Moon Hunt. Plasma on moon sparse, radio communication won’t be hard, finds Chandrayaan-3 instrument

Even while one more instrument onboard the Pragyan rover confirmed the presence of sulfur on the moon, another instrument—a Langmuir probe—on the Vikram lander has revealed that the density of plasma near the lunar surface is rather thin. 

A Langmuir probe—named after its inventor, the American physicist Irving Langmuir—is an instrument that measures properties of plasmas. (Plasma—not to be confused with blood plasma—is a state of matter, when it is super-hot. It is a soup of positively charged ions and negatively charged electrons.) 

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The Langmuir probe on the Vikram lander, which ISRO has christened ‘Radio Anatomy of Moon Bound Hypersensitive Ionosphere and Atmosphere—Langmuir Probe (RAMBHA-LP) has checked out the plasma in the region near the moon’s surface and has assessed that “the plasma near the lunar surface is relatively sparse.” 

This means that there are not many electrons in this region of space, ISRO said in a tweet. The thinness of lunar plasma is important because it affects the way radio waves propagate through space. Radio waves are affected by the presence of plasma—the denser the plasma, the more the radio waves are scattered. 

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The sparseness of lunar plasma means that radio waves can propagate through space with less attenuation, which is important for communication between lunar missions, ISRO said. 

The Langmuir probe

A Langmuir probe, which is essentially an electrode which, when dipped into plasma, can pick up electrons and ions, and measure the electron temperature, electron density and ion density of the plasma by obtaining its current-voltage characteristics. The RAMBHA-LP, developed by the Space Physics Laboratory (SPL), Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, is a 5-cm metallic spherical probe mounted on a 1-meter boom and attached to the Lander’s upper deck. The boom, which opens like a switch-knife, holds the spherical probe away from the Lander, so that the probe is dipped into the undisturbed lunar plasma environment well away from the Lander’s body.  

In a press release issued today, ISRO noted that the system “can detect minute return currents, as low as pico-emperes with a dwell time of 1 millisecond.” 

The system can accurately determine ion and electron densities as well as their energies based on the measured return current. 

“The initial assessment indicates that the plasma encompassing the lunar surface is relatively sparse, characterized by a number density ranging from approximately 5 to 30 million electrons per cubic meter. This evaluation specifically pertains to the early stages of the lunar daytime. The probe operates without interruption, aiming to explore the changes occurring in the near-surface plasma environment throughout the lunar day. These ongoing observations hold significant implications for comprehending the process of charging within the lunar near-surface region, particularly in response to the fluctuations in solar space weather conditions,” the ISRO release says.  

More good news

Meanwhile, another instrument on the Pragyan rover confirmed the presence of sulfur in the region, using a different technique, called ‘alpha particle X-ray spectroscope’. This finding “compels scientists to develop fresh explanations for the source of sulphur in the area—intrinsic? Volacanic? Meteroritic?” tweeted ISRO. Earlier, the ‘laser-induced breakdown spectroscope’ (LIBS) on the rover had “unambiguously” confirmed the presence of sulphur there.  

Presence of sulphur on the moon has been known for decades, through analysis of soil samples brought by the Apollo missions of NASA, but this is the first time that an in-situ measurement was done. This finding may not be new, but it is a ‘technology demonstrator’. The LIBS has also found aluminium, calcium, iron, chromium, titanium, manganese, silicon and oxygen. “Search for hydrogen is underway,” ISRO said. 

Further, the rover was rotated around, in search of a safe route. ISRO has released pictures of the rotation, captured by a camera on the Lander. “It feels as though a child is playfully frolicking in the yards of Chandamama, while the mother watches affectionately. Isn’t it?” tweeted ISRO.