Mars’s Northern Ice Cap is Surprisingly Young

If you’ve ever looked at Mars through a telescope, you probably noticed its two polar ice caps. The northern one is made largely of water ice—the most obvious sign that Mars was once a wetter, warmer world. A team of researchers from the German Aerospace Center (DLR) used that ice cap to make surprising discoveries about it and what it tells us about Mars’s interior.

According to Adrien Broquet and a team of DLR planetary scientists, the northern polar cap on Mars is quite young. They found this out by applying techniques used to measure what ice sheets on Earth do to its surface. The effect that widespread glaciation has is called “glacial isostatic adjustment,” and it’s still happening in places such as Scandinavia. Essentially, it’s a constant movement of land as Earth’s surface deforms in response to the weight of ice. The rate of deformation depends on the specific characteristics of the underlying mantle.

Large areas of our planet have been covered at times by thick glacial sheets. The last time this occurred was during a glacial period that ended about 11,700 years ago. Those sheets “weighed down” the surface, compressing it. As the glaciers melted, the surface began to rise back up in a process called “isostatic rebound”. The rate of both depression and the subsequent rising motion tells something about Earth’s interior, particularly the mantle. Think of pushing down on a sponge and then watching as it expands when you take your hand away.

Studying a Rebounding Ice Cap

Broquet and his team decided to measure glacial isostatic rebound on Mars under the northern ice cap. It’s about 1,000 kilometers wide and three kilometers thick. They studied its formation by combining models of the planet’s thermal evolution with calculations of glacial isostatic adjustment, along with gravity, radar, and seismic observations.

The team concluded that the Martian northern polar cap is quite young, and it’s depressing the ground underneath. “We show that the ice sheet pushes the underlying ground into the mantle at a rate of up to 0.13 millimetres per year,” said Broquet. That’s a fairly small deformation, according to team member Ana-Catalina Plesa. “The small deformation rates indicate that the upper mantle of Mars is cold, highly viscous and much stiffer than Earth’s upper mantle,” she said.

Understanding Planetary Construction

So, how can measurements of ice weighing down planetary surfaces tell us so much? Remember that rocky planets like Earth and Mars are in constant states of change. Those changes can range from short-lived events like volcanic eruptions to long-lived ones like Ice Ages. Each alteration affects the surface, as does the rate at which the surface deforms and “bounces back”. Earth scientists use techniques such as the study of glacial isostatic adjustment to probe deep beneath the surface to understand the characteristics of those layers.

When ice weighs down the surface, the amount of depression depends on the mantle’s viscosity. That’s a measure of how much the mantle’s rocky materials resist flowing. Earth’s mantle rocks are more than a trillion times more viscous than asphalt. They still deform, however, and flow over geological timescales of millions of years. Using radar data and other methods to study the rate of depression and rebound of Earth’s surface, scientists can find the mantle viscosity. As it turns out, when you apply the same methods to Mars, it presents some surprises, including a seemingly cold north pole and the recently volcanically active equatorial regions.

Estimating Mars’s Interior

To understand why the Mars interior is the way it is, you need estimates of Mars’s gravity field (which varies), seismic measurements made by the InSight lander, and other data. They all help to determine rates of depression and rebound on the Red Planet’s surface and interior. The result? It appears that the surface under the Martian north pole has not had nearly enough time to fully deform under the weight of the ice. Broquet’s group estimates that Mars’s north pole surface area is currently subsiding at rates of up to 0.13 millimeters per year. For it to be that slow, the underlying upper mantle viscosity tells us that the Martian interior is quite cold.

The team’s measurements indicate the ice cap is young—well more than any other large-scale feature seen on the planet. It’s most likely to be between 2 and 12 million years.

Other places on the planet may not be quite so frigid as the polar regions. “Although the mantle underneath Mars’s north pole is estimated to be cold, our models are still able to predict the presence of local melt zones in the mantle near the equator,” said study co-author Doris Breuer.

These findings represent the first time that scientists found glacial isostatic adjustment operating on another rocky planet. Future missions to Mars could include more instruments to measure the rise and fall of the Martian surface in response to glaciation.

For More Information

Mars’s Northern Ice Cap is Young with a Cold, Stiff Mantle Beneath
Glacial Isostatic Adjustment Reveals Mars’s Interior Viscosity Structure