Chandrayaan-2 Delivers First Proof: How A Solar Superstorm Inflates Moon's Atmosphere

In the second week of May 2024, the sun reminded humanity, with dazzling intensity, who is truly in charge of our neighbourhood.

A celestial spectacle of staggering proportions unfolded as a series of powerful eruptions, Coronal Mass Ejections (CMEs), slammed into Earth's magnetic shield. Classified as a G5 'Extreme' geomagnetic storm, the highest possible rating, the event painted the skies with auroras visible over latitudes rarely treated to such a sight, from Florida to Mexico.

Yet beneath the beauty, the storm was a serious reminder of cosmic vulnerability: GPS-guided machinery failed, and trans-Atlantic flights were forced into southerly detours to avoid communication blackouts and heightened radiation. Humanity, protected by a thick atmosphere and a powerful magnetic field, was inconvenienced and awed.

But just 384,600 kilometres away, hanging in space like a target, our silent celestial partner, the Moon, took the full, unmitigated brunt of the superstorm.

Lacking the Earth’s double-layered defence, the Moon is utterly exposed to the solar wind's constant breath, which, during a CME, becomes a tempestuous roar. For decades, planetary scientists had predicted a dramatic reaction from the Moon’s environment to such a cataclysm. Computer models suggested the lunar surface would be violently scoured, momentarily swelling its ethereal atmosphere.

Now, pre-emptive scientific readiness met a rare solar storm, allowing a lone orbiter to provide the first-ever direct confirmation of that theory. This ground-breaking observation emerged from the Indian Space Research Organisation (ISRO)'s Chandrayaan-2 spacecraft, a key asset from a mission often solely remembered for its lander failure, proving that superficial setbacks can often conceal dormant scientific achievements waiting for the right cosmic event to be brought to light.

Orbiting 100 kilometres above the lunar surface, India’s Chandrayaan-2 spacecraft, armed with its Chandra’s Atmospheric Composition Explorer-2 (CHACE-2) instrument, acted as the perfect sentinel, revealing in real time how a solar superstorm can fundamentally and violently reshape an airless world.

The Moon’s Ghostly Veil

To appreciate the scale of what Chandrayaan-2 observed is to first grasp the exquisite fragility of the lunar atmosphere, a cosmic secret so often overlooked. It is not an atmosphere in the terrestrial sense, a deep, protective ocean of gas, but rather a surface-bound exosphere, a whisper of gas so tenuous that its constituent atoms, if they had consciousness, would rarely know the company of another.

Imagine standing on the lunar dust, breathing this air, an air so sparse that the pressure measures a millionth of a billionth of Earth's. It is a vacuum cleaner's dream, an environment far emptier than any human laboratory can sustain.

In this desolate space, a liberated atom's journey typically ends not with a collision with a neighbour but with a return to the cold, unforgiving surface itself. The elegance and profound naturalness of this process, a world created by the ceaseless poetry embedded in the equations of physics, offer a richer understanding than any simplistic, conspiratorial fantasy of the Moon as a constructed shell by an alien intelligence.

This ghostly veil is maintained by a delicate, ongoing cosmic process of creation and decay.

How is it built?

Not by architects, but by the relentless physics of the cosmos.

Under normal, or quiet, solar conditions, the air is stirred by two important natural phenomena.

The dominant source, responsible for roughly 70 per cent of the material, is the magnificent inevitability of impact vaporisation. The Moon, a long-term resident of the inner solar system, is ceaselessly sprayed by high-velocity micrometeorites. Each tiny, ballistic fleck vaporises a minute puff of lunar rock, lofting atoms into the exosphere.

The secondary, yet crucial, source is solar wind ion sputtering.

Our Sun, the yellow star, continuously exhales the solar wind, a stream of charged particles travelling at phenomenal speeds. This wind is no mere breath; it is a gentle, persistent sandblaster of protons and electrons, striking the lunar soil called lunar regolith and knocking individual atoms from the surface into the overlying gas. In the context of planetary science, especially concerning airless bodies like the Moon, 'sputtering' refers to the process where atoms or molecules are ejected from a solid surface (here the lunar regolith) when bombarded by energetic particles (here the protons and alpha particles in the solar wind).

The resulting composition, a direct reflection of the lunar soil and the steady solar pressure, rich in argon, helium, and potassium, is a complex chemical portrait painted by time and stellar radiation. For millennia, this delicate, cold balance has been the status quo.

The May 2024 storm, however, was about to upend this dynamic entirely.

