December 09, 2025

Analysis of the Chandrayaan-3 lander data obtained from data from August 23, 2023 to September 03, 2023 has yielded significant, and first-of-its-kind of results on the plasma environment near the Moon’s surface at the Southern higher latitudes, revealing that the electrical environment near the Moon's surface at the South Polar Region is far more active than previously understood.

In physics, plasma is often called the fourth state of matter, consisting of a mixture of charged particles, including ions and free electrons. Despite being electrically neutral overall, plasma is highly conductive and responds strongly to electromagnetic fields. The Moon's thin plasma environment, or lunar ionosphere, is governed by several major processes. Solar wind, which is a continuous stream of charged particles (primarily electrons, Hydrogen and Helium ions) ejected from the Sun's upper atmosphere, constantly impinges on the Moon's surface. This, along with the photo-electric effect—where high-energy photons from the Sun knock out outer-shell electrons from atoms on the surface and in the sparse atmosphere, causing ionization—is the primary mechanism for creating the plasma. The lunar plasma is further influenced by the deposition of charged particles originating from the Earth's magnetosphere (specifically the magnetotail) when the Moon passes through that region (typically 3-5 days during a period of 28 days), resulting in a constantly changing and dynamic electrical environment near the surface.

In this context, the results, obtained by the Radio Anatomy of the Moon Bound Hypersensitive ionosphere and Atmosphere – Langmuir Probe (RAMBHA-LP) instrument onboard the Vikram lander of Chandrayaan-3, mark the first-ever direct, or "in situ," measurements of the lunar plasma at such low altitudes. The key findings include the fact that the electron density near the landing site of Chandrayaan-3, named as Shiv Shakti point (69.3° S, 32.3° E) was measured to be between 380 and 600 electrons per cubic centimeter. This is significantly higher than estimates derived from observations taken at higher altitudes, which are primarily based on observing the changes in the phase of electromagnetic signals from satellites passing the Moon’s thin atmosphere at grazing angles, a technique known as Radio Occultation.

It is further found that the electrons near the Moon’s surface possess remarkably high energy, with equivalent temperatures (called kinetic temperature) soaring between 3,000 and 8,000 Kelvin.

The study uncovered that the lunar plasma is not static but is constantly modulated by two distinct factors, depending on the Moon's orbital position around the Earth. When the Moon is facing the Sun (lunar daytime) and outside the Earth’s magnetic field, changes in the near-surface plasma are driven by particles from the Solar Wind interacting with the sparse neutral gas (exosphere) on the Moon. In contrary, when the Moon passes through the geomagnetic tail, the plasma changes are caused by charged particles streaming from the tapered region of Earth's long magnetic tail (towards the opposite side of the Sun), known as the geomagnetic tail.

Furthermore, in-house developed Lunar Ionospheric Model (LIM) suggests that apart from the elemental ions, the molecular ions (likely originating from gases like CO2, H2O) also play a crucial role in creating this electrically charged layer close to the lunar surface.

These results from the RAMBHA-LP experiment provide essential ground truth needed for the next phase of lunar exploration.

The RAMBHA-LP experiment was designed and developed by Space Physics Laboratory (SPL), Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram.

Reference: “In situ ionospheric observations near lunar south pole by the Langmuir Probe on Chandrayaan-3 lander”, G. Manju et.al., Monthly Notices of the Royal Astronomical Society, 542, 2647 – 2656 (2025);

DOI: https://doi.org/10.1093/mnras/staf1276