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NASA have already sent several surface rover vehicles to the surface of planets, mostly to Mars. Just think about Curiosity.

Compared to the fact that these rovers are remotely controlled and have strongly limited energy resources (not so much they could bring with them, and have "only" solar panels to collect some), they can travel quite big distances. I heard that one of these Mars rovers (don't know which one?) could travel up to 20 kilometres on the surface of Mars.

However, is this enough to make general assumptions about the whole planet? I mean, even on Earth it matters, where those 20 kilometres would be; be it in the Amazonas, in the Himalayas, or in the New York city, etc.

Obviously, prior to organizing a landing project like the Curiosity rover, NASA have monitored the surface of Mars with additional devices — but is it accurate enough, coupled with data gathered from surface rovers?

If I wasn't clear, here's an example: Curiosity measures a temperature inside a crater, but this value may be different inside other craters and valleys.

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    $\begingroup$ That's true of every measurement on every planet by every tool though. A measurement in one ONLY ever proves irrefutably that the measurement at that exact time and spot was the value taken.unless i am misunderstanding your point $\endgroup$
    – user106
    Jul 30, 2013 at 11:26
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    $\begingroup$ Curiosity was mentioned just for an example for Mars rovers. Actually, this is the only one I knew from name. $\endgroup$
    – Zoltán Schmidt
    Jul 30, 2013 at 12:36
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    $\begingroup$ If Martians land a rover in the Sahel, they might conclude Earth is very dry. $\endgroup$
    – gerrit
    Jul 30, 2013 at 13:13
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    $\begingroup$ Indeed! That was I meant in this question. $\endgroup$
    – Zoltán Schmidt
    Jul 30, 2013 at 13:15
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    $\begingroup$ You should distinguish between accurate data and representative data. The data is quite accurate for the places it is measured. The question of whether it represents all of Mars is what you seem to be asking. $\endgroup$
    – Ross Millikan
    Dec 18, 2013 at 0:26

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The general philosophy that NASA and others has taken with landing missions is that they are used to validate something strange seen from on orbit, or to answer questions that can't be observed from on orbit, plus safety considerations in their landing spot. For example, here's the reasons that some of the landers landed where they did:

  • Curiosity - Clay was observed from on orbit, plus interesting layers observed.
  • Phoenix - Selected because of ice observed just below the surface.
  • Opportunity - Observation of Hemetite.
  • Spirit - Signs of erosion, questions relating to volcanic activity

These findings can validate the orbit data, and help us to better understand the orbital data. However, if we send a thousand landers, we can never completely be convinced that there was never life on Mars, for instance. All we can do is say things about the locations we visited. But that can help us better understand Mars as a whole, particularly when combined with on orbit data.

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