The Lovely Lost Landscapes of Luna

That is the title of one of Isaac Asimov’s science essays, collected in his book Is Anyone There?

“The Lovely Lost Landscapes of Luna” was published in 1966, but its overall conclusions are anything but out-of-date. IA grew up with early 20th cy. science fiction, and much of it featured lots of adventures on other Solar-System planets. I recall that someone once claimed that the first SF writer to break out of the Solar System was likely E.E. “Doc” Smith, with his Skylark of Space (1928).

A curious consequence of telescopic observations and acceptance of heliocentrism was the belief that all the Solar System’s planets were inhabited by sentient entities and whole biotas of organisms. In the eighteenth century, that was a very common belief, on the ground that God would not want to waste a world. But in the nineteenth century, some scientists started getting skeptical about that, and by the mid twentieth century, their skepticism had become mainstream. Most of the Solar System seemed very hostile to all but the hardiest microbes, and often to them also. Then spacecraft were sent out, and they returned an abundance of evidence of how hostile the Solar System is.

There was the possibility of planets of other stars, but that was not enough for Isaac Asimov, and he had grown up on the abundantly-inhabited Solar System of 1930’s science-fiction stories.

No, no, the stars are not enough. It’s the solar system we want, the solar system they took away from us thirty years ago.

The solar system we can never have again.

Though since then, hundreds of exoplanets have been discovered. As of 2017 October 5, NASA Exoplanet Archive lists 3529 confirmed planets of other stars, and some of these planets are almost Earthlike.

How it happened is below the fold.

The Sun

In 1800, astronomer William Herschel proposed that the Sun was inhabited. Sunspots were breaks in an outer fiery layer, he proposed. Through them one could see the habitable parts of the Sun.

But sunspots are actually hot enough to glow very brightly (3800 K) — just not as bright as the surrounding photosphere or “surface” (5800 K). That’s where outgoing light can escape the Sun rather than be absorbed by the Sun’s material. The Sun gets hotter and hotter the farther one goes inward, reaching 16 million K at its core.

The Moon

In 1835, the New York Sun entertained everybody with the Great Moon Hoax, alleged discoveries of inhabitants on the Moon by astronomer John Herschel. But over 1834 – 1837, astronomers Wilhelm Beer and Johann Heinrich Mädler concluded that the Moon has no bodies of water or noticeable atmosphere. They presented their conclusion in their 4-volume book Mappa Selenographica and their book Der Mond nach seinen kosmischen und individuellen Verhältnissen (The Moon according to its cosmic and individual circumstances).

H.G. Wells in his 1901 story The First Men on the Moon had the Moon carpeted with plants. But the moonmen lived underground.

But what about the far side of the Moon? Could the Moon be egg-shaped, with the narrow end pointing to the Earth? That would allow the far side’s surface to have low enough gravitational potential to retain an atmosphere without it leaking over to the near side.

Scale height, Lapse rate — there is a problem. The Earth’s atmosphere at 273 K (freezing point of water) falls off with an e-folding distance of 8 km. At 80 km, there isn’t much of it — it’s almost outer space. But the scale height is inversely proportional to the acceleration of gravity, so one would get 480 km for the Moon, not much less than its average radius of 1738 km. There is also the question of the rigidity of the Moon’s material. The pressure on top of Mt. Everest is 1/3 that of sea level, and that mountain’s height above sea level is 8.848 km — a little more than a scale height. So 10 scale heights may be too much for the Moon’s structure.

Scale height = (Boltzmann’s constant) * (temperature) / (molecule mass) / (acceleration of gravity)

Pressure, stress = (density) * (acceleration of gravity) * (height)

So in the early to mid 20th century, one could have deduced that this egg shape is not stable — the Moon would slump into a more round shape.

But when spacecraft were sent to the Moon, they returned pictures showing that the far side of the Moon is just like the near side: airless, waterless, barren, cratered, mountainous, though for some reason, with much fewer lava plains. Those plains are the familiar blotches on the near side. Not just automated spacecraft, astronauts as well.


There seemed to be a fighting chance for habitability. The canals. Percival Lowell had mapped them in detail. They were obviously some engineering work, meaning that they were built by sentient inhabitants — Martians.

In a common planetary-formation scenario back then, the planets were formed outermost to innermost, meaning that Mars was older than the Earth.

This meant that Mars was gradually drying up, and science-fictioneers had fun with various scenarios about it.

  1. The Martians were a wise and advanced race that was resigned to the inevitable, a race that was willing to offer its wisdom to brash youngsters like Earthlings.
  2. The Martians noticed a nice planet to live on, but a planet that already had inhabitants — the Earth. So they tried to conquer it so they could move there. IA recalled enjoying stories of valiant Earthlings defeating villainous Martians.
  3. The Martian race had died out, leaving its artifacts behind, including canals. Arriving Earthlings then built colonies and puzzled over these ruins.

But as IA noted, bad news kept coming from the observatories. Mars’s atmosphere was as thin as atop Mt. Everest, if not thinner. There was no detectable oxygen. Some astronomers could not see the canals. However, there was some water.

