The Negative Ions at the Lunar Surface (NILS) instrument onboard China’s Chang’e-6 probe has detected negative ions on the surface of the Moon. These ions are produced on the lunar surface through interactions with the solar wind.
The South Pole-Aitken Basin on the lunar far side is one of the largest and oldest impact features in the Solar System. It’s easily seen in the elevation data. The low center is dark blue and purple. Mountains on its edge, remnants of outer rings, are red and yellow. Image credit: NASA / GSFC / University of Arizona.
The solar wind is a constant flow of radiation and particles from the Sun. Earth’s magnetic field acts as a shield.
In contrast, the Moon has no magnetic field and a very tenuous atmosphere, called the exosphere.
When the solar wind hits the Moon, the surface reacts, kicking up secondary particles.
These particles may be positively or negatively charged or have no charge at all.
While the positively charged particles have been measured from orbit before, measuring negative particles was a challenge.
Negative ions are short-lived and cannot make it to orbit. This is why ESA scientists needed to operate their instrument close to the lunar surface.
“This was ESA’s first activity on the surface of the Moon, a world-first scientifically, and a first lunar cooperation with China,” said Neil Melville, ESA’s technical officer for the NILS experiment.
“We have collected an amount and quality of data far beyond our expectations.”
“These observations on the Moon will help us better understand the surface environment and act as a pathfinder to explore negative ion populations in other airless bodies in the Solar System, from planets to asteroids and other moons,” said Dr. Martin Wieser, NILS principal investigator.
Chang’e-6 landed successfully in the South Pole-Aitken Basin on the far side of the Moon known on June 1, 2024.
NILS started to collect science data 280 min after landing. The first data collection period lasted for 23 min, until the instrument reverted to low voltage. A few more rounds of data collection followed between communications blackouts and reboots.
“We were alternating between short bursts of full-power and long cooling-off periods because the instrument was heating up,” Melville said.
“The fact that it stayed within its thermal design limits and managed to recover under extremely hot conditions is a testament to the quality of the work done by the Swedish Institute of Space Physics.”
