A lunar rock opera

Chandrayaan-3’s measurements of sulphur open the doors for lunar science and exploration

Good morning! It may well be a bittersweet ending for Chandrayaan-3. Bitter, because its lander Vikram and rover Pragyan aren’t waking up despite Isro’s best efforts. Sweet, because the duo succeeded in collecting valuable data before going into sleep mode. A lot has been said about water ice on the moon’s south pole, but did you know that lunar soil also contains sulphur? The evidence of that, confirmed by Pragyan, holds tremendous significance. Read on to know more. Plus: a curated list of the week’s best longreads.

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In just under 14 Earth days, Chandrayaan-3 provided scientists with valuable new data and further inspiration to explore the Moon. And the Indian Space Research Organization has shared these initial results with the world.

While the data from Chandrayaan-3’s rover, named Pragyan, or “wisdom” in Sanskrit, showed the lunar soil contains expected elements such as iron, titanium, aluminium and calcium, it also showed an unexpected surprise – sulphur.

India’s lunar rover Pragyan rolls out of the lander and onto the surface.

Planetary scientists like me have known that sulphur exists in lunar rocks and soils, but only at a very low concentration. These new measurements imply there may be a higher sulphur concentration than anticipated.

Pragyan has two instruments that analyse the elemental composition of the soil – an alpha particle X-ray spectrometer and a laser-induced breakdown spectrometer, or LIBS for short. Both of these instruments measured sulphur in the soil near the landing site.

Sulphur in soils near the Moon’s poles might help astronauts live off the land one day, making these measurements an example of science that enables exploration.

Geology of the Moon

There are two main rock types on the Moon’s surface – dark volcanic rock and the brighter highland rock. The brightness difference between these two materials forms the familiar “man in the moon” face or “rabbit picking rice” image to the naked eye.

Scientists measuring lunar rock and soil compositions in labs on Earth have found that materials from the dark volcanic plains tend to have more sulphur than the brighter highlands material.

Sulphur mainly comes from volcanic activity. Rocks deep in the Moon contain sulphur, and when these rocks melt, the sulphur becomes part of the magma. When the melted rock nears the surface, most of the sulphur in the magma becomes a gas that is released along with water vapour and carbon dioxide.

Some of the sulphur does stay in the magma and is retained within the rock after it cools. This process explains why sulphur is primarily associated with the Moon’s dark volcanic rocks.

Chandrayaan-3’s measurements of sulphur in soils are the first to occur on the Moon. The exact amount of sulphur cannot be determined until the data calibration is completed.

The uncalibrated data collected by the LIBS instrument on Pragyan suggests that the Moon’s highland soils near the poles might have a higher sulphur concentration than highland soils from the equator and possibly even higher than the dark volcanic soils.

These initial results give planetary scientists like me who study the moon new insights into how it works as a geologic system. But we’ll still have to wait and see if the fully calibrated data from the Chandrayaan-3 team confirms an elevated sulphur concentration.

Atmospheric sulphur formation

The measurement of sulphur is interesting to scientists for at least two reasons. First, these findings indicate that the highland soils at the lunar poles could have fundamentally different compositions, compared with highland soils at the lunar equatorial regions. This compositional difference likely comes from the different environmental conditions between the two regions – the poles get less direct sunlight.

Second, these results suggest that there’s somehow more sulphur in the polar regions. Sulphur concentrated here could have formed from the exceedingly thin lunar atmosphere.

The polar regions of the Moon receive less direct sunlight and, as a result, experience extremely low temperatures compared with the rest of the Moon. If the surface temperature falls below -73 degrees C (-99 degrees F), then sulphur from the lunar atmosphere could collect on the surface in solid form – like frost on a window.

Sulphur at the poles could also have originated from ancient volcanic eruptions occurring on the lunar surface, or from meteorites containing sulphur that struck the surface and vaporised on impact.

Lunar sulphur as a resource

For long-lasting space missions, many agencies have thought about building some sort of base on the Moon. Astronauts and robots could travel from the south pole base to collect, process, store and use naturally occurring materials like sulphur on the Moon – a concept called in-situ resource utilisation.

In-situ resource utilisation means fewer trips back to Earth to get supplies and more time and energy spent exploring. Using sulphur as a resource, astronauts could build solar cells and batteries that use sulphur, mix up sulphur-based fertiliser and make sulphur-based concrete for construction.

Sulphur-based concrete actually has several benefits compared with the concrete normally used in building projects on Earth.

For one, sulphur-based concrete hardens and becomes strong within hours rather than weeks, and it’s more resistant to wear. It also doesn’t require water in the mixture, so astronauts could save their valuable water for drinking, crafting breathable oxygen and making rocket fuel.

While seven missions are currently operating on or around the Moon, the lunar south pole region hasn’t been studied from the surface before, so Pragyan’s new measurements will help planetary scientists understand the geologic history of the Moon. It’ll also allow lunar scientists like me to ask new questions about how the Moon formed and evolved.

For now, the scientists at the Indian Space Research Organization are busy processing and calibrating the data. On the lunar surface, Chandrayaan-3 is hibernating through the two-week-long lunar night, where temperatures will drop to -184 degrees F (-120 degrees C). The night will last until September 22.

There’s no guarantee that the lander component of Chandrayaan-3, called Vikram, or Pragyan will survive the extremely low temperatures, but should Pragyan awaken, scientists can expect more valuable measurements.

Jeffrey Gillis-Davis is Research Professor of Physics, Arts & Sciences at Washington University in St. Louis.


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