SPIEGEL: NASA scientists have discovered strange bacteria in California's Mono Lake. The microbes incorporate arsenic, which is usually poisonous for life forms, into their cells. Are they originally from another planet?

Dirk Schulze-Makuch: No, that can be ruled out. The arsenic bacteria also did not arise independently from the other organisms on Earth. Like all microbes, they multiply best when there is enough phosphorus around. They only use arsenic when there is not sufficient phosphorus -- beggars can't be choosers. The arsenic bacteria are a wonderful example of the adaptability of microorganisms.

SPIEGEL: What does this discovery mean for the search for extraterrestrial life-forms?

Schulze-Makuch: The arsenic bacteria help us broaden our horizons. If we can find such exotic organisms on Earth, what strange beings could exist on other planets? We have to free ourselves from the idea that life-forms will resemble what we know from Earth.

SPIEGEL: What differences can you imagine?

Schulze-Makuch: Our fixation with the idea that oxygen is essential to life is already short-sighted. This aggressive element inflicts damage to our cells in the form of free radicals. Maybe organisms elsewhere in space have found a gentler alternative. When we send space probes to other worlds, we should expect the unexpected. Life can appear anywhere: in poisonous seas or in hot clouds.

SPIEGEL: Where could the resistant arsenic bacteria thrive?

Schulze-Makuch: Arsenic-eating microbes would probably feel very at home on our neighboring planet, Mars. Its conditions are well suited to them. Measurements collected by landing robots on Mars can indeed be interpreted as evidence of bacterial life. However, it could be that any life-forms on Mars aren't actually aliens, but are related to us.

SPIEGEL: How do you mean?

Schulze-Makuch: Almost 4 billion years ago, Mars was a planet well suited to sustaining life, with massive rivers and lakes. Back then, the first primitive organisms appeared on Earth. These single-cell life-forms probably made it to our neighboring planet Mars by way of meteorites and established themselves there. It is possible that descendents of these primitive bacteria could have survived in nooks and crannies on Mars until today. Equally fascinating is the opposite possibility: Life could have started on Mars and then, via a meteorite, made its way to Earth. That would raise the question: Are we all Martians?

SPIEGEL: Which planet in our solar system is most likely to sustain life which contrasts significantly from life on Earth?

Schulze-Makuch: The distant Titan, a moon of Saturn, seems completely foreign to us. Its surface temperature is minus 160 degrees Celsius (minus 256 degrees Fahrenheit), and its atmosphere contains no oxygen. Instead of water, its lakes are filled with liquid natural gas. Methane rains from the sky, and it looks like the aftermath of an oil spill in the Antarctic. If we were to discover life there, it would certainly look completely different from life as we know it on Earth.

SPIEGEL: What are the chances of that?

Schulze-Makuch: Surprisingly good! On the Caribbean island of Trinidad, there is a natural asphalt reserve called Pitch Lake, which is fed by oleaginous substances from the Earth's crust. It's a tourist attraction. Its conditions are similar to that of a lake on Titan. My team and I recently took samples to search for traces of life.

SPIEGEL: And what results did you get?

Schulze-Makuch: To our surprise, we found masses of bacteria which transformed long-chain hydrocarbons into methane in the asphalt lake. How these tough beasts manage to survive with practically no water remains a mystery. Given that so many microorganisms survive in the asphalt lake, then it must be possible on Titan too.