Yes, it is sunlight, but I also believe they process these pictures to accentuate specific details.
Discussion
We actually reflect a lot back! In fact, 100% of it, otherwise if we didnβt the earth would be cooking by now. Itβs about 1KW of energy per square meter at 90 degrees angle. πΆπΎπ«‘
Here is some good educational material about thatπΆπΎπ«‘
So the sun is behind Mercury? π€