Moon’s dark regions, visible to the naked eye, known as the ‘mare’, are remnants of a violent history of the Solar System. There are no records of these violent events on our dynamic Earth.
- Moon, having changed very little in the last billions of years, provides us a window to ponder over the past. The large mare regions on the near side of the Moon that we always see from Earth, mainly consists of basalts which are volcanic rocks. These regions hold the key to how the Moon cooled and evolved and what were the sources of heat that melted and crystallized the material to the present day rocks.
- The Apollo, Luna, and Chang’E-5 missions have brought to Earth an extensive collection of mare basalts.
- Apollo mare basalts date back to the age of 3.8–3.3 Ga (Ga = one Billion years) and were collected from a region unusually rich in potassium (K), rare Earth elements (REE), and phosphorous (P) (together called as KREEP), known as Procellarum KREEP Terrane (PKT). These are rich in radioactive elements that provided the heat to melt rocks resulting in KREEP rich basalts.
- Are there alternate ways for melting on the Moon?
- A team of scientists from Physical Research Laboratory (PRL), Ahmedabad, India, USA and Japan have found a unique group of ancient lunar basaltic meteorites with very low abundance of KREEP. This suggests that these meteorites must have come from a region different from PKT. The samples studied in this work are: (i) Lunar meteorite Asuka-881757 found in 1988 at Antartica, collected by National Institute of Polar Research, Japan, (ii) Lunar meteorite Kalahari 009 found in 1999 at Kalahari Desert in South Africa, (iii) Samples collected by Russian Luna-24 mission (location shown on the map).
- The calculations show that these basalts must be a result of low-pressure melting in the Moon, similar to those in other terrestrial bodies, such as Earth and Mars. Further they show that these basalts originated from a cool, shallow, and compositionally distinct part of the lunar interior.
- This finding suggests that the Moon’s interior melted in the form of basalt magmatism from as early as 4.3–3.9 Ga globally to a more localized scenario in the PKT region later (3.8–3.0 Ga). Fundamentally these new results challenge currently proposed scenarios for the generation of basalts and propose an additional new regime that might be more common on the Moon globally.
