Almost a year after Chandrayaan 3 landed on the Moon, scientists in India have released the findings of studies carried out by one of the instruments on the rover module. The findings comprise the first analysis of the composition of the topsoil in the Moon’s southern latitudes, and support the widely-accepted hypothesis that the lunar surface in the immediate aftermath of the Moon’s formation was covered by a sea of molten material. The data collected by the Alpha Particle X-ray Spectrometer (APXS) also contain new information about the elemental composition of the lunar surface that can help to better understand the evolution of the Moon. The findings were published in the journal Nature on Wednesday. What has the APXS found? Scientists have reported three key findings. * The terrain around Chandrayaan 3’s landing sight is fairly uniform; * The Moon’s crust was formed layer by layer, which adds weight to the lunar magma ocean (LMO) hypothesis; and * The topsoil around the lunar south pole has a greater-than-expected sprinkling of minerals which compose the lower layers of the lunar crust. Taking forward LMO hypothesis The Moon is thought to have been formed after a large asteroid collided with Earth some 4.5 billion years ago. Scientists hypothesise that in its early life, the Moon’s surface was made up entirely of an ocean of magma. As this ocean cooled over millions of years, heavier silicon- and magnesium-rich minerals such as olivine and pyroxene sank to the lower levels of the lunar crust and its upper mantle (which is generally the largest layer inside a planetary body, bounded by the planet’s core on the inside and the crust on the outside). Lighter minerals, composed of calcium- and sodium-based compounds, floated to the top and formed the upper crust. The findings of Chandrayaan 3’s APXS take this hypothesis a step further. They support a class of models under the umbrella of the LMO hypothesis which theorises a stratified lunar crust — where 80-90% of the upper crust is believed to be composed of iron, magnesium, and sodium-rich rocks, and the lower crust of magnesium-rich rocks. “Among the different possible scenarios of lunar crust formation within the premise of LMO, the APXS measurements support the models indicating stratified crust formation,” the study says. ‘Mixing’ of crust’s lower levels The third APXS finding is a new discovery — and suggests that some “mixing” of the various levels of the lunar crust might have taken place. The researchers have proposed that this “mixing” could have been caused by the asteroid impact that led to the formation of the South Pole Aitken (SPA) Basin, the largest and oldest basin on the Moon. The SPA basin has a diameter of approximately 2,500 km — equivalent to the distance between Delhi and Kochi — and a depth of 6.2-8.2 km. It is believed to have been formed 4.2-4.3 billion years ago, when the asteroid hit near the lunar south pole. The researchers theorise that this asteroid impact resulted in the excavation of magnesium-rich material from deeper layers of the Moon, out to the surface of the surrounding areas as ejecta. The Chandrayaan 3 landing site was just 350 km from the rim of the SPA basin. Significance of the findings While these findings more or less support what is already known via modelling, imaging, and various hypotheses, they are novel nonetheless. This is primarily because Chandrayaan 3 landed near the Moon’s south pole, a first in lunar exploration. While the composition of lunar topsoil near the equatorial and mid-latitude regions have previously been studied by earlier Moon missions sent by other countries, this is the first time that such measurements have been carried out near the Moon’s poles. This makes Chandrayaan 3 the first to carry out in situ experiments of any kind at the poles. The rover moved within a 50-metre radius of the landing site, and took readings on different kinds of surfaces, from relatively smooth ones to near the rims of small craters. The overall finding, which suggests that the topsoil near the landing site is fairly uniform, is novel. Notably, the uniformity of the surface is unlike that of Earth, where tectonic plate movements led to the creation of undulating surfaces. With measurements taken at 23 spots along the rover’s path, this is the first time that soil measurements on lunar highlands (light coloured, elevated basins on the Moon standing above dark-coloured basins known as maria) have been taken with such high frequency. Other missions — such as the US’s Apollo 16, Russia’s Luna 20, and China’s Chang’e 4 — too have sampled the lunar highlands, but sparsely. The total observation duration for the APXS measurements was about 31 hours, which were spread out over the mission period of 10 days. Impact on future missions If the mixing of the various layers of the lunar crust was propelled by the impact on the SPA basin and by further impacts subsequently in and around the basin, scientists believe that the APXS measurements will serve as the “first ground truth in the south polar highlands and probably play a key role in the overall understanding of the origin and evolution of the Moon”. The uniform surface also implies that the region can be used as a calibration point for remote sensing operations, and can thus be used for planning future missions. Future missions near the south pole can also evaluate for source-crater pairing of lunar meteorites. Lunar meteorites are meteors that originate from the Moon, ejected upon impact from an asteroid or comet. Caught in the Earth’s gravitational field, the lunar meteorites eventually end up on Earth.
