John,

I suggest that it is not sufficient to look at locales where present terrestrial biota can survive.

One has to take into account whether, in earlier epochs those locales could have allowed biota to *evolve*.

Right now, somewhere at the water-ice interface 10km under the surface of Europa, it is possible that terrestrial organisms could survive.

But they couldn't have evolved there (along the same lines as they did on Earth) and their long-term survival in that distant location is impossible.

So, a more nuanced question would be, 'where in the solar system might conditions have existed where life might have arisen such that it could prosper to this day?'

I'd offer the polar subsurface margins of Mars as a fair starting point. At least, for biota that might use similar metabolic pathways as psychrophiles on Earth.

What is often considered as a suitable environment for life evolution is the stable presence of a liquid reservoir, which in turn is likely to witness the presence of some source of energy available for life. In this sense, besides the examples in the previous answer by James Garry, Enceladus, Venus and Titan are often cited. Enceladus is similar to the Europa's case (subsurface ocean) but with a more evident active geology (geysers at the south pole), Venus offers in its water cloud layer an atmospheric environment thermodynamically quite similar to the terrestrial one, while Titan is the only known case of surface liquids besides Earth (extended polar lakes), even if they are not made of water but liquid methane/ethane.

I actually expected NASA scientists to have answered this question long before they've sent CURIOSITY up to Mars. Maybe they have by starting their project, but I'm certainly not an expert in this field and there are many other conditions to take into account when searching for life.

Imtiyaz, please be kind to provide us a link to that information so that we can access the knowledge. Thanks.

"Life as we know it" requires liquid water, an energy donor, an electron acceptor, and assorted elements to build things with (C, N, S, P, H, etc, etc). There are bugs around on earth that live happily in boiling or freezing conditions, and from pH 1 - pH11.5 (at least), with or without oxygen. Any place in the solar system that has/had these factors can (could have) support(ed) life - possibly Mars and some of the planetary moons.

There are bugs on earth that could grow happily elsewhere in the solar system, but that doesn't mean that we'll find those exact bugs up there - everything will depend on the local conditions. If, as some claim, some of the building blocks of life came to earth from space, life elsewhere in the solar system may be more closely related to life on earth than may previously have been predicted.

"True or false?"? Not very nice.

There are bugs in our freezers that find oxygen so toxic that we can't even use plastic petri dishes to grow them because of the oxygen dissolved in the plastic. Yes, fastidious anaerobes can use or grow in the presence of molecules containing oxygen (not "elements") including CO2 and H2O, but I didn't say anything about oxides, just oxygen.

I really don't see that it's relevant - oxygen is present on Mars as water, CO2 and oxides of one sort or another. There's no reason to think that it won't or didn't occur in other situations that might favour life as we know it.

We were asked to speculate. There may be life on Mars associated with the recently mapped water. There may have been life on Mars in the past when it had more atmosphere. We don't know, and there's a limit to what a couple of robots can search, as yet.

There's not enough evidence for any conclusions yet.

I know this one is not within the Solar System and, instead, is 170 light-years away; but this one adds fuels to our thoughts: http://www.sciencemag.org/content/342/6155/218

Mr Towe, before dismissing oxides as irrelevant, perhaps you should consider the activity of sulphate reducers, denitrifiers, CO2 reducers, and the organisms that use oxides of transition elements, all as electron acceptors. And if they're growing autotrophically, they'll need to assimilate CO2 . I'm not sure how this discussion got hung up on free oxygen.

Before microorganisms can survive, they need to grow. They might have grown and died geological ages ago, they might not have been there.

And to repeat myself, thus far the jury is still out. Not enough samples have been analzed to come to any conclusion, and there won't be for a long time to come.

Are the conditions on Mars such that they seem to preclude life or is it more that we have not found evidence of life on Mars?

Dear All, I have been submerged in the project work that left little time for me to follow RG. Now, there have been a flurry of publications regarding possibility of life elsewhere--including the elegant articles in Science from the Mars Curiosity rover as well as the new "Habitable Zone" article in Nature (see http://www.nature.com/nature/journal/v504/n7479/full/504221a.html and the accompanying article). Not being a geologist limits my grasping ability of these studies, while I am still deeply interested to learn. Now, Kenneth, may I request you to kindly summarize the relevance of those articles here so that this thread moves forward? Many thanks.

Dear All,

This article (http://www.sciencemag.org/content/early/2013/12/11/science.1247051) is coming up in Science, which reveals detection of transient, 7hr-long water vapor over Europa's south pole. Too early (or, too dreamy for a novice like me) to say, but it perhaps reflects a warmer interior (and, arguably, protected from radiation hazards) that might have supported some of the conditions required for emergence of life.

Dear Kenneth, In the context of your post of Oct 24 regarding the origin and evolution of RNA-DNA based life, the following article might interest you; it demonstrates non-enzymatic synthesis of RNA inside model lipid protocells.

http://www.sciencemag.org/content/342/6162/1098.full.pdf