"New kinds of organic compounds" have been found on an alien moon, Nasa says, in a major breakthrough in the search for extraterrestrial life.

The compounds found on Saturn's moon Enceladus are the ingredients of amino acids, the building blocks of life.

Scientists describe the finding as an "important piece of the puzzle" of whether there might be life elsewhere in our solar system.

Enceladus is widely considered one of the best candidates for finding life in our solar system, and the new discovery is likely to lead to further excitement about what could be lurking beneath its mysterious surface.

The new findings come from Nasa's Cassini mission, which ended in September 2017 when the spacecraft flew into Saturn's surface. But before it did so it flew through the plumes that burst out of Encaladus's surface, sending data about them back to Earth that is still being explored by scientists.

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That plumes are thrown out by powerful hydrothermal vents, which spew out material from the core of the moon. That is then mixed with water and then thrown out into space as water vapour and ice grains, which were examined by Nasa's craft.

The molecules were discovered in those ice grains, and were found to be nitrogen- and oxygen-bearing compounds.

On Earth, such compounds are active in chemical reactions that can produce amino acids. Those are the building blocks of life and that process helped create all living things on Earth.

Here, similar hydrothermal vents give energy that fuels those important reactions. The vents on Enceladus could be doing the same, helping create amino acids and giving rise to optimism about whether that could have led to life flourishing beneath the moon's surface.

"If the conditions are right, these molecules coming from the deep ocean of Enceladus could be on the same reaction pathway as we see here on Earth. We don't yet know if amino acids are needed for life beyond Earth, but finding the molecules that form amino acids is an important piece of the puzzle," said Nozair Khawaja, who led the research team of the Free University of Berlin.

The findings were published Oct. 2 in the Monthly Notices of the Royal Astronomical Society.

Scientists already discovered big, insoluble complex organic molecules that are thought to float on top of the moon's ocean. This new work reveals that there are also the ingredients needed to help form amino acids.

"Here we are finding smaller and soluble organic building blocks – potential precursors for amino acids and other ingredients required for life on Earth," said co-author Jon Hillier.