Robyn Williams: As for those folk who don't believe Armstrong and Aldrin walked on the Moon in July 1969, 40 years ago, it's another manifestation of firm belief about the unlikely. This was the theme of a conference of sceptics in Las Vegas attended by Mike McRae of CSIRO in Canberra, on credulity, ET, and funding of science in difficult times.

Mike McRae: In July 1964, NASA's Mariner probe began taking detailed snapshots of the surface of Mars, providing astronomers with close-up images for the first time. The results helped settle a debate that had simmered for over half a century; whether Mars harboured signs of a current or past civilisation.

During the 19th century American astronomer Percival Lowell explored the surface of Mars by telescope from his home on a mountain in Flagstaff, Arizona. He proclaimed that the dark lines and shadows he saw were evidence of canals and agriculture. Most of his peers remained dubious, believing Lowell misinterpreted his observations. It was with the publishing of his ideas that the popular concept of the Martian was born. Never again would people look up at the skies without wondering what might be looking back.

The Phoenix lander is NASA's latest Martian probe. The lander was looking for signs of alien biochemistry by heating and 'sniffing' the soil using a gas spectrophotometer. The presence of just a few molecules would have been an encouraging sign, but sadly none were found. Dr Phil Plait is author of the books Bad Astronomy and Death from the Skies. He has worked on the Hubble Telescope and today he is well known for communicating a better understanding of astronomy to the public. He suggests our efforts in looking for evidence of life on Mars might be born of wishful thinking.

Phil Plait: There is a lot of wishful thinking in it. Mars has always thought to be Earth-like for centuries, and then when we finally went there...it wasn't until the 1960s and the 1970s when we sent probes there and learned that it was dry...we knew there wasn't much air, but it was cold and cratered and looked pretty dead. So it's possible it could go either way. The initial science that we're getting from Mars indicates that life arising there is unlikely, but the Earth formed 4.5 million years ago, and the crust started to cool on the Earth and there was heavy bombardment from space, a tremendous number of comets and asteroids slamming into the Earth keeping the crust molten, making it a living hell on Earth. As soon as that stopped, and I mean within millions of years of that happening, life started to arise. We have fossil remnants of single-celled organisms that are billions of years old. So we know that as soon as the conditions were good on Earth for even a few million years or a few tens or hundreds of millions of years, which isn't that long geologically speaking, life took off.

So sure, we have evidence on Mars that the water flows that we see are what we call catastrophic, it was flooding, there was probably frozen water, ice underneath the surface, and then vulcanism or an impact melted the stuff and you have tremendous floods that were just epic, 100-metre-high tidal waves of water sweeping across the surface. But there was water underground. Mars is a small planet but it's got a lot of surface area, and if Phoenix, which is at about 60 degrees north latitude, which is roughly where Canada is in the northern hemisphere, that's pretty far north, and it's not hard to imagine that in an entire planet if you take a single probe which is the size of a golf cart and land it on a planet and it can only scoop within a metre away from itself and if you kind of picture that on Earth and if it landed in the Mojave Desert or in the outback or in Antarctica, finding signs of life might be a little tough. So it's completely possible that this thing could find nothing and that doesn't tell us anything. It tells us lots of stuff but it doesn't tell us whether life ever existed on Mars.

So Mars is still an open book. Until we get there and really start hammering away at it with robotic probes and maybe people in 20 or 30 years, I don't think we're going to get the answer to this question and confirm it.

Mike McRae: It's important to remember that science relies on funding, and funding relies on what politicians find significant. Microbial farts might lack the romantic air of little green men or Lowell's alien metropolis but could be enough to light the fire for another space race. Neil deGrasse Tyson is an American astrophysicist and host of the PBS TV series Origins. He explains the potential political impact of discovering solid evidence of life on another planet.

Neil deGrasse Tyson: If finding a new biology on another planet is determined to have military value, then yes, there's a race; who's going to get this new life form? Who's going to cultivate it? Who's going to claim ownership of this spot on the planet or on the moon where it was found? Humans are humans. All this talk that, oh, let's go into space and hold hands and kumbaya because space is different...space is different but humans aren't, and I'm not given any reason to believe that we'll be any kinder to each other in space than we are here on Earth where we kill each other for much less.

So if we find life that could be a boon to the budget, but I'd rather it not have to be that, I'd rather people just enjoyed the quest for all that is to be discovered in the universe. So my goal is to get people excited about the entire frontier of science, so that science as an enterprise is well funded. If it's not in this country, it will be funded in other countries and we're going to feel it because innovations in science and technology will define tomorrow's economic growth, and if we refuse to invest in it here, those growth opportunities will happen elsewhere. We'll fade from significance on the international landscape.

Mike McRae: If astronomers have maintained a healthy scepticism on the idea of encountering intelligent life, the general population has continued to embrace the possibility with optimism. An image taken by the Viking I orbiter in 1976 inspired a return to the speculation over Lowell's alien civilisation. For many, a lone rocky outcrop in the Cydonia region looked too much like a large face staring stoically up into the heavens to be considered natural. This was alleged to be irrefutable proof of intelligent life on Mars, a monument carved by creative hands rather than some random geological feature. Despite the image now proving it to be little more than a curious optical illusion, the myth remains.

