Friday, August 1, 2014

Compressing Diamond to Unprecedented Densities

Thousands of exoplanets have been discovered to date. The discoveries show that exoplanets are far more diverse than originally predicted. Knowledge of the behaviour of matter under extreme pressures is important for understanding the interiors of giant planets like Jupiter and other exoplanets such as super-Earths (i.e. exoplanets between 1 to 10 Earth-masses). Using the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) in California, a team has succeeded in crushing diamonds to pressures of up to 5 terapascals, about 50 million times the atmospheric pressure on Earth’s surface. By doing so, the team managed to replicate the extreme conditions found deep within giant planets and carbon-rich super-Earths.

Figure 1: Artist’s impression of an exoplanet.

The technique used by the team to compress carbon (i.e. in the form of diamond) is known as dynamic ramped compression. In the experiment, 176 laser beams with a total peak power of 2.2 terawatts were used to “put the squeeze” on carbon. For each run, a piece of diamond is placed in a small gold cylinder measuring 6 mm in diameter and 11 mm in length. The laser pulses were timed to a precision of 0.02 nanoseconds and focused to strike the interior wall of the cylinder. This caused the gold to produce a burst of X-rays which bombarded and ramp-compressed the diamond. As the pressure increased, properties of the diamond such as density, stress and speed of sound were measured.

From start to finish, each run lasted only about 20 nanoseconds. Still, the compression was slow enough that the diamond remained solid and did not melt. Within that minuscule period of time, the diamond was squeezed to pressures of up to 5 terapascals. Diamond, the least compressible material known, was squeezed to an unprecedented density of 12 g/cm³. As a side note, the normal density of diamond is 3.5 g/cm³. The experiment provided the first ever actual data on diamonds at such high pressures. This data can be used to improve interior models of giant planets and carbon-rich super-Earths. In fact, the peak pressure attained in this experiment is slightly higher than the pressure at the centre of the planet Saturn.

Figure 2: Ramp compression stress and sound velocity measurements. (a) Sound velocity versus density. (b) Longitudinal stress (i.e. pressure) versus density. NIF ramp-compression data with 1σ error bars (solid blue line), together with a number of other models. Central pressures for Earth, Neptune and Saturn are shown for reference. The inset highlights the differences in the models at lower pressures. R. F. Smith et al. (2014).

Figure 3: Mass-radius relationships for planets. Calculations for carbon (based on this study, where 1σ error bars are within the width of the line, dark blue), H2O (light blue), post-perovskite MgSiO3 (green) and iron (red). Lines are dashed when based on extrapolated data. The inset shows the pressure versus density relevant to Jupiter’s core (4.3 to 8.8 terapascals). Mᴇ and Rᴇ are the mass and radius of the Earth, respectively. R. F. Smith et al. (2014).

Carbon-rich super-Earths are a proposed class of planets. 55 Cancri e might be one such planet. A third of the planet’s mass could be comprised of carbon, much of it in the form of diamond. Data from this study can be useful for constructing interior models to determine if planets like 55 Cancri e are indeed carbon-rich. A 10 Earth-mass pure-carbon planet would have a central pressure of about 0.8 terapascals, well within the range of pressures probed in this study. Furthermore, this study might also be applicable for exotic giant planets around pulsars, such as the companion of millisecond pulsar PSR J1719-1438. This object is thought to be a carbon-rich giant planet slightly more massive than Jupiter, with 383 Earth-masses. It orbits the pulsar in a tight 2.2 hour orbit. Its density cannot be lower than 23 g/cm³ or it would be tidally destructed by the pulsar’s immense gravity. If this object is made of pure carbon, it would have a radius of about 4.5 Earth-radii and a central pressure of about 148 terapascals. Such an object can be dense enough to avoid tidal destruction.

- R. F. Smith et al., “Ramp compression of diamond to five terapascals”, Nature 511, 330-333 (17 July 2014)
- Nikku Madhusudhan et al. (2012), “A Possible Carbon-rich Interior in Super-Earth 55 Cancri e”, arXiv:1210.2720 [astro-ph.EP]
- M. Bailes et al. (2011), “Transformation of a Star into a Planet in a Millisecond Pulsar Binary”, arXiv:1108.5201 [astro-ph.SR]