Then Mariner 4 flew by Mars in 1965. Craters but no canals, and a very thin atmosphere. Mariners 6 and 7 also found that out. Mars seemed much like the Moon. But in 1971, Mariner 9 went into orbit around that planet. Despite arriving during a dust storm, the spacecraft mapped the entire planet. No canals, but lots of of things besides craters. A huge rift valley. Huge volcanoes. Numerous dry riverbeds. In the years to come, spacecraft would land on its surface and wander around on it. Its atmosphere is mostly CO2 with a surface pressure less than 1% of the Earth’s, and its surface is a rocky, dusty desert.

Something like scenario #3 has become a mainstream opinion. There is evidence that Mars’s atmosphere was much thicker in the past — isotope abundances. The lighter isotopes evaporate faster than the heavier ones, leaving behind the heavier ones. There is also evidence for former oceans: the Mars ocean hypothesis, with oceans in the northern hemisphere and in the Hellas basin about 4 billion years ago. These oceans then dried up, doing some combination of evaporating into space or becoming permafrost. This has problems with the Faint young Sun paradox, but Mars’s precession has some chaos in it (mars-obliquity.pdf), enough to give it times of high obliquity. That would give the poles hot summers, enough to keep water liquid.

If these oceans existed, then the could well have supported life. Likely primitive organisms like those that lived on the early Earth, but organisms nonetheless. Something like present-day methanogens, which get their energy from

4H2 + CO2 -> 2H2O + CH4

and which make all their biological molecules, as plants do. Some of them could well have survived to the present day, though they would have had to survive in cracks in the planet’s crust, something that some Earth organisms are known to do: Deep subsurface microbes – microbewiki. They would also need enough geological activity to make chemical disequilibria like H2 and CO2 present together, so the organisms have something to extract energy from.


According to that cosmology, Venus was younger than the Earth, and was thus like the Earth in the Carboniferous: covered with a big jungle. IA recalls stories of mold threatening the plants or the plants fighting each other.

Another common notion back then was that Venus was covered with a planetwide ocean. For a Lucky Starr novel, IA liked the idea enough to populate it with lots of weird sea creatures, including an octopuslike monster a mile in size.

Mainstream astronomers also proposed an ocean of hydrocarbons and a desert. In a 1961 paper on the planet in Science magazine, Carl Sagan noted that “those planning eventual manned expeditions to Venus must be exceedingly perplexed over whether to send along a paleobotanist, a mineralogist, a petroleum geologist, or a deep-sea diver.”

Those perpetual clouds? Formaldehyde? Dust? They settled on water for a while.

Then radio observations from the ground and from Mariner 2 — temperatures of 300 C — and 300 C all over. Too hot for liquid water. After the article was written, Mariner 5 and several Venera landers found that it surface was even hotter — 450 C. To Carl Sagan’s list we can add a volcanologist.

The clouds? Mostly sulfuric acid. Battery acid with more of the acid part.


Very close to the Sun, with the sunlit side very hot. It used to be thought that Mercury’s rotation was synchronous, like the Moon’s. This suggested a mild zone between the day and night sides. But radar observations showed that Mercury’s rotation period relative to the stars is 2/3 of its year, and thus its period relative to the Sun is 2 Mercury years.

The outer planets and their moons

“In the Thumping Thirties, we peopled them all.” Including Jupiter and Saturn. IA recalled some stories where Saturn’s surface had big Wild West prairies with huge herds of cattle. He also wrote some stories about menaces from Ganymede and Callisto, two of Jupiter’s four big moons. Titan was another favorite.

Distance was no obstacle. One of the great stories in 1930 was the Solar System facing doom from villains inhabiting Neptune.

But the outer planets have atmospheres of mostly hydrogen and helium and methane and the like, and Jupiter and Saturn are well fit by theoretical models that use a composition not much different from the Sun’s: mostly hydrogen and helium.

Most of the outer-planet moons are airless, and those with atmospheres have mostly nitrogen and methane in them.

Io, however, has active volcanoes, a rather startling discovery.

The visible parts of the outer planets and their moons are also very cold by human standards.

There is a possibility that some of the outer-Solar-System moons may be inhabited, however. Europa and Enceladus have evidence of subsurface oceans, and some other big icy moons may also have subsurface oceans. If they have suitable geological activity, then they will have chemical disequilibria like H2 and CO2 together, something that can power the organisms.

Comets and the Earth

Jules Verne once wrote a novel in which a comet bounced off the Earth and some people ended up living on it for a while. But the visible parts of comets are dust and gases released from their nuclei, and those parts resemble small asteroids. More-or-less resemble, because Comet Churyumov-Gerasimenko has a very complicated surface, more like what a planet would have. Or a big moon.

On the Earth itself, there is no El Dorado in Central America, no She in Africa, no Shangri-La in Tibet. The Earth isn’t hollow, as is evident from earthquake waves traveling all the way through it. The farthest down that we have dug is at the TauTona and Mponeng gold mines in South Africa, about 4 km. The farthest down that we have drilled is at the Kola Superdeep Borehole, about 10 km. Geological effects can get us rocks from further down. Ophiolites, pieces of oceanic crust shoved above continental crust, have a thickness of typically around 10 km. Kimberlite pipes, often mined for diamonds, go down 150 – 450 km. But the Earth’s core is even farther down. The outer core is 2900 km down, and the inner core 5200 km down.

So there we have it. Our homeworld, the Earth, is the only place in the Solar System habitable by us and most of our planet’s biota.

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