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.
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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.
References:
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