Our knowledge of the composition and structure of the Earth’s interior through direct observation and sampling is limited to the top few kilometres of the Earth’s crust. The deepest hole ever drilled into the Earth’s crust is the Kola Superdeep Borehole on the Kola Peninsula in Russia. It was part of a scientific drilling project that reached a maximum depth of 12,262 m in 1989. Drilling deep into the Earth is extremely challenging due to the increasing temperature and pressure. In fact, almost everything that is known about the Earth’s interior comes from the study of seismic waves propagating through the Earth.
In a series of two papers - “Probing of the Interior Layers of the Earth with Self-Sinking Capsules” (2005) and “Exploring the Earth’s Crust and Mantle Using Self-Descending, Radiation-Heated, Probes and Acoustic Emission Monitoring” (2008), the authors propose a novel method of exploring the Earth’s interior down to a depth of more than 100 km. They show that a spherical probe in the form of a tungsten capsule filled with radioactive cobalt-60 can produce sufficient heat to melt its way into the Earth.
The temperature required to melt rocks are in excess of 1000°C and the probe needs to withstand temperatures in excess of 2000°C. Also, the probe needs to be dense so that it is heavier than the surrounding rock material and will therefore sink. As a result, tungsten is an excellent material for the capsule because of its high density and high melting point of 3422°C. Furthermore, tungsten is a relatively inexpensive material and it has a low corrosion rate at elevated temperatures. The tungsten capsule serves to contain the radioactive cobalt-60 and to conduct heat produced from radioactive decay to the probe’s outer surface.
A 30 cm diameter sphere of radioactive cobalt-60 serves as the heat source within the tungsten capsule. Although cobalt melts at 1495°C at one atmosphere of pressure and despite an increase in melting temperature due to higher pressures inside the Earth, cobalt is still expected to turn molten at some of the temperatures envisaged within the probe. An issue with molten cobalt is that it may react and alloy with tungsten, possibly reducing the life of the tungsten capsule. One way to solve that is to coat the internal surface of the tungsten capsule with graphite or a thin layer of refractory ceramics to isolate the cobalt from the tungsten.
Heat generated from the decay of radioactive cobalt-60 allows the probe to melt its way into the Earth. The probe is estimated to melt down to a depth of 20 km in ~1 year. As the probe descents deeper, the rate of descent will gradually slow until the probe reaches a depth of 100 km after ~30 years. By melting its way into the Earth, the probe will leave behind a wake of molten material. Subsequent re-crystallisation of the molten material will generate intense acoustic signals. In addition, the probe can be made to generate its own acoustic signals by using a simple thermo-mechanical generator to transform some of the energy from radioactive decay into mechanical oscillations.
Suitably placed detectors on the Earth’s surface will continuously monitor the acoustic signals from the self-sinking probe. Analysing these acoustic signals will provide information on the composition and structure of the deep layers in the Earth. A self-sinking probe is basically a dumb probe measuring less than 100 cm in diameter - a lump of nuclear waste encapsulated in a tungsten sphere and sunk into the ground. Besides the Earth, self-sinking probes can also be used to study the deep layers of other rocky worlds in our Solar System. Such worlds include Mercury, Venus, the Moon, Mars and Jupiter’s moon Io. This will provide unique and interesting opportunities for comparative planetology.
1. M. I. Ozhovan, F. Gibb, P. P. Poluéktov and E. P. Emets (August 2005), “Probing of the Interior Layers of the Earth with Self-Sinking Capsules”, Atomic Energy 99 (2): 556-562
2. M. I. Ozhovan and F. Gibb (2008), “Exploring the Earth’s Crust and Mantle Using Self-Descending, Radiation-Heated, Probes and Acoustic Emission Monitoring”, Nuclear Waste Research: Siting, Technology and Treatment, Edited by: Arnold P. Lattefer, pp. 207-220