The Perfect Storm for Sputtering

The event that struck the Moon on 10 May 2024 was far from ordinary solar wind. It was a massive, fast-moving Coronal Mass Ejection (CME), a literal billion-ton cannonball of superheated plasma and tangled magnetic field lines explosively ejected from the Sun’s outer atmosphere, the corona. This series of eruptions merged into what astronomers sometimes call a 'cannibal' CME, a superstorm of exceptional energy and complexity that tore through interplanetary space, racing toward the Earth-Moon system at speeds approaching 700 kilometres per second.

Crucial real-time data arrived from the Sun-Earth L1 point, a gravitationally stable vantage located roughly 1.5 million kilometres from Earth directly toward the Sun, where sentinel satellites such as the Advanced Composition Explorer (ACE) provided both an early warning and a detailed breakdown of the storm's composition. Crucially, this data, which precedes the storm's arrival at the Moon, allowed scientists to calculate the exact moment of impact.

First, the number of solar wind protons hitting the Moon per second, the proton flux, spiked dramatically. The background flux suddenly increased by more than an order of magnitude, or a factor of ten, surging to over three billion particles per square centimetre per second. This alone represented a massive increase in the sheer volume of the solar sandblaster.

Let us understand this from an earthly perspective. Assume every square centimetre of a significant number of our GPS satellites' solar panels being hit by three billion high-energy particles every second. The system damage effected by this torrent translates on the ground into a cascade of technical failures: navigation apps will become useless and banking transactions impossible.

Second, the storm carried a hidden weapon, a massive jump in the alpha-proton ratio. Alpha particles, the nuclei of helium atoms, possess two protons and two neutrons, making them four times more massive than a simple proton. An alpha particle imparts vastly more momentum upon striking the lunar regolith than a proton does, making it a far more efficient and destructive sputtering agent. The simultaneous tenfold increase in both the volume of particles and their destructive power created a powerful multiplicative effect. This was not a gentle wind; it was an extreme event perfectly tailored to scour the Moon’s unprotected surface.

CHACE-2 Catches the Wave

While the solar storm was brewing and racing toward the Moon, the CHACE-2 instrument aboard the Chandrayaan-2 orbiter was conducting its routine observations. CHACE-2 is a highly sensitive Quadrupole Mass Spectrometer, the scientific equivalent of a sophisticated sniffer dog capable of identifying every trace gas in its path.

The instrument works by first drawing in the sparse atoms and molecules of the lunar exosphere and ionising them, giving them an electric charge. These charged particles (ions) are then passed into a central filter consisting of four parallel metal rods. By applying precisely tuned direct current (DC) and radio-frequency (RF) alternating current (AC) voltages to these rods, CHACE-2 creates an oscillating electric field.

This field acts as a highly selective 'bouncer', only allowing ions of a single, specific mass-to-charge ratio to pass through to the detector. By systematically sweeping through different voltages, the spectrometer builds a complete, detailed picture of the exosphere's chemical composition. Crucially for this study, the instrument also contains an independent Bayard-Alpert (B/A) gauge that measures the total pressure of all gases combined, providing a quick, robust measure of overall atmospheric density.

On 9 and 11 May, the days immediately preceding and following the storm, CHACE-2's B/A gauge recorded the low, familiar pressure signatures of the normal lunar exosphere. But at approximately 16:59 Universal Time (UT) on 10 May, the measurement saw a dramatic change.

The total pressure gauge began to shoot upwards.

The timing of this observation was the critical piece of evidence. Using the ACE data from the L1 point, scientists were able to calculate that the leading edge of the CME's high-flux, high-alpha-particle plasma stream would arrive at the Moon precisely at 17:00 UT. The dramatic pressure spike observed by CHACE-2, beginning almost exactly on schedule, was the unambiguous, first-ever in situ observation of a Coronal Mass Ejection impacting the lunar exosphere.

An Order of Magnitude Swell

The scientific team, led by M. B. Dhanya of the Vikram Sarabhai Space Centre, meticulously analysed the data. To isolate the CME's effect from the Moon’s normal background atmosphere, they subtracted the total pressure values recorded on the day after the storm (11 May) from those recorded during the event (10 May). This background subtraction yielded the pure enhancement caused by the solar tempest.

The calculations revealed a staggering scale of atmospheric inflation. The total number density of the lunar exosphere, the number of atoms per cubic centimetre, swelled by more than an order of magnitude, or a factor of ten. The density jumped from a background enhancement of approximately 11 billion atoms per cm³ of space to a peak of about 170 billion atoms per cm³.

This temporary atmosphere, violently liberated from the regolith by the enhanced sputtering, was composed primarily of refractory elements abundant in lunar soil, such as silicon, magnesium, aluminium, iron, calcium, and oxygen.

This remarkable observation immediately validated a major theoretical prediction that had remained unverified for over a decade.

