A group of researchers from TU Delft and the Institute for Planetary Research of the German Aerospace Center have been able to the determinate the Mars’s interior structure and the age of the large ice sheet covering its north pole. They have published their findings in Nature. The team used the same geophysical trick on Mars that geologists use here on Earth to: measure how large areas of land are deformed by the ice sheet that presses down on the planet itself. That in turn says a lot about the material and structure underneath all that ice.
Bouncing back from ice
Did you know that Scandinavia rises up out of the sea at rates of a few millimetres per year? This is because of the so-called glacial isostatic adjustment. This part of Earth was covered by kilometres of ice during the last ice age around 20,000 years ago, and the thick ice blocks once deformed and bended the surface of the Earth. Following deglaciation, the surface slowly rebounded up and rose. Measuring this, geologists have been able to determine what’s underneath: the viscosity of Earth’s mantle and its effects on gravity. Similar processes on other planets have remained undocumented – until now.
ESA missions lead the way
In 2003 and 2005, two spacecrafts from ESA (Mars Express) and NASA (Mars Reconnaissance Orbiter) were sent to Mars equipped with radar sounders. These sounders mapped Mars’ north polar cap and allowed to image the shape of the interface between ice and bedrock. The north pole of Mars is covered by an ice sheet, which consists mainly of pure water ice, and that has a diameter of 1,000 kilometres and a thickness of around three kilometres. Surprisingly, while large ice-sheets were able to substantially deform the surface of the Earth 20,000 years ago, the surface of Mars appeared undeformed by such a large mass of ice.
Why the surface of Mars has remained so stiff and undeformed has been unclear for decades. “The formation of ice on Mars differs from earth due to the difference in orbit around the Sun,” states Bart Root, assistant-professor Planetary Exploration at TU Delft and member of the research team. “There are of course also differences in temperature and pressure, which are both lower on Mars than on Earth. For instance, we know that ice on Mars consists largely of CO2.”
Below the surface of Mars
Combining radar data with estimates of Mars’ time-variable gravity field together with measurements collected by the InSight seismometer, the team led by dr. Adrien Broquet can reveal that the key to that conundrum is time. The interior of Mars is so viscous and cold that the surface has not had time to fully deform today. Using numerical simulations, the group estimates that Mars’ north pole is currently subsiding down, at rates of at most 0.13 millimeters per year.
This requires the viscosity of Mars’ mantle to be ten to hundred times greater than on Earth and indicate that the interior of the red planet is extremely cold today. “Although Mars’ north pole is estimated to be cold, our models are still able to predict the presence of local melt zones in the crust and close to the equator” comments Dr. Breuer, co-author of the study. The interior of Mars is full of surprises: with a seemingly cold north pole compared to the volcanically active equatorial regions, as monitored by orbiters and the InSight lander.
Young ice cap
The ice sheet covering Mars’ north pole must be substantially younger than anything else of that scale that is seen on Mars. With an estimated age of 2 to 12 million years, the ice sheet covering Mars’ north pole is now a good candidate to represent the youngest and largest feature we see on the red planet.
The work of Broquet, Breuer, Root and colleagues is the first to document glacial isostatic adjustment on another planet and has profound implications for our understanding of Mars’ interior and geologic evolution. In recent years, multiple GRACE-like gravity missions have been proposed to Mars, such as Oracle and MaQuls. In addition to learn about the climate of the planet, future gravity missions will now have a new goal: providing new measurements of Mars’ rise and fall. Root: “I’m especially excited about the concept of MaQuls, where we’ll use quantum technology to map Mars’ gravity field much more accurately. But what’s even more important, is that quantum technology will enable us to look at changes in the gravity field. And this will kickstart a whole new wave of research, because we can then look at the more dynamic processes underneath Mars’ surface.”
Source: TU Delft.
Image credit: NASA/JPL-Caltech/MSSS.
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