The Chandrayaan‑3 mission continues to yield remarkable insights into the Moon’s southern polar region, with the Vikram lander’s “hop” experiment revealing hidden complexities in the lunar Regolith.
After its historic soft‑landing on 23 August 2023, Vikram carried out a suite of experiments for ten Earth days, including studies of near‑surface plasma and seismic vibrations.
On 2 September 2023, engineers reignited its engines to perform a short vertical hop of about 50 centimetres, using residual propellant. This manoeuvre was not only a demonstration of a critical skill for future sample‑return missions but also provided a unique opportunity to study how the exhaust plume interacts with the lunar surface.
Scientists emphasise that while the term “lunar soil” is often used colloquially, the more accurate description is “Lunar Regolith.” This Regolith is composed of shattered rock fragments and jagged glass‑like shards formed by billions of years of micrometeorite bombardment.
It is abrasive, clings electrostatically to surfaces, and plays a crucial role in thermal regulation on the Moon. Understanding its thermophysical properties—how it absorbs and radiates solar heat—is vital for both scientific exploration and technological planning.
The Surface Thermophysical Experiment (ChaSTE), a rod‑shaped probe with embedded temperature sensors and a heater at its tip, was deployed to penetrate the Regolith. The hop manoeuvre allowed ChaSTE to sample a fresh location, where the engine plume had eroded the upper surface.
This provided measurements during the lunar twilight period, a slow transition lasting hours due to the Moon’s month‑long day‑night cycle. Twilight observations enabled scientists at the Physical Research Laboratory to capture how the Regolith cools in slow motion, offering rare insights into its thermal behaviour.
ChaSTE’s penetration force and temperature profiles revealed that the engine exhaust stripped away the top three centimetres of loose dust, exposing denser material beneath. This disruption removed the “fluff” layer, a change directly recorded in the measurements.
Supporting 3‑D simulations, using Chandrayaan‑2’s OHRC high‑resolution imagery, confirmed the Regolith’s layered stratigraphy.
The top two to six centimetres were found to be hyper‑porous and highly cohesive, acting as a thermal blanket. This layer is critical for trapping water‑ice molecules in the subsurface, making it a key factor in selecting sites for future lunar bases and habitats.
Temperature data collected between 16:20 and 17:17 local lunar time showed sharp cooling trends, with magnitude reversals at depths of 3 mm and 13 mm suggesting a higher‑conducting layer near the surface.
The twilight dataset highlighted stark contrasts between illuminated and shadowed regions, where heat radiates instantly into space due to the absence of an atmosphere. For astronauts, this means the surface can feel like dry flour at the top, but just a few centimetres deeper it behaves like stiff clay, with bulk density increasing from 750 to 1600 kg m⁻³ and cohesion rising from 300 Pa to 1600 Pa.
These findings underscore the heterogeneity of the lunar Regolith at local scales. The Moon’s surface is not uniform but exhibits distinct layering and geotechnical variability shaped by aeons of micrometeorite impacts.
Such knowledge is indispensable for planning surface operations, constructing habitats, and ensuring astronaut safety in the challenging environment of the lunar south pole. Chandrayaan‑3’s hop experiment marks the first time such unique measurements have been captured, setting a precedent for future missions that will probe deeper into the Moon’s secrets.
