If you\u2019ve ever looked at Mars through a telescope, you probably noticed its two polar ice caps. The northern one is made largely of water ice\u2014the 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\u2019s interior.<\/p>\n
<\/p>\n 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 \u201cglacial isostatic adjustment,\u201d and it\u2019s still happening in places such as Scandinavia. Essentially, it\u2019s a constant movement of land as Earth\u2019s surface deforms in response to the weight of ice. The rate of deformation depends on the specific characteristics of the underlying mantle.<\/p>\n 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 \u201cweighed down\u201d the surface, compressing it. As the glaciers melted, the surface began to rise back up in a process called \u201cisostatic rebound\u201d. The rate of both depression and the subsequent rising motion tells something about Earth\u2019s interior, particularly the mantle. Think of pushing down on a sponge and then watching as it expands when you take your hand away.<\/p>\n Broquet and his team decided to measure glacial isostatic rebound on Mars under the northern ice cap. It\u2019s about 1,000 kilometers wide and three kilometers thick. They studied its formation by combining models of the planet\u2019s thermal evolution with calculations of glacial isostatic adjustment, along with gravity, radar, and seismic observations.<\/p>\n The team concluded that the Martian northern polar cap is quite young, and it\u2019s depressing the ground underneath. \u201cWe show that the ice sheet pushes the underlying ground into the mantle at a rate of up to 0.13 millimetres per year,\u201d said Broquet. That\u2019s a fairly small deformation, according to team member Ana-Catalina Plesa. \u201cThe small deformation rates indicate that the upper mantle of Mars is cold, highly viscous and much stiffer than Earth\u2019s upper mantle,\u201d she said.<\/p>\n 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 \u201cbounces back\u201d. 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.<\/p>\n When ice weighs down the surface, the amount of depression depends on the mantle\u2019s viscosity. That\u2019s a measure of how much the mantle\u2019s rocky materials resist flowing. Earth\u2019s 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\u2019s 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.<\/p>\n To understand why the Mars interior is the way it is, you need estimates of Mars\u2019s 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\u2019s 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\u2019s group estimates that Mars\u2019s 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.<\/p>\n The team\u2019s measurements indicate the ice cap is young\u2014well more than any other large-scale feature seen on the planet. It\u2019s most likely to be between 2 and 12 million years. <\/p>\n Other places on the planet may not be quite so frigid as the polar regions. \u201cAlthough the mantle underneath Mars\u2019s 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,\u201d said study co-author Doris Breuer.<\/p>\n 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.<\/p>\n Mars\u2019s Northern Ice Cap is Young with a Cold, Stiff Mantle BeneathStudying a Rebounding Ice Cap<\/h3>\n
Understanding Planetary Construction<\/h3>\n
Estimating Mars\u2019s Interior<\/h3>\n
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Glacial Isostatic Adjustment Reveals Mars\u2019s Interior Viscosity Structure<\/p>\nLike this:<\/h3>\n