MOSCOW, November 4, Vladislav Strekopytov . Based on new data from NASA's InSight mission, scientists have built a refined model of the internal structure of Mars. This allows us to better understand how the Red Planet was formed and why at some point its surface became uninhabitable for life.
Magnetic protection mechanism
Life on Earth would not have arisen without a powerful magnetic field that protects against ionized particles from the solar wind and cosmic radiation. There is no such thing on Mars, and life in the form in which we understand it is now impossible there. To find out how it was in the past, researchers are trying to reconstruct the paleomagnetic history of the planet.
A necessary condition for a magnetic field is the presence of a core or other inner shell that conducts electric current, in which the movement of matter takes place. The Earth has such a shell — this is the outer core. It consists of liquid iron and nickel; convective mixing of the substance occurs in it, associated with the removal of heat from the solid inner core, where nuclear reactions occur.
In the 1970s, the Soviet Mars orbital series detected a small magnetic field around the Red Planet, about 500 times weaker than Earth's. At the same time, its local intensity varied by one and a half to two times, and the magnetic poles did not coincide with the physical ones. Such unevenness indicated that the iron core is motionless relative to the crust, and the magnetic field itself is most likely residual — the magnetization was partially preserved in the minerals from the times when the subsoil was more active.
The results obtained by NASA's automatic space station Mars Global Surveyor, which operated in Mars orbit in 1996-2006, confirmed this assumption — the station's instruments recorded residual magnetism in the oldest rocks of the Martian crust. This means that in the early stages of the existence of the Red Planet, an electromagnetic dynamo mechanism operated in its depths, and then stopped.
To explain why this happened, the following hypothesis was proposed. Initially, a large asteroid orbited Mars, causing tidal effects in the core. Then it dropped to the Roche limit and collapsed. This approximately happened 350 million years after the formation of the planet. Gradually the core cooled down and the magnetic field faded. The remaining Martian satellites were too small to support gravitational disturbances in the interior of the planet.
For a long time, the “asteroid hypothesis” was accepted by almost all researchers, until new data from the NASA InSight mission appeared.
The mystery of the Martian core
InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) is the first robotic module in the history of astronautics, created specifically to study the geology of another planet. Its main instrument is the SEIS seismometer, which records seismic waves that occur during tectonic movements or meteorite impacts. At the boundaries of rock layers with different densities, the speed of these waves changes, some of them are reflected and return to the surface. Based on their parameters, you can get an idea of the structure of the subsoil.
The InSight Mars Lander reached the surface of the Red Planet in November 2018. Over four years of operation, it recorded hundreds of seismic signals. Most of them turned out to be associated with surface tremors, the waves from which propagated only in the Martian crust. But there were also deeper ones, from which geophysicists determined that in the center of the planet there is a liquid core with a radius of about 1830 kilometers — about 30 percent larger than scientists expected.
It has been suggested that the core of Mars, in addition to iron and nickel, contains a large amount of light chemical elements such as sulfur, oxygen, carbon and hydrogen. These elements do combine well with heavy liquid metals, but the problem is that, according to the generally accepted theory of planet formation, in the process of differentiation of the primary protoplanetary matter, all of them should have passed from the core to the outer shells.
< br />
New model of Mars
The InSight mission ended almost a year ago, but experts are still processing the data received. Recently, an event associated with the fall of a large meteorite was discovered in the instrument records of December 2021. The results of the analysis of seismic waves caused by this impact forced us to reconsider the internal structure of Mars. Two independent groups of scientists, including Russian experts, took part in the work.
According to new data, the Martian mantle is heterogeneous — in its lower part, at the border with the core, there is a layer of molten silicates (rock-forming minerals that make up the crust and mantle of Mars and Earth) about 150 kilometers thick. Before this, it was mistakenly included in the core. The radius of the core itself turned out to be smaller than previously thought — 1650-1675 kilometers, and the density was five to eight percent higher (about 6.5 g/cm3). This corresponds to a chemical composition in which 80-90 percent is iron and nickel, and the rest is light elements.
The Earth's core is even heavier. According to the authors, this indirectly indicates that Mars formed a little earlier than the Earth, when there were more light elements in the gas-dust cloud surrounding the Sun.
The new structural model fits well with geophysical observations (for example, how Mars responds to deformation under the gravitational pull of its moon Phobos), ideas about the differentiation of primordial matter, computer simulations and laboratory experiments at high pressure.
The researchers explain the inaccuracy of the initial calculations by the fact that all previous seismic events on which the analysis was based occurred on the same side of Mars where the InSight lander is located. As a result, the seismometer recorded only waves reflected from the boundary of the liquid layer in the mantle, which do not provide any information about the deepest depths. The impact of a large meteorite on the opposite side created waves that traveled through the entire planet.
Blocking layer
Mars is the first cosmic body, besides the Earth and the Moon, where scientists were able to conduct direct seismic sounding. Therefore, it is too early to talk about how typical or anomalous the structure of its interior is. It is only known that in the earth’s mantle, at its border with the core, there is no molten layer.
Researchers speculate that it could be left over from an ocean of magma that once covered Mars. As the magma cooled and turned into solid rock, the heated layer, enriched with radioactive elements, sank lower and lower into the base of the mantle. Eventually, the metal core, according to one of the researchers, University of Maryland geology professor Vedran Lekic, was wrapped in an “insulating blanket” that prevented it from cooling. The convective flows needed to maintain the «thermal dynamo» stopped, and the planet lost the potential for the development of life.
Incidentally, the authors say that the magnetic field that existed in the early stages of the geological history of Mars is not necessarily related to the internal dynamics of the planet's interior . It could also have been created by external sources, such as asteroid impacts. And the movements in the core may have been caused by gravitational interactions with ancient satellites that have since disappeared.