Meteors, commonly known as shooting stars, are overwhelmingly the offspring of comets the Rosetta mission has confirmed, in the process witnessing the way these dust particles form.
Millions of times a day the Earth's skies are lit up by grains of dust burning up from the immense speed (11 to 72 km/s) with which they hit our atmosphere. Some of these come in the form of showers, or even storms, where the meteors all come from the same point in the sky. Others are “sporadics” objects that cannot be predicted and have no obvious source.
The origins of sporadic meteors are less certain, with proposals that the interplanetary dust particles (IDPs) that cause them are also cometary debris, or the remnants of asteroid collisions.
On a visit to the inner solar system the ice that makes up most of a comet starts to sublime, turning into gas. The dust particles held together by the ice are freed to move and can be carried by the escaping gas into the comet's tail. Pressure of sunlight, solar wind and force of gravity from planets gradually perturb orbits until a dust grain's path may have little in common with the comet from which it came.
While still remote from the sun, however, too little gas is produced to push the dust away from the comet. A slow loss of ice still releases a little dust however, and this builds up as a layer on the comet's surface that gets pushed off once gas starts to be released in substantial quantities. The Rosetta mission arrived at Comet 67P/Churyumov-Gerasimenko in August 2014 when dust release was still low.
Cometary grains collected by Rosetta's COSIMA instrument in October were around 50µm across, the European Space Agency announced in Nature. "They are fluffy and porous and rich in Na. (sodium)," said lead author Dr Rita Schulz. So porous many shattered on hitting Rosetta's collection plate, even at less than 10m/s, leaving dust particles behind. These characteristics confirm “that these particles likely represent the parents of IDPs,” said Schulz.
Credit: ESA/Rosetta/MPS for COSIMA Team MPS/CSNSM/UNIBW/TUORLA/IWF/IAS/ESA/BUW/MPE/LPC2E/LCM/FMI/UTU/LISA/UOFC/vH&S. Particles that crumbled (a) and shattered (b) into dust on collection.
"There has been a long-standing dispute of whether the source of IDPs are solely comets or also asteroids," said Schulz. Particles knocked off asteroids would have a different chemical composition, similar to that of larger objects we get to examine when they survive the trip through the atmosphere. "Our results imply that they have cometary origin,” Schulz said. “Another source would not be required."
Rosetta will continue to orbit 67P, observing its closest approach to the sun on August 13. Now that the dust mantle formed during the cometary winter has largely been shed the ESA team expect the particles Rosetta will collect will be more similar to the ice-containing grains collected by previous missions that encountered comets closer to the sun.