Hidden Compartments: The Deep Secrets Revealed by the Mars Gravity Field
Source PublicationNature
Primary AuthorsBerne, Wagner, Matsuyama et al.
"Imagine trying to guess what is inside a wrapped birthday present without opening it. If you shake it and it wobbles like jelly, you know it is soft inside. If it barely moves, it is solid. Similarly, the Sun's gravity 'shakes' Mars. By measuring the Mars gravity field, scientists can tell if the inside is stiff and cold or soft and warm, bringing the planet's hidden internal compartments into focus."
It begins in total silence, millions of miles away. A planetary body harbours secrets deep beneath its crust, locked away under the cover of darkness. For billions of years, the internal structure of Mars has remained a quiet mystery, lingering in the void without sounding a single alarm. The planet does not reveal its inner workings in plain sight. Instead, the clues are buried deep within its rocky mantle. The red planet hides its past inside the very rock designed to keep its history concealed. It creates hidden compartments, turning a seemingly dead world into a secret harbour for ancient heat.
The planet's exterior is a masterclass in stealth. Looking at the surface, one might feel completely certain it is frozen, unaware that a thermal anomaly is ticking near its core. For aeons, the Martian mantle has waited in the shadows. The stakes of understanding this quiet world are incredibly high, as unlocking its past could explain how entire planets live and die. Finding these hidden compartments has always been a massive challenge for planetary scientists. The deep interior is too remote, too well-hidden, and too quiet.
To uncover such a stealthy mystery, science needs a hero: a human endeavour capable of seeing what is buried deep beneath the surface. Surprisingly, the method for mapping these invisible interiors relies on the delicate gravitational dance of the solar system.
Tracking the Mars gravity field
Planetary scientists face a monumental problem. They cannot slice a planet open to see its hidden compartments. Instead, they must measure how a planet reacts to stress. Recently, researchers mapped the Mars gravity field using tracking data from three orbiting spacecraft: the Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter.
Mars has a slightly squashed orbit and a tilted axis. As it moves around the Sun, solar gravity pulls on the planet. This pull creates a seasonal tidal effect, gently stretching and squeezing Mars. By tracking the spacecraft, scientists measured how the planet's gravity changed over time in response to this squeezing.
They expected Mars to act like a perfectly even, symmetrical sphere. But the data revealed a brilliant plot twist. The gravity measurements differed by up to 300 per cent from their standard models. The rock inside Mars is not uniform at all.
A hidden thermal anomaly
The spacecraft data measured changes in the planet's shape. This suggests that the stiffness of the Martian mantle varies by more than 20 per cent. This varying stiffness forms a north-south pattern that perfectly lines up with the surface boundary between the smooth northern plains and the cratered southern highlands.
Because softer rock is usually warmer, researchers infer that a massive thermal anomaly hides deep underground. A hidden compartment of heat, roughly 200 to 400 degrees Celsius warmer than expected, sits in the present-day mantle beneath the southern highlands.
What does this mean for the red planet? Based on these orbital tracking models, the study suggests this temperature difference could reflect regional, slow-churning rock deep within the mantle. Alternatively, the thick crust of the southern highlands might be acting like a heavy blanket, trapping heat for billions of years. Unravelling the ancient heat of a distant planet is a testament to human ingenuity. With the right tools, we can finally bring the invisible into the light.