Photo/IllutrationThe Asahi Shimbun

Long a mystery of lunar science, the presence of oxygen on the surface of the moon has been known to scientists but not its origin.

Now, a team of Japanese researchers has confirmed that oxygen carried from Earth by the "Earth wind," a wind of magnetospheric ions, travels 380,000 kilometers through space to reach the moon's surface.

The Japanese lunar orbiter Kaguya detected the high presence of oxygen ions in space around the moon when it was orbiting behind the Earth and the sun.

Based on the energy level of the oxygen ions measured by Kaguya, the scientists deduced that it was blown from the Earth’s atmosphere.

The team includes experts from Osaka University, the Japan Aerospace Exploration Agency (JAXA) and Nagoya University.

A paper on the finding was published in the online British academic journal Nature Astronomy on Jan. 31.

Once a month, the moon orbiting around the Earth passes through a zone in space called the plasma sheet. This occurs around the time of a full moon when it is on a line with the sun and Earth.

This region is where Earth-derived substances accumulate.

The plasma sheet stretches in the Earth's magnetosphere, which is elongated in a cone shape toward the opposite side of the sun as it meets the solar wind.

The research team studied the plasma in space about 100 km above the surface of the moon using the Kaguya, also called SELENE, which was developed by JAXA and operated until 2009.

The collected data revealed that the amount of oxygen ion surrounding the moon rises when it passes through the plasma sheet region.

The particular oxygen ion detected in the research is usually found in the atmosphere more than 100 km above the Earth.

The team also believes the oxygen found on the surface of the moon was blown from the Earth's atmosphere, based on their similar energy levels.