Moon Shrinking as It Cools, While Dust Adds Mass: Ahmedabad Scientist Explains the Lunar Mystery

A few days ago, a report in the Planetary Science Journal, a US-based publication covering planetary science and astronomy, said the Moon has been getting smaller. The figure it gave was 148 feet over the past 400 million years. In metric terms, that is a contraction of 42 to 46 metres.

Professor Jayesh Pabari, a planetary science expert at the Physical Research Laboratory (PRL) in Ahmedabad, walked Divya Bhaskar through what the finding means. He said the conclusion rests on an analysis of satellite images, which show small scarps, meaning cracks and ridges, across the lunar surface. Their presence indicates that some parts of the surface have risen slightly while others have sunk.

The cause, Pabari explained, is heat. The Moon's interior is losing it, so the Moon is gradually cooling. As it does, material on the surface, including soil and dust, compresses under pressure and becomes more compact. The pace is extremely slow, around 11 centimetres per million years. He said the Moon's total mass remains unchanged in this process; only its surface area decreases, and that reduction could affect its density.

Pabari's own research adds a second thread: the Moon is also gaining weight. Interplanetary dust particles, extremely fine solids drifting between the planets of the Solar System, arrive from outside. The Moon has no atmosphere, so they reach the surface directly and accumulate. Some particles escape the surface and head back into space, but studies tracking both flows found that more arrive than leave. The Moon's mass is therefore rising slowly, the incoming dust settling into a thin layer on the surface.

Pabari said he studied this in 2020, using impact theory, about six years ago. His team reported the lunar escape process for the first time. They found that dust particles, and sometimes even ice, leave the surface, and they also examined the dust arriving from space. Put together, the picture is of a Moon that shrinks as it cools while its mass creeps upward because the dust coming in outweighs the dust going out.

The ice is where the work touches the future. The Moon's North and South Poles have permanently shadowed regions, where sunlight never reaches and where water ice is found. When interplanetary dust strikes these areas, it creates what Pabari called an ejecta fountain, throwing material off the surface. Some of the water ice leaves the Moon with it. "According to our estimates, around six kilograms of ice leaves the Moon and enters space every year," he said. Because the process is so slow, he added, the reserves could decline gradually and might be depleted over a very long period. The six-kilogram figure is the team's estimate.

That matters because water ice is regarded as an important resource for future lunar missions, and could be useful if a human colony is ever established there. Pabari said the gradual loss should be accounted for.

As for daily life on Earth, he said any effects of changes in the Moon's size or mass may become noticeable only after millions of years, a timescale far beyond a human lifespan. There will be no immediate impact on people.

Asked whether the same thing is happening elsewhere, Pabari said the Moon's interior is still cooling, which is what drives the contraction. For other planets and moons, he said, cooling has either already finished or they are not as hot, so this effect is currently more active on Earth's Moon. Pabari has worked in PRL's Planetary Sciences Division for 25 years, studying planets orbiting the Sun and interplanetary dust. The open question for lunar planners is how the slow ice loss squares with plans for a long-term human presence.