An alien scientist injected microbes deep under the ice.

"Teacher, what are you doing?" asked an apprentice.

"It’s a test to see if they'll preserve it."

"Preserve what?”

"This planet. If the ice melts, it'll trigger a tipping point, and the fossilized bacteria's DNA will spread."

"What will that prove?"

"That the dominant species isn't qualified to join us. Only greed will melt the ice. I'm applying the same Great Filter to all habitable exoplanets we discover. Those that survive will eventually make first contact."

"And those that don't?"

"It's obvious. But we can make use of a planet's resources."