An international group of geophysicists studied the appearance of the Earth’s inner core to find out what type of thermal convection it has. During the research, scientists discovered the hottest and coldest spots in the center of the planet.

The modern idea that the Earth’s core is a heterogeneous object consisting of inner (solid) and outer (molten) layers was formulated by Danish physicist Inge Lehmann in 1936. Over time, methods for studying the interior of our planet have improved and understanding has expanded. In particular, the Earth’s inner core plays a passive but important role in mixing the outer core and, as a result, maintaining the stability of the magnetic field.

But for objective reasons: the inner core is located at a depth of more than five thousand kilometers, not everyone knows the center of the Earth. For example, the melting point of iron, the main component of the inner core, at pressures at this depth varies between 5000 Kelvin (4726.85°C) and 7000 Kelvin (6726.85°C), depending on impurities and conditions. Additionally, the thermal conductivity of the core remains unclear.

A group of geophysicists from Australia, China and Europe decided to find out what type of thermoconvection is characteristic of the Earth’s inner core. For this, experts collected data on the time and length of PKIKP and PKiKP waves (they pass through the center of the planet, only the first penetrates the border of the inner core and appears earlier on seismograms, and the second is reflected from it and appears later). After calculating the time difference of two waves (5,477 in total), scientists created a three-dimensional model of the upper layer of the Earth’s inner core, complete with temperature signatures. The results of the scientific study were published in the journal Scientific Reports.

The resulting tomogram of the outer layer of the inner core with a depth of 100 km revealed various features. Compression waves (labeled P) traveled faster through the Eastern Hemisphere than through the Western Hemisphere; however, there was a ridge in this region from the mid-Atlantic to Central America where P waves moved rapidly. By measuring the decay temperature of waves, researchers found the coldest region beneath the northern part of the Atlantic Ocean; The surface temperature of the inner core is 200 Kelvin below average.

The hottest spots were located at the edges of the cold one; In the northern part of South America, where temperatures were 600 Kelvin warmer than average, and in South Asia, a large area from the Mediterranean to eastern Australasia was 300 Kelvin warmer. Based on these data, the authors suggested that iron crystals in the inner core are probably oriented radially away from hot spots along the surface. These anisotropic features indicate a complex thermal convection device at the Earth’s center: there are both internal, localized heat sources and non-localized ones.

The authors explained the peculiar anisotropic orientation of the iron crystals by the fact that perhaps the entire surface of the inner core was subjected to convection in the past. As a result, a stable structure emerged, and when the core cooled, convection in the core stopped and the crystals froze in the direction detected by the seismograph sensors. Meanwhile, the outer shell continues to participate in heat exchange.