With surface temperatures estimated at minus 230 degrees Fahrenheit, Jupiter's moon Europa has long been considered far too cold to support life, its 60 square miles of water thought to be frozen from top to bottom.
Why there is no conjunction precedes the last clause. I think a conjunction is indispensable between two independent clauses. So in my view, the last clause should be: and its 60 square miles of water thought to be frozen from top to bottom.
GMAT Forum