?
The
appearance of Wells’ Martians and their livestock may very well be seen as
commentary of the then anthropocentric view of aliens of the time. Even in the
novel, most people awaiting the opening of the Martian cylinder expected a
creature much like a human, with only minor differences, to crawl out, to then
be greatly shocked to see a bear-sized, cephalopodous creature. In view of the
many humanoid aliens that were designed even long after Wells’ time, this aspect
of the story still remains subversive. In fact, one could argue that the
appearance and behaviour of the Martians, as well as the general story of the
book, laid out the groundwork for the later cosmic horror genre. One H.P.
Lovecraft would have been an eight-year-old boy at the time of the novel’s
release and it is more than likely that he read Wells’ works.
Fig. 5: Forest life on Mars as imagined by H.G.
Wells.
On
the other hand, the design of the Martians was also greatly influenced by
Wells’ own visions of what evolution might lead humanity towards. In Man of
the Year Million, Wells wrote that he imagines future humans to have
greatly reduced all of their organs, except for the brain and the hands, which
are both instrument and teacher of the brain. He also believed future humans to
externalise most of their digestive function. The Martians might therefore be
seen as the extreme endpoint of that development, with the body having become
all brain, the hands having become all tentacles and digestion having been
reduced to drinking the blood of animals. Indeed, the narrator of the novel
does speculate that the Martians may have once been humanoid in body form at
one point in their evolution. In this light, the Martians’ relation to their
humanoid livestock becomes interesting. Perhaps they were once closely related,
but had become starkly different through a similar relationship as seen between
the Eloi and Morlocks in Wells’ other novel, The Time Machine, just with
even more time for evolutionary divergence. All of this should also be viewed in
the light of Wells writing The War of
the Worlds as a form of critique of the colonialist geopolitics of his
time. As noted multiple times throughout the novel, the Martians in large part
do to the people of the British Isles what the British Empire had done a few
decades prior to the native people of Tasmania and many other colonized
countries. And is the way in which the Martians consume humans really all that
different from what we humans do to
animals and other forms of life we regard as “lower”? One could use all this to
argue that these Martians are only alien in their appearance, but not in their
nature.
Fig. 6: Mammal- and bird-like Martians as
imagined by H.G. Wells.
After
his short story and hit novel, Wells was not done with Mars quite yet. For a
March 1908 issue of the Cosmopolitan Magazine, Wells wrote an article
titled The Things which live on Mars.
Wells wrote his article in response to Percival Lowell’s then newest book Mars as the abode of life. While Wells enthusiastically
agreed with Lowell’s vision of Mars, he lamented that the astronomer in his
work never went into detail about the exact appearance of Mars’ biosphere,
especially regarding the question what sort of creature the intelligent Martian
civilization would have evolved out of. Thus, he engaged in something that
would today be called speculative evolution, with the creatures he imagined
being beautifully illustrated by William R. Leigh, who was otherwise known for
his Wild West art. Wells argued that plant life on Mars would be tall and thin,
with small leaves and needles and in general resemble plant life from Earth’s
deserts and mountain regions. Further he noted that something comparable to
insect life would exist, though he remained open to the question of if it would
be larger or smaller than on Earth. He also argued that there would be no
permanent aquatic life, such as fish, as the astronomy of his time indicated
that large bodies of water existed on Mars only in temporary form during the
summer, when the polar caps melted. Wells further reasoned that with
tall-growing plants there would be climbing animals and for an animal to be an
efficient climber it would need a spine, meaning there must be vertebrate life
on Mars. From here on out Wells’ vision of life on Mars becomes somewhat
chauvinistic in favour endothermic vertebrates, such as himself. He argues that
reptilian- or amphibian-type life may have long been outcompeted by now, as the
bird- or mammal-like Martians would be adapted a lot better to Mars’ current
cold climate (it should be noted that in real life, endothermic and ectothermic
animals on Earth do about equally well in both ends of the temperature-scale,
with warm-blooded small mammals and cold-blooded reptiles both thriving in
deserts and warm-blooded seabirds and cold-blooded fish and krill both thriving
in the polar regions).
Fig. 7: Wells’ highly advanced Martians,
similar to those from The Crystal Egg, but disappointingly more humanoid
(though this may be more due to the illustrator
William Leigh).
