Garnish: if you want a moon side dish, prepare a Mars-size body for a collision towards the end of the cooking process.

This was the main course served up at the University of Hong Kong last week by Nasa research scientist Max Bernstein, and recommended as dish of the day to replace an overcooked primordial soup that had been on the back burner since the 1950s.

Dr Bernstein, who is deputy chief of the space science and astrobiology division at Nasa's Ames Research Centre, was giving a public lecture on 'The Search For Life In The Solar System: Lessons From Studies Of Meteorites And The Origin Of Life'. He was passing through Hong Kong on his way to the 36th scientific assembly of the Committee on Space Research, which concludes tomorrow at the Friendship Hotel and Beijing Institute of Technology.

The second astro-scientist to visit in as many months - hard on the wheels of superstar physicist Stephen Hawking - Dr Bernstein specialises in photochemistry and cometary ice. Although his audience - at about 100 - was much smaller than Professor Hawking's draw of 1,800 at Hong Kong University of Science and Technology, his lecture was no less cutting edge.

He explained that primordial soup first made it on to the cosmic menu when Charles Darwin wrote a letter to a friend at the end of the 18th century, speculating that life may have started in 'some warm pond' filled with phosphoric acid, salts and other nutrients, sparked off by lightning.

'Somehow in this hospitable environment you get these small molecules coming together to make a bigger one that can replicate itself,' he said. 'To go from that to you driving on the highway is trivial, that's just a set of modifications. The hard part is making that first self-replicating molecule and it wasn't very clear how that happened.'

Darwin's speculation led to a 1953 experiment by Stanley Miller, who ran electricity through a cocktail of molecules believed to have been in the early atmosphere, such as methane and water vapour, and produced a 'goo' that contained amino acids, 'the basic components of everything alive today'.

Dr Bernstein said the experiment, conducted in the same year as the structure of DNA was published, came at a time of confidence in scientific progress and appeared to support the primordial soup theory. Unfortunately, it was fatally flawed.

'The problem is that they thought the Earth began as Jupiter is now, with a thick atmosphere rich in hydrogen and, in addition, a nice warm water ocean.' he said.

'In fact, it's believed the Earth started as a molten ball of rock devoid of atmosphere. In addition, we're almost positive the Earth-Moon system formed when a Mars-sized object collided with the Earth. If at that point we had an atmosphere and ocean it would have boiled away. The early Earth was a really harsh environment.'

Dr Bernstein said life's origins were more like a cake mix than a soup, needing additional ingredients, in this case organic material falling to Earth via asteroids and meteorites from 'giant chemical factories' in the form of dense icy clouds way out in the universe.

The dense clouds 'from which all solar systems are formed' contained molecules suspended in icy screens exposed to ultra-violet radiation and cosmic rays.

'Normally, when people think about stuff coming to Earth they think about a big asteroid killing the dinosaurs, but what I'm talking about here is not the impact of a giant asteroid, but rather the slow accumulation of tiny bits of dust just millionths of a metre across, that float relatively gently into the Earth's atmosphere. We sweep up literally tonnes every day,' he said.

Dr Bernstein said he and his colleagues replicated the conditions found in 'chemical factory' dense clouds in the laboratory and produced results that had both positive and negative implications for the search for extraterrestrial life.

'If we're correct and all solar systems formed from these clouds, then that means all solar systems form with many of these molecules ready made, so perhaps it's more likely there is life in the universe off the Earth,' he said.

On the other hand, there was bad news for much of the evidence - called bio-markers - that people had put forward to support claims that there could be life in space.

Dr Bernstein said he and his fellow researchers could reproduce most of the so-called evidence from their experiments, meaning the molecules were already in space, produced by the 'chemical factories' and were not, therefore, generated by, or indicators of, life. The Allen Hills Martian meteor was a good example.

'One of the [lines of evidence] was that they saw functionalised aromatic molecules and they claimed these were bio-markers . . . but the truth is the chemistry we study shows you will see these molecules anywhere.'

Dr Bernstein said there were also difficulties about what would constitute evidence of extra-terrestrial life.

Some argued it would be the discovery of DNA, but he 'wasn't so sure'. DNA had evolved from RNA (Ribonucleic acid) on Earth, which showed that it could mutate, and it was possible that extra-terrestrial DNA, if it existed at all, could be completely different to accommodate different conditions in the same way as temperature modified DNA on Earth.

'It's not at all obvious that alien life would use DNA, so it is important for us to try to keep an open mind about what to search for when we are looking for life on other planets,' he said.

Dr Bernstein said the 'devil was in the details' and it was important to analyse organic material closely. Indicators of life could include the 'handedness' of molecules (a specific kind of composition - life on Earth tends to use only 'left-handed' amino acids), a particular kind of 'meta-chemical' pattern to molecules, or finding something 'really odd that didn't make any sense, for example if you saw a molecule with just one kind of sugar, but never any other'.

The bad news, he said, was that there hadn't been any indicators of life so far.

'I remain quite hopeful of finding some kind of life, microbial for example. That seems quite possible on Europa or sub-surface Mars, but as for finding stuff like us, that strikes me as very unlikely,' he said.

HKU dean of science Sun Kwok said it was important for the public and students to have access to multi-disciplinary science of the kind presented by Dr Bernstein.

'It is a great idea to introduce Hong Kong students to a broad sense of science because traditionally they tend to think of it as being narrow. Here we are talking about astrobiology, which is a combination of astronomy, biology, chemistry, physics and geology,' he said.

'This is the inter-disciplinary approach. The new way of doing science.'