Adas is marginally warmer than Rannoch despite being further from its sun. Volcanic activity spews methane into Adas's atmosphere, and this haze retains heat in a greenhouse effect. Historically, the quarians used the geth to mine the planet, and when the geth rebelled, the small quarian population on and around Adas was quickly overrun.
It is clear that the quarian armada has not forgotten or forgiven. The remains of geth space stations litter Adas's orbit, now all shrapnel and fused metal. The Normandy's sensors pick up a strange croaking sound, probably sound kind of distress call from geth survivors left floating in space, condemned to the eternal cold.