Thus,
Wells imagines the intelligent Martians to have descended from such mammal-like
Martian animals. Furthermore he thought they would almost assuredly have big
heads with big brains and forward-facing eyes atop an erect spine, as well as
big chests to breathe in the thin Martian atmosphere. He also thought it was
probable that they would be bipedal. To give him credit where it is due, Wells
is honest about not being able to answer any other details. He notes that they
could have more limbs than he speculates, that it is just as likely for the
Martians to have fur or feathers rather than naked skin and instead of arms
with hands they might use any other form of manipulatory organ, such as
tentacles or even trunks. Nonetheless, Leigh chose to illustrate Wells’
Martians as naked humanoids, though with bird-like feet, wings and tentacles
instead of fingers. In some ways this resembles the winged Martians
from The Crystal Egg and might indeed be an homage, though it is still disappointingly
anthropocentric compared to the cephalopods of The War of the Worlds. As
for the nature of Martian civilization, Wells wrote that most of the animal and
plant life he imagined previously in the text may already be extinct, as, per
Lowell’s writings, the Martians may have expanded their urban and agricultural
areas across the whole surface of the planet, in the process wiping out all
wildlife. This process he again deduced from what modern humanity is doing (and
continues to do) to its natural habitats.
Fig. 8: A green Martian from Barsoom. In many ways an ancestor to Warhammer/Warcraft-type orcs (and predating the Tolkien ones too while we’re at it)
At
the beginning of the 20th century, another man also became famous for his
writings on Mars, when in 1912 Edgar Rice Burroughs, the creator of Tarzan,
published the novel A Princess of Mars. The story follows the adventures
of the American civil war veteran John Carter, whose mind and body are mysteriously transported to the planet Mars, a world of ruins, fantastic
animals and various humanoid races. Burroughs’ Mars, named Barsoom by its
inhabitants, was greatly inspired by Percival Lowell’s writings (who in those texts fancifully imagined what it would be like for a human to stand on
Mars, for illustrative purposes to the reader). Like in Lowell’s vision, the
inhabitants of Barsoom transport water from the poles to the more equatorial
regions by use of massive canals, however, due to environmental degradation,
the global civilization of Mars has already collapsed and control over the
canals is quarrelled over by various warring city-states, while many other parts
of the planet have degraded into post-apocalyptic barbarism. The atmosphere has
also become so thin that the inhabitants have built atmosphere generators to artificially
keep it from dissipating, which may be Burroughs’ concession to more advanced
spectroscopy measurements of Mars’ atmosphere at the time. Life on Barsoom
differs in many respects from Wells’ vision, on account of Burroughs claiming
to have never actually read Wells’ work (Holtsmark 1986, though read on). Except for the native
race of the Green Martians, which have four arms, deep chests and tusked mouths, most
of the intelligent Martians are just humans with odd skin-colours, while most
of the Martian wildlife are close analogues to Earth-life with extra limbs. The exception may be the kaldanes, a race of crab-like brain-beings that bear more than a passing resemblance to Wells’ invaders, perhaps making Burroughs’ above claim a bit doubtful. The
Barsoom series, spanning eleven books that were released even after the main author’s death (1950) all the way into 1964,
would not become influential through any imaginative biology, but rather
through its storytelling and fun, escapist sci-fi concepts. It would go on to inspire
many people, among them the likes of Carl Sagan, to become astronomers and it founded the planetary romance genre. It would go on to inspire Perry Rhodan and
Flash Gordon, which in turn would become the basis for massive movie-industry
giants like Star Wars. The terms Jed(i) and Sith actually seem to have been directly lifted by George Lucas from the John Carter novels (Robert Zemeckis in fact once claimed that John Carter has become unfilmable today because Lucas had already gutted the books for all they were worth for his own movies). The character
Superman also originally began as an inversion of John Carter. Carter, being a human from higher gravity Earth, has superhuman strength on Mars relative to
its native inhabitants and can jump extraordinarily high. Superman, who in his
original iterations could not fly but just jump very high, has likewise superhuman
strength on Earth because he is an alien from a planet with even higher
gravity. Lastly, Burroughs’ Mars was the original desert planet of
science fiction, probably having one or two influences on Frank Herbert’s
Arrakis. In 1996, Barsoom would finally cross over with Wells’ Martians in the
tribute anthology War of the Worlds:
Global Dispatches, in which in one story, John Carter fights the original
Martians on the homefront. In 2012, literally a century after the original
book, John Carter finally received a movie adaptation. While it bombed at the
box office, I personally really enjoyed it and recommend watching it for the
escapism and the love it shows to the source material.
