What researchers are trying to do involves global warming, a sort of greenhouse effect on the cold planet Mars. Scientists may be able to test their hypotheses about global warming in their attempts to elevate Mars' surface temperature. Likewise, once theories, they may be applied to our own planet in an attempt to reverse environmental damage done by pollution and deforestation.
I don't really understand the part in red, especially the underlined part 'once theories'.

Grammatically, what is that 'once' and what does 'once theories' mean here?
Likewise, once theories, they may be applied
Similarly, these hypotheses about global warming, which were once/sometime only theories, may be applied
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Ah, that 'once'. I see. Thanks!

By the way, did you forget that I was wondering where you were from?
Taka, is it at all possible that some of the text was left out? Like "once these theories are proven" or something like that?

EDIT: Oh, nevermind. I had this window open for so long without posting that two more comments have joined in, and I think that MH's explanation makes perfect sense.
It's perhaps important to point out that likewise makes the link between the application of those theories from Mars to Earth.

GG, I don't think there's anything missing, but it's not easy to digestEmotion: sad
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Grammar GeekTaka, is it at all possible that some of the text was left out?
I don't know...It's from NASA's official home page: