The artwork featured detailed Nasa images of the planet and at an approximate scale of 1:1million, each centimetre of the internally-lit sculpture representing 10km (6.2 miles) of the surface of the planet.
The piece is on show until 2 November, with tickets to it also offering free entry to the cathedral.
One boy who spoke to the BBC while with his family said: “I didn’t expect it to be that big.”
Visitors take in cathedral's 'big' Mars sculpture https://t.co/u80pd8FDro
— BBC Cornwall (@BBCCornwall) October 28, 2025