The Age of Uncertainty and the Great Disappointment
Despite
such fanciful speculations in fiction and popular science magazines, actual
science became increasingly more skeptical of Earth-like conditions and life on
Mars as the 20th century wore on. In the 1920s, better telescoping
and spectroscopic measurements showed that Mars’ atmosphere was even thinner
than assumed, had significantly less water vapor and oxygen than thought and
that temperatures ranged on the surface from -68 degrees Celsius to +7 degrees
at best. At least a lifeform such as John Carter would not have been able to
live here. There was also increasing doubt about the validity of Lowell’s
observations, with better telescopes indicating that the features he saw were
an optical illusion where the mind drew straight lines connecting dark patches
on the surface. But such explanations remained controversial, as no earth-based
telescope was powerful enough to see the red planet in crisp detail. And even
in the 30s, weird happenings on the red planet pointed towards possibly
intelligent activity, with large, green, flares being reported in 1937 (Davydov
1969). Was this a communication-attempt with Earth, a nuclear explosion or
simply volcanic activity? Until the age of spacecrafts, the question of life on
Mars in the mind of the scientist remained in a murky limbo. The general view
was that the conditions on the planet were too harsh to house spectacular
megafauna and a civilization of intelligent beings (and if such things ever did
exist on Mars, they would now be extinct), but the conditions were still
comparable enough to some extreme regions on Earth that they would permit the
existence of primitive plants and animals on the surface. A great example of such
a low-complexity Martian biosphere comes from the 1957 documentary Mars and Beyond, directed
by animation legend Ward Kimball as part of the Disneyland TV series. Apart
from hilariously pointing fun at sci-fi tropes of the time, the program also
presents a fantastic 5-minute segment that shows in a starkly realistic
style the possibilities of extremophile critters on the Martian surface. A big
argument at the time for the existence of surface life on Mars was that
telescopes showed green-blue patches across the surface of the planet change
shape, extent and colour with the changing of the seasons. This was often taken
as an unmistakeable sign of patches of vegetation responding to the winter and
summer seasons, with scientific papers on the Martian biosphere basing
themselves off such observations all the way into the early 60s (Salisbury
1962).
Fig. 9: Changing colour patterns on Mars, once taken
to be signs of vegetation, but now known to be caused by dust storms.
With
the age of robotic spacecraft came a great punch to the gut for any such speculations.
Mariner-4 successfully launched in 1964 and provided the first close-up
photographs of Mars’ surface in 1965. What they showed was a lifeless, moon-like
desert, with an atmosphere even thinner than previously expected and no
magnetosphere to shield the surface from harmful radiation. Despite this, prominent
figures such as Carl Sagan still defended the idea of surface life on Mars. Sagan
argued that Mariner-4 had photographed only a tiny part of the Martian surface
and at such a low resolution that, had the probe orbited Earth, it would not
even have been able to detect human civilization (Kilston et al. 1966). A year
later, he wrote an
article for National Geographic, accompanied by an illustration of Martian
surface life he deemed realistic. At night the plantlife folds up to protect
itself and the animals shield themselves with siliceous shells from harmful
UV-radiation.
Fig. 10: 1975 renderings of Martian surface
life, made by an unnamed artist at the JPL.
Despite
Sagan’s arguments, further probing by robotic spacecrafts increasingly showed
that Mars was more inhospitable than anyone would have thought, the last hopes
for surface life likely dying out in 1976 with the (albeit still somewhat
controversial) findings of the two Viking Landers. Only a year prior an unknown
artist at the Jet Propulsion Laboratory had produced his artwork of sessile,
silicon-based life on the surface of Mars. While conditions may have once been
more hospitable in the deep past, today we know that Mars’ modern, mainly
carbon dioxide atmosphere has only 0.6% of Earth’s air pressure, which,
combined with the extremely low temperatures, means that apart from toxically
salty brines, water can only exist in a solid or gaseous state. Its surface is
constantly baked in harmful radiation, which furthermore produces perchlorate
salts that are toxic to most life. The supposed patches of vegetation turned
out to have been seasonal dust storms instead, Lowell’s canals were nowhere to
be found and the Face on Mars of the Cydonia region was but an ordinary hill
viewed at a funny angle. While the search for life on our neighbouring planet
still valiantly continues, nobody expects anymore to find it on its surface. If
life exists on Mars, then today only in shielded and warmer sub-surface
habitats deep underground and, in all likelihood, only in microbial form.
