The plumes were first spotted in March and April 2012 by more than a dozen amateur astronomers using ground-based telescopes. Each time, the plumes in Mars's southern hemisphere were visible for more than a week, reported an international team of scientists in Nature this week.
The plumes took about 10 hours to develop above the surface of the same part of Mars, covering an area of up to 1,000 kilometres by 500 kilometres. They rose up to 250 kilometres into the sky — about 150 kilometres higher than other plumes previously spotted on Mars.
"At about 250 km, the division between the atmosphere and outer space is very thin, so the reported plumes are extremely unexpected,” said Agustin Sanchez-Lavega of the University of the Basque Country in Spain, lead author of the report, in a statement posted on the European Space Agency website.
None of the Mars orbiters circling the planet were at the right place at the right time to capture images of the plumes.
Hubble spotted plumes in 1997
However, the researchers hunted through Hubble Space Telescope images and found similar plumes in an image from May 17, 1997.
In the new paper, the scientists came up with two possible explanations: Clouds produced by frozen water or carbon dioxide particles.
Auroras similar to the northern lights have previously been observed in this location owing to an anomaly there in Mars's magnetic field.
But the researchers said "neither explanation … is fully consistent with the plume's enormous size."
According to the European Space Agency, scientists are hoping to get more answers after the arrival of the agency's ExoMars Trace Gas Orbiter, which is scheduled to launch in 2016.