Maybe NASA believes that the little green men really are green - one of its scientists has collaborated on a report suggesting that the Earth's rising levels of greenhouse gases could provoke an alien attack.
Using spectrometry, extraterrestrials could detect changes in Earth's atmosphere and deduce that we're out of control, the report suggests. It's one of a number of scenarios discussed in the paper, written by Shawn Domagal-Goldman of Nasa's Planetary Science Division and Pennsylvania State University academics.
"These scenarios give us reason to limit our growth and reduce our impact on global ecosystems. It would be particularly important for us to limit our emissions of greenhouse gases, since atmospheric composition can be observed from other planets," the authors write.
"A preemptive strike would be particularly likely in the early phases of our expansion because a civilization may become increasingly difficult to destroy as it continues to expand."
In some places the authors get really carried away - one wonders how many of them are frustrated would-be Hollywood screen-writers.
If we did get attacked by aliens, they suggest, it's possible that another bunch might fly in to save us.
"In these scenarios, humanity benefits not only from the major moral victory of having defeated a daunting rival, but also from the opportunity to reverse-engineer ETI technology," they say with glee.
We should also be very careful about giving away any information about our biology, they say, as this could be used to genetically engineer biological weapons.
The authors even worry that our planet might not be up to the paperwork and bureaucracy of joining a Galactic Federation.
It's hard to believe that aliens would really need evidence of greenhouse gas increases to decide that we're a pretty despicable species. It's been widely pointed out before that anyone within 75 light-years of here has already seen Hitler in full flow.