Recent findings by rover Curiosity of seasonal methane and oxygen releases may
support such subterranean biospheres, as well as possible microscopic
ichnofossils inside Martian sediments and meteorites (Baucon et al. 2020).
But what if?
With
apologies to any microbiologist reading this, but where is the fun in some bacteria?
What if we turn back the clock to 1964 and have Mariner-4 first lay its eyes on
a different, slightly more habitable Mars, with just a little more air and a
little more heat, and have the probe detect some oases of life inside the deserts and
craters? Nothing too fancy, no civilizations, no giant beasts, just
extremophile fauna and flora trying to make its best out of living on a harsh,
but still liveable Martian surface. In the words of the Artilleryman: We’ll
start all over again!
On
this website you will explore this different Mars through the accounts of the manned
space mission Horus 2, which will take you across the dunes, dust plains, glacial lakes and
cratered canyons of Mars and showcase all its unique wildlife. This will be a
Mars as envisioned by the scientists and artists of the 50s and 60s, though it
will attempt to take into account some modern data about the red planet as well.
Fig. 11: A primitive antitrematan, during a time
when Mars still had an ocean. You will not get to meet this critter on
your journey through this site, but will instead meet some of its descendants.
Before
I release you onto Mars, I need to mention that all of what you will be reading
here will merely be a teaser for what is to come. I have been working for a
couple of years now on a physical book about Martian life, however in billion-year-old
fossil form. While Har Deshur will be a throwback to an alternate modern Mars, Life on a Dead Planet, as the work is
currently titled, will play on a Mars that is the same today as in our own timeline
and will instead be a fictional future paleontology textbook that will deal
with the fossils that have survived from the planet’s deep past when it still
had oceans. Nonetheless, you will see many familiar clades and lifeforms across
both works. When Life on a Dead Planet will come out I cannot say, as it
is undergoing a major rework, however you can read some of the earliest drafts
of some of the chapters on my Patreon if
you are interested enough.
Now do like Arnold Schwarzenegger and get your ass to Mars!
References:
- Baucon,
Andrea et al.: Ichnofossils,
Cracks or Crystals? A Test for Biogenicity of Stick-Like Structures from Vera
Rubin Ridge, Mars, in: Geosciences, 10, 2020.
- Davydoc,
V.D.: New Interpretations of Mayeda’s flare on Mars, in: Soviet Astronomy, 13,
1969.
- Heuser, Marie-Luise: Transterrestrik in der
Renaissance: Nikolaus von Kues, Giordano Bruno, Johannes Kepler, in: Michael
Schetsche/Martin Engelbrecht (Hrsg.): Von Menschen und Ausserirdischen.
Transterrestrische Begegnungen im Spiegel der Kulturwissenschaft, Bielefeld
2008.
- Holtsmark,
Erling: Edgar Rice Burroughs, Boston 1986.
- Kepler,
Johannes: Harmonice Mundi, Linz 1619.
- Kilston,
Steven; Drummond, Robert; Sagan, Carl: A
search for life on Earth at kilometer resolution, in: Icarus, 5, 1966, p.
79 – 98.
- Lowell,
Percival: Mars as the abode of life, Flagstaff 1908.
- Salisbury,
Frank: Martian
Biology. Accumulating evidence favors the theory of life on Mars, but we can
expect surprises, in: Science, 136, 1962, p. 17 – 26.
- Schulze-Makuch,
Dirk; Crawford, Ian: Was There an Early Habitability Window for Earth's
Moon?, in:
Astrobiology, 18, 2018, p. 985 – 988.
- Wells,
Herbert George: The things which live on Mars, in: Cosmopolitan Magazine, 44,
March 1908, p. 334 – 342.
Image Sources:
