Four things Chandrayaan-3 has taught us about the lunar south pole. The Indian Space Research Organisation’s (ISRO) Moon lander Vikram and robotic rover Pragyan have now been told to go to sleep. ISRO hopes to awaken them at lunar dawn on 22 September

But in their two-week sojourn around the Moon’s south pole, they provided insights that have planetary scientists abuzz. Here are some of the first remarkable findings.

A thin soup of ions and electrons swirls near the lunar pole

A probe onboard Vikram made the first measurements of the density and temperature of Moon’s ionosphere. ISRO reports a “relatively sparse” mix of ions and electrons in the 100-kilometre-thick layer of electrically charged plasma that surrounds the Moon’s surface near the south pole.

Initial measurements of the plasma indicate a density of about 5 million to 30 million electrons per cubic metre. And the density seems to vary as the lunar day progresses, an ISRO scientist analysing the Chandrayaan-3 mission's data told Nature. The peak density of a similar layer in Earth’s upper atmosphere is one million electrons per cubic centimetre.

The density of the ionosphere would affect lunar communication and navigation systems if humans were to inhabit the Moon — the higher the electron density, the longer radio signals take to travel through the ionosphere. The sparse plasma means that potential delays would be “minimal”, the scientist says, and would not pose a problem for transmission.

Temperature Variations With Depth

Understanding lunar soil, including its temperature and conductivity, will be important when considering settlement on the Moon. The soil “is an important in situ resource for construction”, says Anil Bhardwaj, director of ISRO’s Physical Research Laboratory in Ahmedabad.

The lander is fitted with a temperature probe containing 10 sensors and able to reach 10 centimetres below the surface of the Moon. Its preliminary data show that during the day, the temperature 8 cm down is around 60 ºC lower than at the surface.

Planetary scientist Paul Hayne at the University of Colorado Boulder, says that a steep decline in temperature is expected during the lunar daytime, because the heat does not conduct downward from the warm sunlit surface. “This is similar to the effect one experiences when visiting a beach on a hot day — dig down just a few centimetres and the sand is much cooler,” he says.

Measurements so far have found that the temperature at the surface is significantly warmer than recorded by NASA’s 2009 Lunar Reconnaissance Orbiter, adds Hayne.

The temperatures “are far too warm for water ice to be stable”, says Hayne, explaining that water converts from solid to gas at a very low temperature in the vacuum of space — at about −160 ºC. Chandrayaan-3’s data indicate temperatures warmer than −10 ºC at all depths sampled. Further down "we expect temperatures to flatten out at close to the average surface temperature of about −80 ºC,” says Hayne.

A Suspected Moonquake

Among many vibrations recorded by the lander’s seismograph, one in particular caught the attention of scientists. The instrument “seems to have recorded a very small seismic event that decayed to background in about 4 seconds”, says planetary geochemist Marc Norman at the Australian National University in Canberra. ISRO scientists suspect it was a small moonquake or the impact of a tiny meteorite.

Such perturbations are expected on the Moon. “Small impacts and local tectonic adjustments related to tidal forces are common on the Moon, but we really need a global seismic network on the Moon and longer-term observations to understand the significance of any particular event,” says Norman.

Sulphur Confirmed

Testing by the rover unambiguously confirms the presence of Sulphur in the lunar surface near the south pole, ISRO reports. It also found aluminium, silicon, calcium and iron, among other elements.

“Sulphur, being volatile, is not generally expected,” explains Bhardwaj. Confirmation of its presence is really important, say scientists. Sulphur is a key element of molten rock, and researchers think that the primitive Moon was covered with a thick layer of hot molten rock, which crystallized to form the Moon’s surface. Measurements of Sulphur concentrations can provide insight into that process, says the ISRO scientist. However, it’s also possible that the Sulphur came from asteroids that bombard the Moon’s surface. The ISRO scientist says they hope to add their findings to those of the US Apollo missions to better understand the Moon’s geochemistry.

Vikram-Pragyan Hibernation

After a ground breaking two-week mission, India's robotic explorers are fast asleep in the frigid darkness of the moon's south pole region. Whether they'll wake up when the sun shines down on them at the end of this lunar night mostly whittles down to luck.

Temperatures near the moon's poles can drop to as low as -424°F (-253°C or 20 K). Yet neither Chandrayaan-3's lander, Vikram, nor its rover, Pragyan — which made a historic touchdown on Aug. 23 — are equipped with heaters otherwise common for moon missions.

These heaters, called radioisotope heater units (RHUs), work by passively radiating heat to keep the hardware onboard spacecraft at sustainable operating temperatures. Most commonly, RHUs used in space missions convert heat generated from the natural decay of radioactive versions of plutonium or polonium into electrical power. This process ultimately warms spacecraft hardware, though mostly just enough to help it survive very cold temperatures.

But without such power systems, the survival of Chandrayaan-3's robotic duo is left to chance. 

RHUs have been used in moon landing missions since the 1970s. 

Lunokhod-1, which was the first successful lunar rover that covered over 10 kilometers (6 miles) in just 10 months, had powered itself using solar cells mounted on a large lid. During nights on the moon, it closed that lid to keep itself warm until the next sunrise with the energy supplied by a polonium-210 radioisotope heater.

ISRO hasn't publicly discussed why similar radioisotope heaters were not fitted onboard Chandrayaan-3's Vikram lander and Pragyan rover – nevertheless, the robotic duo splendidly accomplished its science goals in a region of the moon that has firmly become a hotspot for space exploration, thanks to apparently harbouring reservoirs of frozen water. In fact, it was the first to successfully land there at all.

The lander even exceeded its mission objectives when it managed to "hop" on the moon's surface, flinging itself upward by about 16 inches (40 centimetres) and a tad closer to its companion Pragyan, which had already been put into sleep mode at that point.

As for preparing the robotic duo for its first lunar night, the batteries onboard Chandrayaan-3's rover Pragyan were fully charged before it was put to sleep, the Indian Space Research Organization, India's space agency operating this mission, said in a post on X (formerly Twitter).

"After sunset, there is no power," Arun Sinha, a former senior scientist at ISRO, told prior to Chandrayaan-3's launch. "However, there are faint chances of extra-efficient battery charge. If this is good, another 14 days might be available."

"Else, it will forever stay there as India's lunar ambassador," ISRO said Sept. 2 in a post on X.