Butting heads with Moon-landing denialists and defenders of UFO conspiracies is something few astronomers ever avoid. Phil Plait has made a career out of it. He is president of the James Randi Educational Foundation, an organisation focused on promoting scepticism and critical thinking. For all of its beauty and awe, astronomy is complex, based on principles that, to you and to me, might seem weird. It attracts misunderstanding, misperception and misinformation. Considering some people believe the Moon landings were hoaxed and even that complex civilisations thrive on Mars, I asked Phil Plait to predict what the Phoenix mission might have inspired should it have found signs of life.

Phil Plait: Trying to predict the craziness that people come up with is a losing game because there are people out there who are more clever and more bent that rational people can be. I'm always shocked. I'll see some Hubble picture coming down and I'll look at it and I'll think that's gorgeous and here's the science behind it, and then a week later something pops up that's totally whacky, that aliens are manipulating atmospheric gases on Saturn or something like that. But if we find life on Mars...and we're not really geared to do that yet. The Phoenix lander which is on Mars is looking for the conditions where life can arise, it's not really looking for life itself. If there are bacteria in the rock on Mars, Phoenix is not going to find it, but there are going to be future missions that will.

It's hard to say what people will say. Religious people, especially in America, the fundamentalist religious people will probably go nuts because special creation is very important to the young Earth creationists, that God created man in his image and this was a one-time event that life is on Earth. And there are creationists who are saying we won't find extraterrestrial life. Recently actually one of the more famous ones came out and said this, and that's easy to show wrong, all you have to do is find a bacterium from space and it's all we have to do. It's not as easy as all that, but if it happens these guys will go ballistic. Other people say this validates their beliefs in UFOs and aliens abducting them and all that sort of thing when of course that's not true at all.

If we find life on Mars it's going to be living bacteria today, very primitive, or it's going to be fossils from hundreds of millions or billions of years ago. I'm not saying we're going to find life on Mars, I actually think it's pretty unlikely, but I'm saying if life is found, if you go with that idea, that's probably the way it's going to be. But what it really means is that life can arise in more than one place. We don't know how easy it is for life to arise. We know that once it showed up on Earth it took hold and it was everywhere and it was really hard to get rid of it, but that doesn't mean it's easy to come up with in the first place, and maybe it started on Mars and was brought here, like on the Mars meteorite, and maybe it started on Earth and moved to Mars, although physically that's harder. But it tells us a lot about the conditions that life arises in the universe, and I think that's one of the most profound things we can wonder about.

Mike McRae: It's easy to become cynical and feel that while life might be relatively common in our universe, the sheer diversity of planetary environments means we're on a fool's quest to find it sitting close by. Yet being optimistic might not simply be a case of pure wishful thinking. Neil deGrasse Tyson.

Neil deGrasse Tyson: It depends what you mean by 'fool's quest'. There are many places still in our own solar system we have yet to look for life, like deep beneath the Martian soils where we know there is water that's left over from long ago that is now part of the permafrost of Mars. There might be fossil evidence of life that once thrived. Mars, all evidence showed, was once wet and fertile with liquid rivers of water running over its surface. You can see the dried riverbeds and the meandering river patterns and flood plains and river deltas. So Mars had a very different past from what it has today. Mars still needs to be searched.

There are other places, for example, one of the moons of Jupiter, Europa, where the outer surface is icy and it's got this liquid ocean of water, it's been liquid for billions of years. Every place on Earth where there's liquid water there is life. So that's one of several other places that can be sighted in our own backyard, but beyond that you can look to other planets with a spectroscopic technique that allows you to search for what are now called biomarkers in the atmosphere.

For example, methane...this is the gas that is in many people's stoves, methane is a prime ingredient in the flatulence of cows and the burps of termites. It's highly flammable, it is unstable in an atmosphere, 'unstable' meaning it breaks apart, it bonds with other things, it goes away. If you find methane in an atmosphere there's something sustaining it there, there's some process, and we know that life has a way of sustaining methane.

Oxygen is not stable in an atmosphere. If you find it then there is some active chemical processing. Life is a very easy guess for what might be sustaining it. So there's a whole movement underfoot right now setting up tables of biomarkers so that you can infer what could be going on on the planet's surface, and these would be planets on other solar systems outside of our own.

Mike McRae: Even if Mars proves to be sterile, the solar system holds several other potential aces up its sleeve. Beneath the icy shell of Jupiter's moon Europa lies an immense watery ocean which could teem with microscopic life. Failing that, Saturn's moon Titan presents even stranger possibilities with its thick atmosphere and pools of liquid hydrocarbon. In either case, given what we know about life here, it's unlikely the planet will harbour entities with an appreciable IQ. That said, discovering any new life form will revolutionise our understanding of how life began on Earth. Never mind that ET won't want to phone home, for many people even a single dead amoeba would still be the discovery of the millennium.