As I mentioned before, the absurd idea from the original V series that the aliens had arrived to steal our water made sense in the 80's, when widespread water in the Solar System had not been discovered. Science since has proben that one of the primary constituents of asteroids (and several planetary bodies including the moon Europa (which may contain three times the amount of water on Earth) is water, and it would be much simpler to extract water from a low-gravity moon than from the surface of a strong gravity planet like Earth.
So I was, and still am, curious what the premise will be of the aliens arrival on Earth in the newest iteration of the V series.
So after careful consideration, I have developed two possible reasons that aliens would arrive on Earth and not just irradiate the surface immediately. Because honestly, if an alien race had the tech to actually get to Earth they would either not need to come here, or need to come here for non-diplomatic reasons.
1. Beryllium/Aluminum Harvesting: It turns out that in the known univese, Beryllium (element 4) is not formed in the furnaces of stars; most beryllium was formed during the Big Bang, or when the gamma bursts of star supernovae hits interstellar dust particles. These are both fairly rare events (the Big Bang only happened once...or did it???). The aliens arrive on Earth, and inform us that their interstellar drives rely on the spontaneous oxidation of water when in contact with beryllium, and they need more beryllium for the war fleet they are building to attack the other aliens.
Earth, as it turns out, has a decent amount of beryllium. So the aliens begin harvesting beryllium. Unfortunately, that means they basically dig up and superheat the crust of the planet, which wipes out human civilization and causes wide-spread tectonic chaos. Once the aliens extract all the beryllium they can, they move on, leaving our planet a barren, lifeless rock.
Or alternately, the aliens could show up, and mention that 96% of the matter in the universe is useless dark matter and energy, and of the 4% of the Universe that is matter, 98% of that is hydrogen and helium (which doesn't do them much good considering their star drive runs on beryllium), and of the 2% of matter in the Milky Way galaxy that isn't hydrogen or helium, about 0.29% percent of the remaining matter is aluminum, which they use to build their interstellar warships. As it were, Earth contains tons and tons of aluminum, so the aliens proceed as above, mining and superheating the crust of the Earth to extract the metal, turning our world into one big barren, toxic strip mine. Then, with all the usable aluminum mined, the aliens depart. Or worse yet, they set up their shipyard in space, and plunder our atmosphere for gasses for their welding equipment.
2. Xenoforming: Turns out the aliens that arrive above our cities are outlaws from an interstellar union, and have been driven from their homes. They arrive at Earth and find the humans both pathetically low-tech and easily manipulated. They quickly and easily set up a slave state for themselves, and whenever an uprising occurs, they just brutally and quickly vaporize the city in which the uprising occurred, forcing the kowtow of humanity. When the interstellar union forces arrive at Earth and find the aliens deeply ingrained in the human culture, they simply glass the planet, eliminating both the outlaw aliens and us.
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