November 23, 1963 the first episode of the British science-fiction television programme "Doctor Who*" was broadcast. The series follows the adventures of the "Doctor", last survivor of the Time Lords, an incredible advanced alien race once native to the planet Gallifrey. In his 50 years long history the abilities of the Doctor to manipulate space and time made physicists and astronomers alike speculate about the science behind Doctor Who, but what about the geology as depicted in the series?

As Time Lord the Doctor should be confident in the basics of terrestrial history and if he wishes to travel further back in time also in chronostratigraphy. Geologists are also sort of Time Lords, even if their instruments are not so sophisticated as a TARDIS (and lack self-consciousness), they were able to produce a map of time. But not only stratigraphy, also seismology and volcanology made a guest appearance in the series.

As it seems the 10th doctor and his companion Donna Noble are responsible for the eruption of Mount Vesuvius in A.D. 79 - but this was necessary to prevent the invasion of earth by a silicon-based life form thriving inside the dormant volcano.

In 50 years the Doctor meet many strange creatures, but maybe silicon-based species are the most fascinating for a geologist. Silicon is one of the most abundant elements on earth and combined with oxygen forms the great variety of silicate minerals - but despite their beauty they are lifeless...

In 1891 the German astrophysicist Julius Schreiner was one of the first to propose silicon (Si) based life forms. Three years later H.G. Wells speculated in an article of popular science about the potential of this idea:

"One is startled towards fantastic imaginings by such a suggestion: visions of silicon-aluminium organisms - why not silicon-aluminium men at once? - wandering through an atmosphere of gaseous sulphur, let us say, by the shores of a sea of liquid iron some thousand degrees or so above the temperature of a blast furnace."

In 1934 Stanley Weinbaum (who had studied chemistry, but never graduated) published his short story "A Martian Odyssey", where he introduced the first silicon-based life forms in popular culture. He even describes their metabolism:

"Those bricks were its waste matter... We're carbon, and our waste is carbon dioxide, and this thing is silicon, and its waste is silicon dioxide-silica. But silica is a solid, hence the bricks. And it builds itself in, and when it is covered, it moves over to a fresh place to start over."

His readable style influenced many other science fiction authors and the idea of alternative biology was soon adapted to various movies and television episodes.

In the science-fiction/monster movie "The Monolith-Monster" (1957) fragments of a meteorite are discovered in the desert of California. The strange extraterrestrial mineral from outer space starts to grow to gigantic crystals when it comes in contact with water and a storm is approaching fast (the movie is also worth to watch for the used geo-babble). The idea of minerals as an invasive species is also used in the computer game "Command & Conquer: Tiberium", where the alien mineral Tiberium grows by extracting nutrients from organic life forms. Even the smallest crystal of these minerals can grow to gigantic proportions, however they can´t be classified as true aliens, as they lack the ability to actively self-replicate or evolve.

More evolved are the creatures from the "Island of Terror" (1966). Here the artifical "silicates" can move and even hunt for the calcium of human bones.

In fact the very incarnation of an extraterrestrial predator - H.R. Giger´s "Alien" - possesses a shell of Si-compounds to protect it from environmental factors. This resistant shell is also very useful to contain the acid blood of this monster.

In the TV series "Star Trek" the crew discovers a silicon-based life-form during mining activities on the planet Janus VI, however soon we discover that the "Devil in the Dark" is less hostile than other xenomorphs.

"Life as we know it, is universally based on some combination of carbon compounds. But what if life exists based on other element. For instance silicon."

In the late 20th century adaption of "Star Trek: The Next Generation" some episodes feature a living "Crystalline Entity", however this time again a creature hostile to human life.

In the X-file episode "Firewalker" a mushroom like parasite infests some volcanologist, unfortunately killing them by filling their lungs with silicon oxide. We discover that this time the life form is a result of convergent evolution, adapted to live inside partially molten rocks.

Despite such promising visions, it seems improbable that we will soon encounter some of these fascinating creatures like the Doctor does. The structure of silicates as we known them is limited mostly to long chains or sheets. Silicon doesn't form complex molecules, like enzymes needed in carbon-based life forms and it would be difficult to achieve a metabolism with such a simple chemistry. But in the end “I am and always will be the optimist. The hoper of far-flung hopes and the dreamer of improbable dreams.”

Bibliography:

REYNOLDS, J.E. (1893): Address to the British Association for the Advancement of Science. Nature 48(477)

WELLS, H.G. (1894): Another Basis for Life. Saturday Review: 676

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