Re: That conclusion seems a bit fast to me
Well, there is plenty of Earth-based experience to back it up. Back in the 1980s, UK researchers (mainly at Scott Polar Research Institute) found similar evidence of lakes beneath the Antarctic Ice Cap. Lake Vostok was the big one, but many more lake candidates were identified. Of course, at the time the possibility of actually verifying that they were lake using drilling techniques was science-fiction! But there are plenty of ways of checking that the bright reflections are from a water layer:
1) Is the top surface smooth compared with surrounding areas?
2) Is the dielectric constant estimated from the strength of the echo compatible with water?
3) Is the combination of pressure (derived from the thickness of the ice column) and temperature (derived from knowledge of surface temperature plus estimates of geothermal heat flux) within the liquid part of water's phase diagram?
4) Is there a surface expression of the lake (not found until 1996 by SAR imaging)
Since then at least one of these lakes has been drilled and they are water bodies.
Finally, there's the question of what else could it be? It looks like water, it behaves like water and it's beneath a 2km column of (mostly) water!
I used to do this stuff for a living!