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Venus and Mars are both on the edges of the Goldilocks Zone in our solar system, but why don’t they show liquid surface waters and large apparent biospheres like we have here on Earth?
Venus is has the hottest planetary surface in our solar system. It’s about 850°F there and the pressure is about 92 times more than what we have at sea level here on Earth. That makes Venus a very different place. Venus may have once had oceans (and maybe even a biosphere!), but it appears that the entire surface of Venus heated up high enough that all of the rocks melted and turned into lava at some point long ago. On top of that, Venus has what we call a “runaway greenhouse”, where the buildup of greenhouse gases (especially CO2) in the Venusian atmosphere made it get hotter, which caused more greenhouse gases to build up, which made it warmer, and so on. Venus is a very interesting place!
We think early Mars likely had lots of water, in rivers and lakes and maybe even in an ocean. That’s because early Mars likely had a much thicker atmosphere. But, these days, the surface of Mars is very cold, very dry, and the pressure is very low. Without a thick enough atmosphere, Mars cannot sustain liquid water at its surface, even though it’s within the Goldilocks Zone.
Even though Venus and Mars are on the edges of the Goldilocks Zone, they don’t have abundant biospheres that we can see on their surfaces. This tells us that being within the Goldilocks Zone alone probably isn’t enough to guarantee that a world will have liquid water or life. However, it’s still an important place for us to look around other stars when trying to find Earth-like worlds in our galaxy.