Why do space probes and robots need to send samples back to Earth when they have analytical equipment on board?

Question

I’ve seen some of the Martian rovers are collecting samples to be sent back to Earth to determine the presence of microbes on Mars. But why do they need to be sent to Earth? Why can’t the analytical equipment on board detect it?

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Answer 1

In addition to what’s already been said about limited capabilities on the spacecraft, physical samples can also be sent to different labs to repeat observations. This means they can double check each other’s results, and be more confident that they are seeing genuine data and not errors or instrument faults.