ice
Ice is a common resource in Xenominer. It will not be a big burden on your inventory as you will often use it to refill your oxygen supply. Do not save it up as this is all it is good for.
You can find ice in abundance pretty much anywhere. The best place to look for ice is at the top of mountains but you can find it buried underground occaisionaly. Either way it is quite common. If you cannot find any ice you can use Regolith to refill youroxygen.