The laws of physics do not care about size. The same equations that describe a drop falling from a leaf also describe a thunderstorm sweeping across a continent. Scale changes. The physics does not. That principle, known as scale invariance, has found a spectacular new demonstration 225 million kilometers (140 million miles) from home, courtesy of NASA’s late, great MAVEN orbiter.
In a study published this month in Nature Communications, a team led by Shaosui Xu of the UC Berkeley Space Sciences Laboratory reports that the same magnetic engine responsible for Earth’s auroras, the legendary “Dungey cycle” also operates on Mars. It does so not at planetary scale, as on Earth, but in dozens of miniature arenas scattered across the Martian Southern Hemisphere, each the size of a small country rather than an entire world.
The discovery rewrites what scientists thought they knew about Martian auroras. It also suggests the Dungey cycle may be a universal phenomenon wherever a stellar wind meets a magnetic field, regardless of whether that field envelops a whole planet or just a patch of ancient crust.
The Dungey Cycle at Full Scale
To understand what MAVEN found, it helps to understand how Earth makes its own light shows. The Dungey cycle, named after British physicist James Dungey who theorized it in 1961, describes the interaction between the solar wind, a stream of charged particles constantly blowing off the sun, and a planet’s magnetic field.
On Earth, the solar wind compresses the magnetic field on the dayside, stretching it into a long magnetotail on the nightside. Magnetic field lines on opposite sides of that tail snap together and reconnect. This injects energy into the magnetotail and accelerates electrons that spiral down into the upper atmosphere, where they collide with oxygen and nitrogen and produce the aurora borealis and australis.
Until MAVEN’s final act, this was a process believed to be essentially Earth’s exclusive domain among the terrestrial planets.
Mars and the Ghost of a Magnetic Field
Mars today has no global magnetic field. Its liquid iron core cooled and solidified billions of years ago, shutting down the internal dynamo that once generated a protective magnetosphere. But that dynamo left behind a ghost.
About 4 billion years ago, while the dynamo was still active, it magnetized large swaths of the Martian crust. When the dynamo stopped, those regions remained magnetically imprinted, like iron filings frozen after the magnet was removed. The result is a patchwork of crustal magnetic anomalies, primarily in the Southern Hemisphere, that produce weak but measurable magnetic fields reaching up into space.
“The Martian crustal fields create localized what you might call mini-magnetospheres,” Xu explained. “They are not connected to each other. Each one is its own little bubble of magnetic protection, rising maybe a few hundred kilometers above the surface.”
For years, planetary scientists knew these mini-magnetospheres could generate auroras. The Hubble Space Telescope and Mars Express both detected ultraviolet auroral glows coming from the Southern Highlands. But the mechanism behind those glows remained uncertain. Reconnection was one candidate, but no one had been able to show it followed the Dungey cycle in particular.
“We knew that magnetic reconnection was happening at Mars but did not expect it to be like the Dungey cycle,” Xu said.
MAVEN’s Final Gift
The answer came from a spacecraft that had already outlived its expected lifespan by years. MAVEN, the Mars Atmosphere and Volatile Evolution orbiter, arrived at Mars in September 2014. Its primary mission was to study how the Martian atmosphere escapes into space, helping scientists understand how Mars transformed from a warm, wet world into the cold desert it is today.
MAVEN carried a suite of instruments designed for exactly this kind of work: a fluxgate magnetometer to measure magnetic fields, a Solar Wind Electron Analyzer to track incoming electrons, and the Suprathermal and Thermal Ion Composition instrument, known as STATIC, to measure the energy and composition of ions. Together, these instruments gave Xu’s team everything needed to diagnose the reconnection process in action.
By analyzing data from multiple MAVEN passes over the Southern Hemisphere crustal fields, the team observed the signature of Dungey-like reconnection unfolding in miniature. Solar wind magnetic field lines approached Mars, encountered the localized crustal fields, and reconnected with them. The reconnection injected energy into mini-magnetotails downstream of each crustal patch, accelerating electrons that rained down into the thin Martian atmosphere, producing local auroras.
The entire process mirrors Earth’s Dungey cycle almost exactly, but compressed in scale by three orders of magnitude. Where Earth’s magnetotail stretches hundreds of thousands of kilometers behind the planet, a mini-magnetotail behind a crustal field region on Mars might extend just a few hundred kilometers. Where Earth’s auroral ovals span entire continents, Mars’ Dungey-cycle auroras are localized glows that would appear, to a hypothetical Martian skywatcher, as isolated patches of light in the Southern Hemisphere sky.
A Universal Mechanism
The finding carries implications far beyond Mars. If the Dungey cycle operates at subplanetary scales over crustal magnetic fields, then it likely operates wherever a stellar wind meets a magnetic obstacle of any size. That includes moons like Ganymede, which has its own intrinsic magnetic field. It may also include exoplanetary systems, where interactions between stellar winds and planetary magnetic fields are difficult to observe directly but may now be more predictable.
“That this process runs at such different scales tells us something fundamental about how magnetic reconnection works,” Xu said. “It is not a planetary phenomenon. It is a plasma phenomenon that happens wherever the conditions are right.”
The universality of the Dungey cycle also helps explain a puzzle that has dogged Mars researchers for years. Spacecraft had detected auroras at Mars before, but the timing and location were not always consistent with simple particle precipitation models. The new results suggest multiple auroral mechanisms are at work on Mars simultaneously. Some auroras come from direct electron impact. Others, following the Dungey cycle, come from reconnection-driven acceleration, and their timing depends on the local orientation of the crustal field and the solar wind, a combination that changes constantly.
MAVEN’s Legacy
The paper in Nature Communications is one of the last major discoveries from MAVEN. The orbiter lost contact with Earth in December 2025, after more than 11 years in orbit, nearly three times its planned mission duration. NASA declared the mission concluded in June 2026. But the data MAVEN collected over that decade-plus continues to yield new science.
“This is a remarkable result that changes how we think of Martian auroras and is another important step toward understanding why Mars and Earth have evolved so differently despite being governed by the same underlying physics,” said Shannon Curry, MAVEN Principal Investigator at the University of Colorado Boulder.
That evolution question is the deeper backdrop to everything MAVEN did. Earth and Mars started with similar ingredients: both formed in the inner solar system, both had liquid water early in their history, and both were subject to the same solar wind. But Earth retained its magnetic field, atmosphere, and oceans, while Mars lost all three. The discovery adds a new piece to that puzzle, showing that even a partial, localized magnetic field is enough to engage with the solar wind in the same fundamental way a full planetary field does. The difference between the two worlds is not one of kind, but of degree.
The Pattern in the Noise
In physics, scale invariance often points toward a deeper truth. When the same equations govern phenomena at wildly different sizes, it usually means those equations capture something essential. The Dungey cycle operating at planetary scale on Earth and at mini-magnetosphere scale on Mars suggests that magnetic reconnection is one of those essential processes, as fundamental to the interactions between stars and planets as gravity is to the motions of those bodies themselves.
The Martians would not see curtains of green and red light dancing across their sky. Their atmosphere is too thin and too different in composition. But if anyone were standing in the Southern Highlands on a clear Martian night, looking up at the right moment, they would see a glow, a localized, evolving patch of ultraviolet light, driven by the same invisible engine that lights up the skies of Earth.
Scale changes. The physics does not. MAVEN spent 11 years proving that point, and with this final discovery, it has left planetary science a richer field than it found.