In 2012, researchers Rosemary Killen and Dana Hurley published seminal computational models demonstrating that an extreme solar event would enhance the density of the Moon’s sputtered atmospheric elements by a factor greater than ten. The CHACE-2 measurement provided the first experimental ground truth, demonstrating that the theoretical models were remarkably accurate in predicting the scale of the lunar exosphere's response to extreme solar events.

The CHACE-2 data provided the first direct empirical validation, proving the theoretical models were accurate in predicting the scale of the Moon’s atmospheric response. Furthermore, the data showed that this enhanced atmosphere was global; it persisted even beyond the dayside-nightside terminator, suggesting the super-energetic solar wind protons were able to reach the non-sun-facing side, or the nightside, and sputter the surface there as well, before the effect rapidly subsided once the CME had passed. The effect of the coronal discharge from the Sun thus created a phenomenon on the entire lunar surface.

Implications for the Artemis Era

The Chandrayaan-2 findings carry sustained and immediate implications for the future of human space exploration.

As NASA's Artemis programme aims to establish a long-term, sustainable human presence on the Moon, understanding the lunar environment in all its extremes becomes a matter of astronaut safety, instrumental non-vulnerability, and mission success. The discovery confirms that the Moon is not a static, dead environment; it possesses 'weather', and that weather can be violent.

The temporarily inflated, denser exosphere created by a CME poses several quantifiable hazards for lunar explorers.

First, the intense solar wind can create a highly charged plasma environment near the surface.

This electrical charge is known to levitate the fine, abrasive lunar dust, regolith, which is notoriously hazardous, capable of damaging spacesuits and electronic equipment. A CME, by intensifying the plasma, could significantly worsen this problem, creating a thicker, more pervasive charged dust cloud. The resulting respiratory risk to astronauts, often likened to a potential "black lung disease" for lunar miners, is a serious concern that must be accounted for in habitat design and mission protocols.

Second, the violent sputtering process threatens to contaminate crucial scientific targets.

A primary goal of future missions is to explore and sample the deposits of priceless water ice hidden within the Permanently Shadowed Regions (PSRs) near the lunar poles. The intense blasting effect of a G5-class CME, as observed by CHACE-2, could potentially blast atoms from sunlit regions and transport them across the entire lunar surface. These displaced atoms could eventually settle in the pristine, ancient PSR craters, potentially contaminating the very ice that astronauts are sent to study.

Finally, this discovery solidifies the Moon’s role as a unique natural physics laboratory.

By lacking a strong magnetic field or a thick atmosphere to complicate the interaction, the Moon’s exosphere acts as a direct, integrated particle detector. Its dramatic 'puff up' in response to the CME provides a clean, quantitative measurement of the storm's intensity and composition. This new understanding will directly shape future mission planning, ensuring that astronauts and sensitive equipment are protected during the passage of the next solar superstorm.

A Universal Process

Beyond its immediate relevance to the Moon and the Artemis programme, the Chandrayaan-2 observation provides a vital benchmark for understanding space weathering across the inner solar system.

The same solar wind sputtering that temporarily inflated the Moon’s exosphere is a fundamental, constant process sculpting the surfaces of countless airless bodies, from the smallest asteroids to large planets.

The implications for Mercury are particularly striking. Orbiting far closer to the Sun, Mercury is subjected to a solar wind that is naturally an order of magnitude more intense than what the Moon experiences. Mercury also possesses a similarly tenuous exosphere and only a weak global magnetic field. The Chandrayaan-2 findings provide a quantitative glimpse into the extreme space weather Mercury must endure not just during a storm, but perhaps on a near-constant basis. The physical processes are universal; only the intensity and frequency change.

In sum, the Chandrayaan-2 orbiter, often overlooked, delivered the definitive empirical data: a solar superstorm violently inflates the Moon's ethereal atmosphere by an order of magnitude, confirming a decade-old theoretical model and providing critical intelligence for future human missions.

This finding, revealing the Moon as a dynamic space weather laboratory and offering a benchmark for airless worlds like Mercury, is a towering achievement of basic science.

This success follows a trajectory of decisive scientific contributions from India, from the Chandrayaan-1 discovery of lunar water to the precision landing of Chandrayaan-3 near the South Pole. These are the quiet, persistent achievements of an equal knowledge producer, built not upon the historical advantage of colonial surplus or the centralised dictate of a totalitarian state, but through the patient, democratic evolution of scientific institutions. In this sense, India's journey to the Moon represents a model for genuine global scientific progress, rooted in intellectual merit and independent capability.

Journal Reference: Dhanya, M. B., Yadav, C. M., Thampi, S. V., Das, T. P., Thampi, R. S., & Bhardwaj, A. (2025). Impact of a coronal mass ejection on the lunar exosphere as observed by CHACE-2 on the Chandrayaan-2 orbiter. Geophysical Research Letters, 52, e2025GL115737. Click here to read or download.