The successful hop experiment conducted by the Vikram lander during India's Chandrayaan-3 mission has opened a new window into the Moon's south polar region, courtesy of a groundbreaking study from the Physical Research Laboratory (PRL) in Ahmedabad. This maneuver, performed in the mission's final days, not only demonstrated the lander's propulsion capabilities but also exposed subtle layers in the lunar regolith that were previously hidden. Researchers analyzed data from the Chandra's Surface Thermophysical Experiment (ChaSTE), revealing a complex soil structure that challenges uniform models of the lunar surface and promises vital information for upcoming explorations.
Chandrayaan-3, launched on July 14, 2023, achieved a historic soft landing on August 23 near the lunar south pole at Shiv Shakti Point (69.37°S, 32.35°E). Over its operational lifespan, the mission gathered unprecedented data from this shadowed, volatile-rich terrain. The hop experiment, executed on September 3, involved firing the lander's engines briefly to lift it about 40 cm and relocate it roughly 50 cm northwest, with a slight 2.6° rotation. This short journey allowed ChaSTE—a probe designed by PRL in collaboration with Vikram Sarabhai Space Centre (VSSC)—to penetrate fresh regolith at the new site, measuring temperatures and properties during the lunar twilight phase from 16:25 to 17:30 LT.
Understanding the Vikram Hop Experiment
The hop was more than a technical demo; it mimicked scenarios for sample return or repositioning in future missions. Vikram's engines generated a plume that interacted with the surface, displacing loose material and compacting subsurface soil. Lander Imager (LI) images confirmed the shift, showing rover tracks and craters as reference points. This disturbance provided a natural 'excavation,' stripping away the topmost regolith to reveal what lay beneath.
At the post-hop site, ChaSTE's 10 sensors penetrated up to 6.5 cm, with five fully operational. Initial penetration data, captured via motor current, showed divergences from pre-hop profiles in the top 3 cm but perfect alignment when adjusted for a 3 cm shift downward. This confirmed plume-induced erosion of approximately 3 cm of loose surface soil, a first direct in-situ evidence of such localized change near the south pole.
ChaSTE: Probing Lunar Soil Thermophysics
ChaSTE, principal investigator-led by PRL's Anil Bhardwaj, is a temperature probe with a tapered penetrator tip for hammering into regolith. It measures vertical profiles up to 10 cm deep, capturing thermal conductivity, heat flow, and diurnal variations. During Chandrayaan-3, it operated across lunar day-night, but twilight data post-hop proved especially revealing. Sensors recorded steady cooling until sunset, followed by a sharp drop, with intriguing reversals at shallow depths (3 mm and 13 mm).
Discovery of Two-Layer Regolith Structure
The star finding is a distinct two-layer stratigraphy in the top 6.5 cm. The upper layer (0-3 cm), compacted by the plume, exhibits higher bulk thermal conductivity of 0.0201 W m⁻¹ K⁻¹, allowing faster heat dissipation. Below, from 3-6.5 cm, conductivity drops to 0.0125-0.0128 W m⁻¹ K⁻¹, indicating looser, more porous material originally overlain by the eroded fluff.
Cooling curves post-penetration equilibrated quickly in the top 3.5 cm but showed steep gradients deeper, underscoring this dichotomy. Bulk density ramps from ~750 kg m⁻³ at surface to 1600 kg m⁻³ at depth, with porosity falling from 75% to 45%. Such variability means surface operations must account for 'footprint' effects from lander plumes.
Temperature Profiles and Local Heterogeneity
Twilight temperatures revealed faster-than-expected cooling in the upper layer, influenced by nearby craters creating shadowing. Model-derived diurnal profiles matched observations closely, but discrepancies highlighted topography's role—a 2.6° slope and northward depression caused sudden dips around sunset. Top sensors showed inversion post-sunset, signaling denser surficial regolith.
This is the first direct evidence of meter-scale regolith heterogeneity at high latitudes, differing from equatorial sites like Apollo landings where soil is more uniform. South polar terrain, with potential water ice, demands such granular data for thermal modeling.
For precise details on the thermal profiles, refer to the full study in The Astrophysical Journal.
Geotechnical Properties and Modeling Insights
Penetration motor currents yielded geotechnical metrics: cohesion rises from 300 Pa surface to 1600 Pa at 6.5 cm, friction angle from 40° to 45°. These inform rover mobility and habitat stability. 3D thermal simulations, incorporating local slope and shadowing, validated ChaSTE data, predicting equilibrated layers and probe perturbations.
- Surface layer (0-3 cm): Compacted, high conductivity, low cohesion.
- Subsurface (3-6.5 cm): Pristine, lower conductivity, higher strength.
- Implications for plumes: Erosion depth scales with thrust, critical for lander design.
PRL's Pivotal Role in Indian Space Research
Physical Research Laboratory (PRL), established in 1947 by Dr. Vikram Sarabhai, spearheaded ChaSTE development. Lead author K. Durga Prasad and team from PRL, including G. Ambily, Chandan Kumar, and director Anil Bhardwaj, conducted the analysis. PRL's expertise in planetary sciences positions it as a hub for lunar studies, fostering PhD programs and university collaborations.
This work exemplifies India's self-reliant space ecosystem, blending academia and ISRO. PRL researchers often hold adjunct faculty roles at IITs and universities, bridging research to higher education. More on PRL's contributions at their official site.
Implications for Future Lunar Missions
These insights refine models for volatile retention—higher conductivity top layers could accelerate ice sublimation. For Chandrayaan-4 (sample return, 2027) and Artemis, they guide plume mitigation, ISRU for water extraction, and base site selection. Non-uniformity suggests south pole's regolith evolves via micrometeorites, outgassing, and plumes.
Engineers now prioritize low-thrust landings to minimize disturbance. Academically, it spurs curricula in planetary geotechnics at Indian institutes like IITs and IISc.
Broader Scientific and Educational Impact
The study, published April 10, 2026, in ApJ, underscores India's rising lunar prowess post-Chandrayaan-2/3 successes. It inspires STEM students, with PRL offering fellowships. In higher education, it highlights interdisciplinary research—thermal physics meets geomechanics.
Stakeholders from ISRO to global NASA note its value for human landings. Indian universities integrate such findings into astrophysics courses, preparing next-gen scientists.
Challenges and Future Research Directions
Challenges include modeling micro-topography effects and scaling erosion to larger plumes. Future probes may target deeper profiles or volatiles. Collaborations with NASA/ESA loom, boosting Indian researchers' profiles.
PRL plans ChaSTE follow-ups on Chandrayaan-4, eyeing water-ice interfaces.
Photo by Bhavya Pratap Singh on Unsplash
Conclusion: Paving the Way for Lunar Habitation
Vikram's hop unveiled the Moon's south pole as dynamically layered, informing sustainable exploration. PRL's study cements India's leadership, urging higher ed investments in space tech. As missions evolve, these insights ensure safer, smarter lunar ventures.
