NASA: Comet Dust –“Interstellar Fossil Evidence of the Origins of Life”



Orion-interstellar-dust-diffraction-pattern (1)


"By analyzing interstellar dust, we can understand our own origins," said  Andrew Westphal, a planetary scientist at the University of California at Berkeley. "Just as people go to Africa to look for fossil hominids, say, 4.5 million years old, trying to understand the origins of humanity, we want to look at stuff that helped form the solar system 4.5 billion years ago."

The image at the top of the page image shows a diffraction pattern from the first interstellar dust candidate the Comet Orion, collected by NASA's Stardust spacecraft in 2004 (which we're posting as a companion to today's fascinating feature: Life In the Universe –"Precursor Molecules to DNA Could Have Been Delivered Millions of Years Ago")

Seven tiny grains of rock captured by NASA's comet-chasing Stardust probe in 2004 may be visitors from the vast reaches of interstellar space, researchers say. These interstellar dust motes from Stardust shown below are fluffier and more diverse than expected, findings that could one day shed light on the origins of the solar system, scientists added. Interstellar dust motes are bits of rock that permeate the enormous spaces between the stars.

The primary goal of the Stardust mission was to collect samples of a comet and return them to Earth for laboratory analysis. Comets are ancient bodies of frozen ice and dust that formed beyond the orbit of the most distant planet. They were expected to contain materials that the solar system formed from, preserved in ice for billions of years. When the international team of 200 scientists began examination of the returned particles, they found that the particles were indeed ancient building blocks of the solar system but the nature and origin of the particles was quite unexpected. Before the mission, there were very good reasons to believe that we knew what comets would be made of and there was a general expectation was that the particles collected from comet Wild 2 would be mainly be dust that formed around other stars, dust that was older than the Sun. Such particles are called stardust or pre-solar grains and this was the main reason why the mission was named Stardust.




What the probe found was remarkable: Instead of rocky materials that formed around previous generations of stars they found that most of the comet's rocky matter formed inside our solar system at extremely high temperature. In great contrast to its ice, the comet's rocky material had formed under white-hot conditions. Even though we confirmed Comets are ancient bodies with an abundance of ice, some of which formed a few tens of degrees above absolute zero at the edge of the solar system, we now know that comets are really a mix of materials made by conditions of both "fire and ice". Comet ice formed in cold regions beyond the planet Neptune but the rocks, probably the bulk of any comet's mass, formed much closer to the Sun in regions hot enough to evaporate bricks. The materials that the probe collected from comet Wild 2 do contain pre-solar "stardust" grains, identified on the basis of their unusual isotopic composition, but these grains are very rare.

Among the high temperature materials some are already well known components of primitive meteorites; rocks from asteroids that formed between Mars and Jupiter. These include odd rounded particles called chondrules and white irregular particles known as Calcium Aluminum Inclusions (CAIs). Chondrules are the dominant material in many primitive meteorites and they are rounded droplets of rocks that melted and then quickly cooled as they orbited the Sun. CAIs are much rarer than chondrules and are distinguished by their unusual chemical and isotopic composition. They are also the oldest solar system materials and are composed of exotic minerals that form at the very high temperature.

It was very exciting to find that pieces of CAIs and chondrules in the comet and the scientific implications of this are profound. When the Stardust team first presented the discovery of comet CAIs at the annual Lunar and Planetary Science conference, just three months after Stardust landed, jaws dropped in the room crowded with 600 scientists. It was just phenomenal to discover something this profound, right in the beginning of the analysis program.

The discovery of chondrules and CAIs proves that matter abundantly formed in the inner solar system was somehow transported to the edge of the young solar system where comets formed. There are some theories that suggest that CAI's formed just a few radii from the surface of the Sun, 4.567 billion years ago. The finding that inner solar system materials, formed at very high temperature, were transported all the way to the edge of the Solar System to the region where Pluto is one of the major scientific findings of Stardust.

In other words, instead of being dominated by particles formed around other stars, our comet's rocks were predominantly formed close to the Sun. Thus, these comet sample studies have provided a direct look at the nature and origin of the building blocks of planets, materials that were sprayed all over the young solar system and must have been incorporated into all planets and moons.

One of the most unexpected discoveries was the 2009 discovery of the amino acid glycine by a team of scientists from the Goddard Space Flight center. While perhaps not totally unexpected that a comet would contain amino acids it was unexpected that this molecule could be detected in the tiny particles that were collected at such high speed (six times the speed of a rifle bullet!). It was quite a technical triumph to develop the methods that made the detection possible and incorporated the use of isotopic composition to prove the glycine was not a contaminant from our own planet. The significance of this discovery is that comets must have delivered at least one amino acid to our planet before it had life. Because most stars have comets it suggests that all Earth-like planets obtain important pre-biotic molecules from space.

Another surprise from the 2004 comet flyby came when we flew through the dust escaping the comet. It had been expected that the impact rate of particles on the spacecraft would increase with time, reach a peak, and then decline as the comet nucleus disappeared "in the rear view mirror". Instead, the rate of impact rate changed in spurts, probably caused by entering and exiting "jets" of dust flowing off the nucleus and also the breakup of "cometary dirt clods" as they drifted away from the nucleus and lost ice that had served as glue to hold them together.

The Daily Galaxy via NASA/JPL

Image Credit: Zack Gainsforth

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