Solar wind strikes the Moon differently depending on which hemisphere faces away from Earth. Samples returned by China’s Chang’e-6 mission from the lunar far side demonstrate that energetic particles from the Sun penetrated deeper into the regolith there than in material collected from the near side.
The core finding comes from measurements of noble gases trapped in the soil. Neon isotope ratios in the Chang’e-6 regolith average 11.34 ± 0.22 for 20Ne/22Ne, a lower value than seen in near-side samples and one consistent with stronger fractionation by faster, more energetic solar wind on the far side.
Krypton and xenon released from the grains during stepped heating experiments showed a single sharp peak at high temperature. That pattern indicates the particles were implanted at greater depth than in near-side material examined previously.
Earth’s magnetic influence
Earth’s magnetosphere slows and deflects a portion of the solar wind that reaches the lunar near side. The far side, shielded from that effect during much of the Moon’s orbit, receives the full force of the undisturbed stream. As a result, solar wind particles arrive with higher average energy and bury themselves more deeply before coming to rest.
Approximately 25 percent of total solar wind exposure recorded at a near-side site involved slower flow moderated by Earth’s magnetosphere, according to ScienceDaily coverage of the peer-reviewed research. Noble gases preserved in lunar soil may therefore serve as fossil records of past interactions between the solar wind and our planet’s magnetic field.
The Chang’e-6 landing site lay within the South Pole-Aitken basin on the southern mid-latitude far side. These were the first direct samples from that hemisphere, allowing a clean comparison with near-side regolith returned by Chang’e-5 and the Apollo missions.