Where were comet crystals formed?

Silica crystals can only form in a red-hot environment. Whereas comets originate in the icy cold outer regions of the solar system. Then how do these crystals come to be found in comets? A team of researchers provide the answer in Nature.

An instability in the gravity of the protoplanetary disc causes large amounts of gas and dust to be deposited in the star. (Source: NASA/JPL/Caltech)

It had long been a puzzle for scientists how small silica crystals come to be found in the interior of comets. These crystals can only form in a red-hot environment , while comets originate in the icy cold outer regions of the solar system. Originally these crystals must have floated around as non-crystallised silica particles in the dense clouds of gas and dust which make up the solar system.  Researchers from Leiden, Budapest and Heidelberg have found a new explanation for how and where thse crystals are formed, using NASA's Spitzer Space Telescope. The results of their research were published on 14 May in Nature.


EX Lupi

The researchers discovered that silica crystallises when a star erupts.  They discovered the infra-red fingerprint of silica crystals on the disc of dust and gas around the young star EX Lupi during one of its brightest peaks, observed by Spitzer in April 2008. These crystals were not visible during earlier observations using Spitzer during one of the star's calmer periods. 

New scenario

This caused the star to erupt, leading to over-heating of the layers on the surface of the disc. (Source: NASA/JPL/Caltech)

The researchers believe that the crystals are formed through heating and annealing of small particles on the surface of the innermost gas and dust disc, caused by the heat of the eruption. This is a completely new scenario for how this kind of material is formed. Scientists have so far assumed two different scenarios for the formation of silica crystals that are found in comets and in discs of gas and dust from young stars. According to one scenario, long-term exposure to the heat of a young star in the centre of the disc causes the silica particles to crystalise. According to the other theory, the shockwave of a large object in the disc rapidly heats the silica particles to the right temperature, causing them to crystallise, after which the crystals rapidly cool again. What the researchers from EX Lupi discovered does not match either of these theories. They therefore conclude that there must be a third - never previously considered - way for silica crystals to form.


Brightness

The non-crystallised particles on the surface of the disc are heated, then cool down, acquiring the crystal structure and the green hue of forsterite. (Source: NASA/JPL/Caltech)

EX Lupi is a young star that strongly resembles the sun as it was four and a half billion years ago. The star has two characteristics that are typical of very young stars. It is surrounded by a dense disc of dust and gas in which planets are formed, and it peaks every four to five years in terms of its brightness, which lasts several months. The star's light intensity is increased ten-fold during these peaks. Such peaks are caused because the protoplanetary disc is instable and large amounts of material collide with the star.  Every fifty years there is an extremely strong peak. Our sun in its younger years probably had an equally active period with irregular peaks in brightness. 


Infrared spectrograph

In April 2008 astronomers were observing EX Lupi using the Spitzer's infrared spectrograph. Although the star's peak in clarity that was observed in January of that year had already started waning, it was nonetheless thirty times brighter than normal. When these new observations were made using Spitzer in 2005 before the eruption started, some remarkable differences were observed.

Forsterite

Repeated eruptions of the young star lea to an increase in forsterite crystals in the disc. They then become part of the comet material. (Source: NASA/JPL/Caltech)

In 2005 the silicate on the surface of the protoplanetary disc comprised non-crystallised dust paticles. In 2008, the spectrum showed the presence of silica crystals above the non-crystallised dust. The crystals proved to be made of forsterite, a material often found in comets and protoplanetary discs. The crystals were hot, evidence that they were formed in a process involving high temperatures, but not as a result of a shockwave. If this had been the case, they would have cooled down.


Crystal-forming zone

As a result of the eruption, EX Lupi became about a hundred times brighter. The crystals were formed on the surface of the disc, precisely at the distance to the star where the temperature was high enough, around 1000 Kelvin (725 °C) but lower than 1500 Kelvin (1100 °C). All the dust particles would vaporise at higher temperatures.  The radius of this crystal-forming zone is comparable with the span of the earth-like planets in our own solar system, in other words as far as the orbit taken by Mars. 

Film
The origin of forsterite crystals in the protoplanetary disc. This video clip shows the crystallisation process.  (Source: NASA/JPL/Caltech)


Publication Nature 14 May 2009
Title: Episodic formation of cometary material in the outburst of a young Sun-like star
Authors: P. Ábrahám, A. Juhász, C.P. Dullemond, Á. Kóspál, R. van Boekel, J. Bouwman, Th. Henning, A. Moór, L. Mosoni, A. Sicilia-Aguilar, N. Sipos

(19 May 2009)

Last Modified: 20-05-2009