In an exciting milestone for lunar scientists around the world, India’s Chandrayaan-3 lander landed 600 km from the moon’s south pole on August 23, 2023.
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 first results with the world.
While data from Chandrayaan-3’s rover, called Pragyan, or “wisdom” in Sanskrit, showed that the lunar soil contains expected elements such as iron, titanium, aluminum and calcium, it also showed an unexpected surprise: sulfur.
Planetary scientists like me know that sulfur exists in moon rocks and soils, but only in very low concentrations. These new measurements imply that there may be a higher sulfur concentration than expected.
Pragyan has two instruments that analyze the elemental composition of the soil: an X-ray spectrometer with alpha particles and a laser-induced degradation spectrometer, LIBS for short. Both instruments measured the sulfur in the soil near the landing site.
Sulfur in the soil near the moon’s poles could one day help astronauts live off the land, making these measurements an example of science enabling research.
Geology of the Moon
There are two main types of rocks on the moon’s surface: dark volcanic rocks and the brighter highland rocks. The brightness difference between these two materials forms the familiar ‘man in the moon’ face or the ‘rabbit picking rice’ face to the naked eye.
Scientists measuring the composition of moon rocks and soil in laboratories on Earth have found that materials from the dark volcanic plains tend to contain more sulfur than the brighter material from the highlands.
Sulfur mainly comes from volcanic activity. Rocks deep within the moon contain sulfur, and when these rocks melt, the sulfur becomes part of the magma. When the molten rock approaches the surface, most of the sulfur in the magma becomes a gas that is released along with water vapor and carbon dioxide.
Some of the sulfur remains in the magma and remains in the rock after cooling. This process explains why sulfur is mainly associated with the moon’s dark volcanic rocks.
Chandrayaan-3’s measurements of sulfur in soil are the first to take place on the moon. The exact amount of sulfur 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 may have a higher sulfur concentration than the 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 geological system. But we will have to wait and see if the fully calibrated data from the Chandrayaan-3 team confirms an increased sulfur concentration.
Atmospheric sulfur formation
Measuring sulfur is interesting to scientists for at least two reasons. First, these findings indicate that the highland soils at the lunar poles may have a fundamentally different composition, compared to the highland soils at the lunar equatorial regions. This difference in composition likely results from the different environmental conditions between the two regions: the poles receive less direct sunlight.
Second, these results suggest that there is somehow more sulfur present in the polar regions. Sulfur concentrated here could have formed from the extraordinarily thin moon atmosphere.
The moon’s polar regions receive less direct sunlight and therefore experience extremely low temperatures compared to the rest of the moon. If surface temperatures drop below -73 degrees C (-99 degrees F), sulfur from the moon’s atmosphere can collect on the surface in solid form – like frost on a window.
Sulfur at the poles could also come from ancient volcanic eruptions on the moon’s surface, or from meteorites containing sulfur that struck the surface and vaporized on impact.
Moon sulfur as a resource
For long-term space missions, many organizations have thought about building some kind of base on the moon. Astronauts and robots could travel from the South Pole base to collect, process, store and use naturally occurring materials such as sulfur on the moon – a concept called in-situ resource use.
Using resources on site means fewer trips back to Earth for supplies and more time and energy for research. Using sulfur as a resource, astronauts can build solar cells and batteries that use sulfur, mix sulfur-containing fertilizers and make sulfur-containing concrete for construction.
Sulfur-containing concrete actually has several advantages over the concrete normally used in earth-based construction projects.
First, sulfur-based concrete hardens and becomes strong in hours rather than weeks, and is more resistant to wear and tear. There is also no need for water in the mixture, so astronauts can save their valuable water for drinking, making breathable oxygen and making rocket fuel.
Although there are currently seven missions on or around the moon, the moon’s south polar region has not previously been studied from the surface. Pragyan’s new measurements will thus help planetary scientists understand the moon’s geological history. It will also allow lunar scientists like me to ask new questions about how the moon formed and evolved.
For now, scientists at the Indian Space Research Organization are busy processing and calibrating the data. On the moon’s surface, Chandrayaan-3 will hibernate during the two-week lunar night, where temperatures will drop to -184 degrees F (-120 degrees C). The night lasts until September 22.
There is no guarantee that Chandrayaan-3’s lander component, called Vikram or Pragyan, will survive the extremely low temperatures, but if Pragyan wakes up, scientists can expect more valuable measurements.
Jeffrey Gillis-Davis, research professor of physics, arts and sciences at Washington University in St. Louis
