Why are photometric observations of giant planets and their satellites, and not the Sun itself, carried out to study

Why are photometric observations of giant planets and their satellites, and not the Sun itself, carried out to study the constancy of the solar radiation power?

The largest valley currently known in the solar system is the Mariner Valley on Mars. It is a huge canyon over 4000 km long, 120 km wide and 5-6 km deep.

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