Hello Mirana, You are correct that 'evidence' is an uncountable noun. Both sentences are grammatically correct. The second sentence would be used when you had previously referred to several examples of life on mars, hence using 'these are'. Hope this helps.

Evidence is non-count and you can not use the plural verb "are" with it as-is. However using a word like "pieces" makes it countable. Therefore, you can say something like this, "These are some of the most complicated pieces of evidence I've seen." (plural) This piece of evidence is more important than that one. (singular) The same is true of the word, "advice." "Let me give you a piece of advice. (singular) "Some advice is better than others." (plural) http://www.englishpracticeonline.com/make-uncountable-nouns-countable/