New ideas on comet tails blow in the solar wind

Engineers and scientists gathered around a screen in an operations room of the Naval Research Laboratory in Washington, DC, eager to take a look at the initial data from NASA's STEREO probe. We were in January 2007 and the two satellites STEREO, which were launched a few months ago, are the abbreviation of the Observatory of Solar and Terrestrial Relations, opened the eyes of their instruments for the first time. First place: STEREO-B. The screen blinked but, instead of the vast starry field they hoped for, a smear of pearly white feathers – like an angel's wing – filled the frame. For a few minutes of panic, the NRL astrophysicist Karl Battams was worried about the problem with the telescope. Then, he realized that this shiny object was not a defect, but an apparition, and these were the first satellite images of the comet McNaught. Later in the day, STEREO-A would return similar observations.

The comet C / 2006 P1 – also known as the comet McNaught – owes its name to astronomer Robert McNaught, who discovered it in August 2006 – was one of the brightest visible comets of the Earth in the last 50 years. Throughout the month of January 2007, the comet was deployed in the southern hemisphere sky, so bright that it was visible to the naked eye, even during the day. McNaught belongs to a group of rare comets, nicknamed the great comets and known for its exceptional brightness. However, McNaught stands out from its competitors by its highly structured tail, composed of many distinct bands of dust called streaks, or streaks, which extend more than 100 million kilometers behind the comet, longer than the distance between the Earth and the Sun. A month later, in February 2007, a spacecraft called Ulysses from ESA (European Space Agency) and NASA would meet the long tail of the comet.

"McNaught was a huge deal when he came because it was so ridiculously bright and beautiful in the sky," said Battams. "His streaks were dusty fingers that stretched out over an immense expanse of sky.Structurally, it's one of the most beautiful comets we've seen in decades."

How exactly the tail was broken in this way, the scientists did not know. This recalled the stories of another legendary comet of long standing: the Great Comet of 1744, which is said to have spectacularly unfurled in six tails above the horizon, a phenomenon that astronomers could not explain. By unraveling the mystery of McNaught's tail, scientists hoped to learn something new about the nature of comets and solve two cosmic mysteries in one.

An essential difference between the study of comets in 1744 and 2007 is, of course, our ability to do so from space. In addition to the incidental observation of STEREO, another mission, SOHO from ESA / NASA – the solar and heliospheric observatory – made regular observations while McNaught was flying in the sun. The researchers hoped that these images could contain their answers.

Now, years later, Oliver Price, a doctorate in planetary sciences. A student at the Mullard Space Science Laboratory at University College London in the UK, has come up with a new image processing technique to exploit from the wealth of data. Price's findings – summarized in a recent publication Icarus paper – offers the first observations of the formation of streaks and an unexpected revelation about the effect of the sun on comet dust.

Comets are cosmic crumbs of gas, rocks and frozen dust from the formation of our solar system 4.6 billion years ago. They can therefore contain important clues about the beginnings of our solar system. These clues are unlocked, as in a time capsule, whenever the elliptical orbit of a comet brings it closer to the Sun. Intense heat vaporizes the frozen gases and releases the dust inside, which passes behind the comet, forming two distinct tails: a tail of ions carried by the solar wind – the constant flow of charged particles from the Sun. – and a tail of dust.

Understanding how the dust behaves in the tail – how it fragments and agglomerates – can teach scientists a lot about similar processes that formed dust into asteroids, moons and even planets, there are billions d & # 39; years. Appearing as one of the largest and most structurally complex comets in recent history, McNaught was a particularly interesting topic for this type of study. Its brilliance and its high dust production greatly facilitated the resolution of the evolution of fine structures in its tail of dust.

Price began his study by focusing on something that scientists could not explain. "My supervisor and I noticed strange phenomena in the images of these streaks, a disruption of the lines otherwise clear," he said. "I began to investigate what could have happened to create this strange effect."

The fault appeared to be at the heliospheric current layer, a limit where the magnetic orientation, or polarity, of the electrified solar wind changes direction. This made the scientists perplexed because, while they had long known that the tail of a comet ions was affected by the solar wind, they had never seen the solar wind touch dust tails before.

The dust in McNaught's tail – roughly the size of cigarette smoke – is too heavy, scientists say, for the solar wind to move. On the other hand, the tiny, electrically charged ions and electrons of a tail of ions navigate easily along the solar wind. But it was difficult to say exactly what was happening with McNaught dust and where, because at about 60 miles per second, the comet was moving rapidly between STEREO and SOHO.

"We got very good data sets with this comet, but they came from different cameras placed on different spacecraft, all located in different places," said Price. "I was looking for a way to combine everything to get a complete picture of what's going on in the queue."

His solution was a new image processing technique that compiles all data from different spacecraft using a tail simulation, where the location of each tiny dust is mapped by solar conditions and physical characteristics. such as its size and age. it had been since he'd gone off his head, or coma, from the comet. The end result is what Price called a time card, which superimposes the information of all the images taken at a given moment, allowing it to track the movements of the dust.

Time charts allowed Price to see the streaks forming over time. His videos, covering a two-week period, are the first to follow the formation and evolution of these structures, showing how fragments of dust fall from the comet's head and collapse into long streaks.

But the researchers were delighted to find that Price's charts made it easier to explain the strange effect that had drawn their attention to the data. Indeed, the current leaf has been at the origin of disturbances of the dust layer, breaking the smooth and distinct lines of each striation. During the two days, it took the full length of the comet to navigate the current sheet. Whenever the dust encountered the changing magnetic conditions, the position of the comet was unstable, as if it were crossing a cosmic retarder.

"It's as if the feathers of the striation were crumpled when it crossed the current leaf," said Geraint Jones, planetary scientist at University College London. "If you imagine a wing with a lot of feathers, while the wing passes through it, the lighter ends of the feathers get deformed.For us, this is an obvious proof that the dust is electrically charged and that the solar wind affects the movement of this dust ".

Scientists have long known that solar wind affects charged dusts; Missions like Galileo Cassini and Ulysses watched her move electrically charged dust into the space near Jupiter and Saturn. But they were surprised to see the solar wind affect larger grains of dust like those of McNaught's tail, about 100 times larger than the dust ejected around Jupiter and Saturn, because they are much heavier to bear for the solar wind. around.

With this study, scientists gain new insights into long-standing mysteries. The work highlights the nature of the striated comet tails of the past and provides a crucial goal for the study of other comets in the future. But it also opens up a new line of questioning: what role did the sun play in the formation and the first history of our solar system?

"Now that we see the solar wind changing the position of the dust grains in McNaught's tail, we may ask ourselves: could this have been the case so early in the history of the solar system, the wind solar also played a role in the organization of old dust? " "Jones said.

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