The spacecraft is in a hugely elliptical orbit and only approaches Mars for a small part of thetime. ESA didn't spend the money to have enough fuel to circularise the orbit and don't have the chutzpah to try an aerobraking circularisation. (Maybe don't even have the fuel to raise periapsis after aerobraking, either!).

Rather than think about changing the orbit, they're waiting until the precession of the orbit means that closest approach will be in the nighttime (I know that's almost what you said, but it's subtly and mportantly different).

What is interesting is the one figure in that article that shows the ground reflection. There's a lot of interesting information in there - the range appears to decrease slightly with increasing frequency (? getting first arrivals from the tops of hills rather than the naverage ground elevation?). There are also waveform changes with frequency that will require a careful analysis.

What is clear, is that there is no obvious subsurface layer generation any reflectivity, so in this area (wherever on Mars it is), at the time of observation, there was no obvious water.

We can thereforealmost rule out models of abundant subsurface water. The game has shifted to looking for sparse subsurface water or even, reluctantly, pointing to a lack of liquid water in the subsurface. :-)