Planet Zorg had a lot of things, but one thing it didn't have a lot of was sodium chloride. Its ecosystem had evolved without it, and its species — sentient and non-sentient alike — couldn't tolerate a lot of it. They were far more comfortable with the beautiful yellow sulphur clouds that swirled through the atmosphere, and ammonia-rich but otherwise "fresh" water of the planet's lakes and oceans.
Therefore, when a planet came up on their scanners that featured not just one, but five large fresh-water lakes in a single watershed, the Zorgians were ecstatic. Here was a planet which they could not just invade, but perhaps even settle. The resource harvesting teams might get to take their helmets off now and then. Low morale in other off-world invasion forces had been a chronic problem. This planet was two-thirds water with high levels of sodium chloride, to be sure, but if the fresh-water inland lakes were used as a home base for the first attack wave, the lowered risk had an excellent chance of returning a more lucrative reward.
A Zorgian mother ship was dispatched, and it sent down scouts the night after the lake area's winter solstice, the better to utilise the cover of darkness.
Scouts are partly bioengineered, and emit their own natural light. To avoid attention, they were directed to head for the indigenous civilisation's major habitations, since those areas already had so much natural light pollution a few more lumens wouldn't be noticed.
The first reports back from the scouts to the mother ship were enthusiastic. The atmosphere and solid surface of the planet were cold, but well within the Zorgian tolerance range. Likewise, the atmosphere was mostly nitrogen, not sulphur, but nitrogen was an easily-tolerated element, and not a concern.
Most amazing of all, there was frozen fresh water carpeting the carbon-based ground, with more falling from the sky! The atmospheric water fell in very-cold droplets, landed on various land features, and froze solid.
The scouts sent back data on their findings: chemical analyses, photographs, further observational reports. When their vibration sensors detected something large moving nearby, they hid under some plant life, beautifully coated in the alien planet's low-ammonia atmospheric fresh water.
One scout was able to successfully send back moving photographs before it was destroyed like all the others. The data showed a large moving object — possibly sentient if alive, or controlled by a sentient if a machine — working its way slowly down a clear area coated with a thick layer of frozen water.
And that's when the Zorgians realised that the sentient life on this alien planet was more than ready for them, and was already mounting a counter-attack. What the large moving object was spewing out from its sides were large crystals of near-pure sodium chloride.
Only one crystal needed to make contact with a scout to destroy it. Most scouts encountered several. The sodium chloride reacted with the ammonia and sulphur in their systems and ruptured their outer carapaces, exposing their innards to the alien atmosphere and killing them.
The Zorgian mother ship commander quickly calculated the loss of scouts against the viability of the Plan B site — a single large lake located in a much less hospitable area — and aborted the mission. It immediately ordered the crew to initiate a hyperjump back to the home planet, and settled in to write an indignant report shaming the military researchers and their poor viability analysis.
"Spread pattern okay?" said Frank.
"Looks like it," said Bill. He glanced in the passenger-side mirror. "What the hell is that?"
Frank shrugged. "Temperature must have climbed up to just over freezing. Sounds like the ice popping off." He navigated the salt truck past several large fallen tree branches. "I don't envy the Hydro people. They'll be working around the clock past Christmas on this mess."
"Yeah, but that green glowing thing we went by..."
"I didn't see it, but I'm guessing it's a lawn light. This neighbourhood seems to still have electricity overall."
"You're right," said Bill. "It just looks weird under the snow like that."
Frank grunted and navigated the next corner.