Life on Mars? Water on the moon? These have become hardy perennials of space science journalism over the past few years.
Martian microbes raised their head again today, with the publication in the US online magazine Spaceflight Now of a story that Nasa scientists have found new evidence of bacterial remains in an ancient meteorite.
The rock in question, known as Allen Hills (ALH) 84001, was discovered in Antarctica in 1984. Geologists say it is a chunk of the Martian surface blasted into space by an asteroid or comet impact on the red planet 16m years ago.
ALH 84001 originally hit the headlines in 1996, when Nasa and the White House made the sensational announcement that the meteorite contained microscopic traces of ancient bacteria.
The supposed micro-fossils certainly looked lifelike to the lay eye, but the excitement faded as independent scientists said the structures could have other causes, such as terrestrial contamination and/or heat shock as the rock blasted off Mars into space.
The new research, which Spaceflight Now says Nasa will publish very soon, is based on the latest high resolution microscopy, which was not available 13 years ago.
This apparently rules out other causes and strengthens the original conclusion that the structures really are microfossils. Best evidence is the alignment of tiny crystals of a mineral called magnetite, which the Nasa scientists could only have been laid down by bacteria.
But no study, however convincing, of a meteorite discovered on Earth can prove the existence of microbes on Mars. That proof can only come from a space probe examining material on the planet itself.