New observations from NASA’s Mars Reconnaissance Orbiter indicate that the crust and upper mantle of Mars is stiffer and colder than previously thought, implying that liquid water might be located at greater depths than originally suspected.
The discovery was made using the Shallow Radar instrument on the spacecraft, which has provided extraordinarily detailed pictures of the interior layers of ice, sand and dust that make up the north polar cap on Mars. The images also reveal a smooth, flat border between the ice cap and the rocky Martian crust; the weight of a similar stack of ice on Earth would cause the planet's surface to sag. The fact that the Martian surface is not bending means that its strong outer shell, the lithosphere, which comprises the crust and upper mantle, must be very thick and cold.
"The ability of the radar to see through the ice cap and determine that there is no bending of the lithosphere gives us a good idea of present day temperatures inside Mars for the first time,” says Suzanne Smrekar, deputy project scientist for Mars Reconnaissance Orbiter.
The radar image (top) reveals the layers of ice, sand and dust that make up the north polar ice cap on Mars and shows that the boundary between the ice layers and the surface of Mars is relatively flat (bottom white line on the right). The coloured map below shows the topography of the corresponding terrain (red and white are high ground, green and yellow are lower ground). Image: NASA/JPL-Caltech/University of Rome/SwRI.
Temperatures in the outer portion of a rocky planet like Mars increase with depth toward the interior, and the thicker the lithosphere, the more gradually the temperatures increase. Therefore, the discovery of a thicker Martian lithosphere means that any potential aquifers have to be deeper than previously calculated where temperatures are warmer, and by default, so would any possible organisms living in that water.
A view of the north polar ice cap on Mars taken by the Mars Reconnaissance Orbiter. The ice cap is about 2 kilometres thick. Image: NASA/JPL/Malin Space Science Systems.
The discovery of alternating layers of material also allow scientists to trace the climatic history of Mars. The radar pictures reveal four zones of finely spaced layers of ice and dust separated by thick layers of nearly pure ice. Scientists think this pattern of thick, ice-free layers represents cycles of climate change on Mars on a time scale of roughly one million years. Such climate changes are caused by variations in the tilt of the planet's rotational axis and in the eccentricity of its orbit around the Sun. The observations support the idea that the north polar ice cap is geologically active and relatively young, at about 4 million years.
With baited breath scientists await the arrival of the Phoenix Lander, which is scheduled to touch down in proximity to the north polar cap on 25 May, where it will examine the history of water and habitability potential in the Martian arctic’s ice-rich soil.