NASA’s InSight has spent several years on Mars with an (almost literal) ear on the ground. Its sophisticated seismometer has been recording Marsquakes, which have been used by scientists to get the first direct measurement of the interior of a planet that is not Earth.
The work is reported in three different papers in Science (here, here, and here) that focused on the crust, mantle, and core of the planet respectively. The data currently limits scientists from having a complete understanding of the Red Planet, but the work provides crucial constraints on what goes on beneath the dusty rust-colored soil. These papers expand not only our understanding of what we think Mars is like today, but also how it used to be, and even when it formed.
“We can now constrain the crustal thickness at the InSight landing site with seismological measurements for the first time, and – together with the previous information on gravity and topography – map the thickness of the crust across all of Mars. This is something that researchers have been waiting for for decades,” one of the lead authors, Dr Brigitte Knapmeyer-Endrun, from the University of Cologne, told IFLScience.
The team highlights how the current data supports two models. Either the crust is thin and made of two layers – a porous top layer that might be chemically altered and a deeper layer closer to the original composition – or it could be a three-layer affair. In the first scenario, the crust underneath InSight would be much thinner than expected, around 20 kilometers (12 miles), and the latter would almost double that.
Globally, the crust is expected to be on average between 24 and 72 kilometers (15 to 45 miles) and it might be enriched in radioactive elements that heat this region more, at the expense of the interior of the planet. The team also reports a very thick lithosphere – the upper layer of the planet – of about 500 kilometers (310 miles), twice that of our planet. A possible explanation of why Mars unlike Earth (and maybe marginally Venus) might never have had plate tectonics.
Deeper inside the planet we have the mantle. On Mars, this is made of a single layer, unlike the two of our planet. Low frequency marsquakes allowed for a probe of its properties down to 800 kilometers (500 miles) – over half of its thickness. The composition of the mantle is similar to Earth’s own, being rich in olivine, but up to a point. Structures found in the deeper mantle are absent on Mars because the planet doesn’t reach the pressures we observe here – and the cause is found in the core.
InSight’s data has revealed that the core is much larger than previously thought, with a radius of 1,830 kilometers (1,137 miles). The research also confirms that the core is liquid, something that has been suspected for a while but without certainty. The data also suggests a surprising composition for the innermost region of Mars. It turns out that it’s less massive than expected, suggesting that within the molten iron and nickel, there are also lighter elements that bring the density down.
“That is something that puts a lot of constraint on the formation of Mars. For Mars to have accumulated this large amount of light elements like sulfur, carbon, oxygen, and hydrogen in its core, it must have formed very early. Maybe even as early as when the solar nebula was still around. This is not the case for Earth which formed later,” another of the paper lead author Dr Amir Khan, from ETH Zurich, told IFLScience.
It is unclear if Mars has a smaller solid inner core, but finding this out is on the agenda. Actually, there’s a lot on the agenda for the InSight extended mission, which is expected to end in December 2022. Improving the constraints produced by these papers is key, but there are many mysteries left to solve. This includes answering the question: what is the cause of the strong quakes detected from Cerberus Fossae?
“We are hoping for additional marsquakes that are different from previous ones to provide the missing information, but we are also using different analysis methods on the available data,” Dr Knapmeyer-Endrun told IFLScience.
Marsquakes further away would lead to better constraints on the deeper mantle. Stronger events too will allow the InSight science team to improve the new model of the inside of Mars. Most of the quakes have been of such low magnitude that we wouldn’t feel them on Earth.
“We have the gross structure of Mars now right, but we still have a lack of details,” Dr Khan told IFLScience. “The next step is to try and understand what these things mean for the formation of Mars and how it went from there on to evolved to what we see today.”