Saturn’s moon Enceladus has a rocky core and an icy surface.
Conclusion (must be)
There must be a lake of liquid water between these two layers of Enceladus (the rocky core and the icy surface).
Evidence (and)
The Cassini space probe measured the density of Enceladus: between the rocky core and the icy surface is something denser than ice, and that could only be liquid water.
Evaluate
Whenever we do Main Conclusion (or Role), we prime our brain before we read the paragraph with the reminder that the Conclusion is almost always earlier than the Supporting ideas. In fact, a ton of the time the Conclusion will be the very first claim. We read these listening for the first opinion we hear from the author.
The word must is being used to convey opinion in the second sentence. It's a weird word, because in conditional logic contexts it means factual necessity. But in "Solve the Mystery / Explain the Curious Fact" contexts, must is used to convey that speculative use of must that we use.
Pretend you were hosting a party, you left to go to the bathroom, and when you came back you see that two of your friends (a married couple) left without saying goodbye. Your brain would want to Solve the mystery / Explain the Curious Fact.
You might scratch your temple, confused, and say,
That's the speculative "must" being used in the 2nd sentence of this paragraph. We would hear more naturally as announcing the author's causal hypothesis if it occurred at the end of the paragraph:
We probed Enceladus. We found that there's something in between the ice layer and the rocky layer. It's denser than ice. Liquid water is only plausible thing that could be there that's denser than ice. Thus, there must be a lake between the ice layer and rock layer.
So one thing assisting our ability to find this conclusion is our knowledge that the Conclusion almost always occurs earlier than the Support (on Main Conclusion and Role questions). Another thing helping us is our awareness that must gets used to convey a speculative causal hypothesis (i.e. an opinion from the author).
Finally, if the two claims are joined by the word "and", then they are almost always both Supporting ideas. The "and" is showing us that these ideas work together, they blend to let us derive the Conclusion.
Goal
We just want to replicate the meaning of the 2nd sentence. On Main Conclusion, we make sure to import "borrowed language", if there are any pronouns or implied nouns.
On Enceladus, there is a lake of liquid water between the rocky core layer and the icy surface layer.