Paper of the month:  Neutrino tomography of the Earth

The knowledge about the structure of the Earth comes mainly from seismological data [3, 4]. Although there are millions of earthquakes every year and the study of the propagation of seismic waves is a well established method to measure the density of the Earth, its determination is a non trivial problem, being especially difficult the determination of the core’s density, that would benefit from complementary information. Given the relevance in understanding the Earth’s internal dynamics and evolution, using a completely different technique to infer the Earth’s density distribution is important. The method used in [1] is based on the study of the atmospheric neutrinos that travel through the Earth.

 

An isotropic flux of high energetic atmospheric neutrinos is produced when cosmic rays, high energetic particles coming from outer space, interact with the atmosphere. Although neutrinos interact very rarely, for energies around and above a few TeV the interaction rate of neutrinos with nucleons is large, being the Earth partially opaque for them. The more energetic is the neutrino and the bigger the number of nucleons along its path through the Earth, the higher the probability of undergoing inelastic scattering and get absorbed. That will imprint a dependence in the observed flux with both the energy and zenith angle with respect to the detector, see Fig. 1. By studying this attenuation it is possible to do a “radiography” of the Earth.

 

This work is based on a one-year muon sample of the IceCube telecospe [2]. IceCube is a km3 neutrino detector based in the South Pole capable of measuring the Cherenkov light emitted by muons traveling through it. These muons are produced when neutrinos, after traversing the Earth, interact with the ice surrounding the detector. The energy and direction of the incoming neutrinos can be extracted from the direction and energy deposited by the muons. The density profile of the Earth obtained using this data is shown in Fig. 2.

 

The fascinating part of this method is that they are able to determine the mass of the Earth and its moment of inertia using weak interactions only, independent from gravitational measurements. After parameterizing the Earth’s density profile, the mass of the Earth as weighed by neutrinos is given by M^ν_⊕ = (6.0+1.6 −1.3)×10²⁴ kg, while the gravitational measurement up to date is M^grav _⊕ = (5.9722 ± 0.0006) × 10²⁴ kg. The resulting moment of inertia I^ν_⊕ = (6.9±2.4)×10³⁷ kg m² is also in agreement with I^grav_⊕ = (8.01736±0.00097)×10³⁷ kg m².The mass of the Earth’s core has been estimated to be M^ν_core = (2.72+0.97 −0.89)×10²⁴ Kg which is a bit bigger than the result from geophysical density models, which estimate it to be ∼ 33% of the total Earth mass. Using neutrinos they are also able to confirm that the core is denser than the mantle, which is necessary for the Earth to be gravitationally stable (please see pdf attached).