The craters.
The meteorite impacts smoothed the craters to a point they reflect the light of the sun. Frozen ice flows would simply melt under the impact, given that the impacts had to occur after the "ice" froze.
If it were ice, the dust coating would again just dull the reflection in the crater, clinging to the released moisture. If it were some sort of volcanic flow, the polished and heated surface would resist dust clinging to it, which is what we see in the image
The material in that sea is almost definitely not water or ice.
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