Mars Exploration Rovers
Why did they go to Mars?
The two Mars Exploration Rovers (Spirit and Opportunity,) landed on Mars in January 2004 to discover what the weather was like on Mars and to see whether it was favourable to life or not. Both rovers lasted longer then they were expected to and returned information that transformed our understanding of Mars.
What did Spirit discover?
Spirit explored for years beyond it's original 92 day mission and discovered that Mars was a lot wetter than it is now. The fantastic rover still managed to provide information even as it was stuck in deep sand at a place called Troy at Gusev Crater. On the 25th of May 2011, NASA stopped contacting the marooned rover because it had been silent since March 2010. They declared it's mission complete.
What did Opportunity discover?
Opportunity discovered that it's area of Mars stayed wet for ages and that the conditions could've been suitable for sustaining microbial life. Scientists believe that Opportunity's landing site was once a shore line to salty sea on Mars. Since March 2010, Opportunity holds a driving record of more than 20km .
Discovering that Mars was a lot wetter than it is today, shows that conditions on the planets are changing all the time.