What observational evidence supports the idea that Mercury once shrank by some 20 kilometers in radius?

Name: What observational evidence supports the idea that Mercury once shrank by some 20 kilometers in radius?
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The presence of many long, tall cliffs, known as scarps, supports the idea that Mercury once shrank by about 20 kilometers in radius. These scarps are thought to have formed as a result of the planet's cooling and contraction, causing the crust to break and shift.

What Observational Evidence Supports the Idea That Mercury Once Shrank