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Comets are fascinating objects that have captivated humans for millennia. They are often described as "dirty snowballs" or "icy dustballs", but what exactly are they made of? And what can they tell us about the history and evolution of our solar system?
One way to answer these questions is to study the dust that comets release into space as they approach the Sun. This dust is composed of tiny grains that reflect sunlight and form the bright tails that we see from Earth. Dust also carries information about the chemical and physical properties of the comet nucleus, as well as the conditions in which it formed billions of years ago.
To get a closer look at comet dust, the European Space Agency (ESA) launched the Rosetta mission in 2004. Rosetta was the first spacecraft to orbit a comet and deploy a lander on its surface. Its target was comet 67P/Churyumov-Gerasimenko, a 4-kilometer-wide irregular-shaped body that orbits the Sun every 6.5 years.
Rosetta arrived at comet 67P in August 2014 and spent more than two years studying its structure, activity, and environment. During this time, Rosetta collected and analyzed thousands of dust grains using various instruments on board the orbiter and the lander. Some of these instruments were designed to capture dust particles directly, while others used cameras, spectrometers, or mass spectrometers to measure their properties remotely.
The results of Rosetta's dust studies have revealed many surprising and intriguing facts about comet 67P and its origin. Here are some of the main findings:
Comet 67P is made of two distinct lobes that likely merged in a low-speed collision early in its history. The dust grains on each lobe have different compositions and sizes, suggesting that they came from different regions of the protoplanetary disk, the cloud of gas and dust that surrounded the young Sun and gave birth to the planets.
The dust grains on comet 67P are very diverse in shape, size, and structure. They range from micrometers to centimeters in diameter, and from fluffy aggregates to compact pebbles. Some of them have complex layered structures that indicate multiple episodes of heating and cooling, while others have smooth surfaces that imply gentle processing. Some grains are rich in organic molecules, such as amino acids and sugars, while others are dominated by minerals, such as silicates and sulfides.
The dust grains on comet 67P are very old and pristine. They have not been significantly altered by thermal or chemical processes since they formed more than 4.5 billion years ago. They contain isotopic ratios and elemental abundances that are similar to those found in primitive meteorites, which are thought to represent the building blocks of the solar system. They also contain presolar grains, which are even older than the solar system and originated in other stars.
The dust grains on comet 67P are very dynamic and responsive to the changing environment around the comet. As the comet approaches the Sun, its surface temperature increases and its ice sublimates, releasing gas and dust into space. The dust grains are then accelerated by the gas flow and the solar radiation pressure, forming jets, streams, and tails that shape the comet's appearance. The dust grains also interact with each other through collisions and electrostatic forces, forming clusters and clumps that can affect their trajectories and lifetimes.
The Rosetta mission ended in September 2016 with a controlled crash-landing of the spacecraft on comet 67P's surface. However, the data collected by Rosetta will continue to be analyzed and interpreted for years to come, providing new insights into the nature and origin of comets and their role in the evolution of our solar system.
Read the following articles for more information on the dust properties of comet 67P studied by Rosetta and other researchers around the world.