A specially equipped NASA 747 plane has detected atomic oxygen in Mars' upper atmosphere, the first time that such an observation has been made by the agency in 40 years. The unique plane that obtained the measurements is called the Stratospheric Observatory for Infrared Astronomy (SOFIA), which used a special instrument to determine that the planet's upper atmosphere, also called its mesosphere, contains atomic oxygen, although only half of the expected amount.

"Atomic oxygen in the Martian atmosphere is notoriously difficult to measure," said Pamela Marcum, a project scientist with SOFIA. "To observe the far-infrared wavelengths needed to detect atomic oxygen, researchers must be above the majority of Earth's atmosphere and use highly sensitive instruments, in this case a spectrometer. SOFIA provides both capabilities."

Atomic oxygen is of interest to scientists because it influences how other gases escape Mars, affecting the planet's atmosphere is a significant way. The lower-than-expected oxygen count suggests variations in Mars' atmosphere and scientists will continue to use SOFIA to examine these variations and use the findings to better understand the planet's atmospheric characteristics.

The last time that atomic oxygen was measured on the Red Planet was during the Viking and Mariner missions of the 1970s. However, the new observations were made from a more ideal distance - an altitude of between 37,000 and 45,000 feet - that is above most of the moisture in the Earth's atmosphere that blocks infrared light, allowing for more accurate measurements.

Although the data was published in 2015, the recent discussion comes shortly after scientists used rocks on the surface of Mars to determine that the planet used to have an atmosphere that was rich with oxygen, much more than its current content.

"We found 3 per cent of rocks have high manganese oxide content," said Agnès Cousin of the Research Institute in Astrophysics and Planetology in Toulouse, France. "That requires abundant water and strongly oxidizing conditions, so the atmosphere may have contained much more oxygen than we thought."

As of now, Mars' atmosphere is approximately 95 percent carbon dioxide, with only trace amounts of oxygen.

SOFIA is built from a Boeing 747SP jetliner that was modified to carry a 100-inch diameter telescope and is based at NASA's Armstrong Flight Research Center's hangar 703 in Palmdale, Calif.

The findings were published in 2015 in the journal Astronomy and Astrophysics.