NASA's Stardust mission reveals secrets of comet Wild 2

NASA's Stardust mission to Comet Wild 2 has finally revealed its secrets. The results show that the outer solar system in its early days was not what was thought. However, it remains a mystery how a comet got dust from different parts of the early solar system.

These images of Comet Wild 2 from the Stardust mission look like any other comet, but the samples collected reveal something much more intriguing. (NASA/JPL)

Before the Rosetta spacecraft landed on comet 67P/Churyumov-Gerasimenko or space agencies reported samples from asteroids, NASA sent a mission to collect pieces of comet Wild 2. This comet, after an encounter with Jupiter in 1974, follows a more typical asteroid orbit between Mars and Jupiter. The mission was launched in the last century (1999) and brought its sample back to Earth in 2006.

Instead of landing on a comet, NASA took advantage of the property of comets to produce tails of gas and dust. The Stardust mission inserted a gel-covered plate shaped like a tennis racket into the tail of comet Wild 2 and collected whatever stuck. The preliminary analysis was released shortly after the first samples were analyzed, but now a more detailed picture is emerging.

According to Dr. Ryan Ogliore of Washington University in St. Louis, "Comet Wild 2 contains things we've never seen before in meteorites, such as unusual carbon-iron assemblages and the precursors to the igneous spheres that make up the most common type of meteorite. And all of these objects are exquisitely preserved in Wild 2. The comet was a witness to the events that shaped the solar system into what we see today.

Unlocking the secrets of comet Wild 2 was a lengthy process because the samples were trapped in the aerogel. NASA had to call in an expert to examine microscopic images of the collection plate to locate the pieces. "Almost every particle from Wild 2 is unique and has a different story to tell," Ogliore said. "It's a very time-consuming process, but the scientific payoff is huge."

The samples show that comet Wild 2 is not just composed of unaltered dust from the supernova, but has a mixture of sources. In addition to interstellar dust, the samples contain traces from many parts of the cloud that became planets and asteroids after the Sun formed. This includes material from both sides of the space created by Jupiter in the cloud. "Comet Wild 2 is unlike any known asteroid," Ogliore writes in the article.

The discovery came as a surprise, since little was thought to happen beyond the orbit of Neptune, where Wild 2 is thought to have formed.

The pieces that became part of comet Wild 2 remained almost unchanged, sitting on ice. Since the comet was only about 3 km wide, it did not undergo any geological processes, which would have caused the grains to become compressed or altered by reactive chemistry.

Today, after 18 years of studying the comet, we have a better view of the dynamic formative years of the solar system, Ogliore said. However, it remains unknown whether comet Wild 2 is typical or just one of the more interesting ones.

The analysis was published in the journal Geochemica.