In the new sci-fi movie "Oblivion," Earth's most precious resource is Tom Cruise. But running a close second (spoiler alert) is water. Aliens want it. All of it.
This is old hat, science fiction-wise. In "The War of the Worlds," H.G. Wells had Martians coming to Earth to quench their thirst. The extraterrestrial lizards (cleverly disguised as human catalog models) in the 1980s TV series "V" came here to steal our water too -- though they wanted it in part to wash down the meal they intended to make of us. In the more recent "Battle: Los Angeles," pillaging Earth's oceans was the only motivation we're given for why aliens were laying waste to humanity.
The first problem with this plot device is that it's pretty dumb. Hydrogen and oxygen are two of the most common elements in the universe. An alien race is savvy enough to master interstellar travel but too clueless to combine two Hs with one O to form H2O? C'mon.
At least in "Mars Needs Women," the precious resource in question -- Earth girls -- by definition can be found only here, just as "real" Champagne must come from the region that bears its name. And though I have no doubt that Earth women really are the best, the logic of evolution suggests that compatibility issues for aliens would be a hurdle not even Match.com could overcome.
In "To Serve Man," the famous "Twilight Zone" episode, the motivation was far more plausible: They wanted to eat us ("To Serve Man" -- it's a cookbook!). And who knows -- maybe we're delicious.
One rule of thumb in sci-fi is that the aliens are really us too. They reflect a good trait in humanity -- think E.T., Spock or Mork -- or a bad one. That's why writers recycle ancient human motives -- the desire to plunder, colonize, rape, enslave -- as the motives of futuristic aliens.
That's all fine. But science fiction is also supposed to raise ambitions for what humans can accomplish. And in that, Hollywood is failing.
For a while now, filmmakers have been churning out fare -- like the horrendous remake of "The Day the Earth Stood Still" -- based on the Malthusian assumption that resources are finite and if we keep going the way we are, the Earth will be "used up" (to borrow a phrase from the opening monologue of the canceled cult sensation "Firefly"). Either that or we'll be invaded by aliens who appreciate our stuff more than we do.