What is the main scientific reason for prioritizing the return of samples to Earth labs?
Answer
Earth-based laboratories possess instruments orders of magnitude more sensitive than those mobile platforms can carry.
The decision to return samples stems from the limitations of in-situ analysis; the capabilities available in large, sensitive Earth-based laboratories cannot be replicated on a lightweight, mobile platform operating millions of miles away.

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How does a Mars Rover work? (Perseverance) - YouTube
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