The ice vaporized, then re-solidified where it was colder, leaving dark deposits on the surface. Radar imagery shows the dark patches subside at the coldest parts of the crater, where ice can exist on the surface.

The areas where the dark patches are seen are not cold enough for surface ice without the overlying layer of what is believed to be organics.

So remote was the idea of organics on Mercury that MESSENGER got a relatively easy pass by NASA’s planetary protection protocols that were established to minimize the chance of contaminating any indigenous life-potential material with hitchhiking microbes from Earth.

Scientists don’t believe Mercury is or was suitable for ancient life, but the discovery of organics on an inner planet of the solar system may shed light on how life got started on Earth and how life may evolve on planets beyond the solar system.

“Finding a place in the inner solar system where some of these same ingredients that may have led to life on Earth are preserved for us is really exciting,” Paige said.

MESSENGER, which stands for Mercury Surface, Space Environment, Geochemistry and Ranging, is due to complete its two-year mission at Mercury in March.

Scientists are seeking NASA funding to continue operations for at least part of a third year. The probe will remain in Mercury’s orbit until the planet’s gravity eventually causes it to crash onto the surface.

Whether the discovery of organics now prompts NASA to select a crash zone rather than leave it up to chance remains to be seen. Microbes that may have hitched a ride on MESSENGER likely have been killed off by the harsh radiation environment at Mercury.

The research is published in this week’s edition of the journal Science.