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

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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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I am not sure if a sample return is required to detect microbes or not.

The general idea is that the analytic equipment onboard is limited in the number of tests and what tests can be done. So if you return the sample to earth more types of analysis can be done

If you look at the website of a proposed sample return mission they describe it as the following [https://www.jpl.nasa.gov/missions/mars-sample-return-msr](https://www.jpl.nasa.gov/missions/mars-sample-return-msr)

>Mars Sample Return is a proposed mission to return samples from the surface of Mars to Earth. The mission would use robotic systems and a Mars ascent rocket to collect and send samples of Martian rocks, soils and atmosphere to Earth for detailed chemical and physical analysis. The mission is being planned jointly with the European Space Agency.

why do you check the oil level on your car by pulling the dipstick out, wipe it then dip it again

if you did the double dip and an actual qualifed and time served mechanic said hey can i dip it again then tell you what i think most of us would say yup go for it dude let me know what you think

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the stuff we send to other planets does the manual work but its super expensive to send even 1 gram of weight into space. So we give them enough tools to give us an indication of stuff

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we still want that stuff sent back to earth so some super clever people in labs can take a look at and go yo send your next many millions and hundreds of millions of monies and all that time it will take into this place we have figured out we need to look at next based on what we have been able to study

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for the now. the future will change and we can go all sci fi on this and have probes that do many more things, or send their information back to more advanced space stations.

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we have one ISS. it can’t do everything we can do on earth. the probes can’t do everything we can on earth. so for now everything needs to come back to earth

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.

We have much better and more capable equipment on earth in labs, than what can be sent on a rover.