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There are plenty of similarities between the two volcanoes, despite the discrepancy in scale – Etna is around 200 times smaller. Tharsis is surrounded by titanic gouges in the landscape radiating outwards for hundreds of miles, known as fossae. The biggest of these structures, Valles Marineris, averages 4km deep and stretches some 4000km. On a smaller scale, comparable faults radiate into the landscape around Etna. Both Etna and Tharsis show similar patterns of rifting running through the body of the volcano. And the basic proportions of the two structures are surprisingly alike.