The electrical environment near the Moon’s surface at the South Polar Region is far more active than previously understood. The findings provide essential ground truth needed for the next phase of lunar exploration, the Indian space agency said.
According to Indian Space Research Organisation (ISRO), the analysis of the data obtained from the Vikram lander between 23rd August 2023 and 3rd September 2023 has resulted in a first of its kind about the plasma environment near the Moon’s surface at the Southern higher latitudes.
The Vikram lander was part of ISRO’s Chandrayaan-3 or the third moon mission and it had the payload Radio Anatomy of the Moon Bound Hypersensitive ionosphere and Atmosphere-Langmuir Probe or RAMBHA-LP in short. The objective of RAMBHA-LP was to measure the near surface plasma (ions and electrons) density and its changes with time.
The other six payloads are: Chandra’s Surface Thermo physical Experiment (ChaSTE); Instrument for Lunar Seismic Activity (ILSA); Laser Retroreflector Array (LRA) Rover;vAlpha Particle X-Ray Spectrometer (APXS); Laser Induced Breakdown Spectroscope (LIBS) Propulsion Module; Spectro-polarimetry of HAbitable Planet Earth (SHAPE).
A Matter of Plasma
ISRO said, 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, ISRO said.
This, along with the photo-electric effect, is the primary mechanism for creating plasma.
Sunlight carries tiny packets of energy called photons. When very energetic sunlight hits atoms on a surface or in the thin atmosphere, it can knock electrons off those atoms. This process is called the photoelectric effect.
When electrons are knocked loose, the atoms become electrically charged (this is called ionisation). A collection of these charged particles is called plasma.
So, strong sunlight knocks electrons off atoms, turning the gas into plasma, and this is the main way that plasma is created in this situation.
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, the Indian space agency said.
ISRO said, the results obtained by RAMBHA-LP 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 – named as Shiv Shakti point (69.3° S, 32.3° E)- was measured to be between 380 and 600 electrons per cubic centimeter.
The measurements were higher than the earlier estimates taken at higher altitudes, 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, ISRO said.
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, ISRO said.
That apart, the ISRO 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.
The RAMBHA-LP experiment was designed and developed by Space Physics Laboratory (SPL), Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, India.
