Category Archives: comet dust

astronomy, chemistry, comet dust, cosmic chemistry

Before the Beginning

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Comets As Probes of Pre-solar System History

When comets do a turn around the Sun, they leave behind streams of dust particles that Earth eventually intersects in its own orbit around the Sun. Most of the time we see these particles as they enter our atmosphere and burn up. It’s rare to get samples of these dusty bits, but when planetary scientists DO get them, they’ve basically gotten their hands on very old, very primitive bits of material that existed LONG before the Sun and planets did. This is because comets formed out of the materials in the protosolar nebula — essentially they’re orbiting deep-freezes of ice and dust.  Scientists have long known about comets and their treasure troves of ancient stuff.  In 2003, they managed to gather up good samples of Comet 26P/Grigg-Skjellerup and have been studying them since then.

Interplanetary dust particles showing pre-solar grains of silicates and organic matter that originated in interstellar space. Courtesy H. Busemann. Click to embiggenate.
Interplanetary dust particles showing pre-solar grains of silicates and organic matter that originated in interstellar space. Courtesy H. Busemann. (Click to embiggenate.)

The findings are amazing. According to Dr. Henner Busemann of the University of Manchester, who is presenting these results on Tuesday at the European Week of Astronomy and Space Science being held at University of Hertfordshire in the U.K., the dust grains have all the signs of being very ancient — predating the birth of the Sun and planets. Some of it is true stardust, floating in interstellar space after being ejected during the process of birth, life and death of other stars. “We found an extraordinary wealth of primitive chemical fingerprints,” he said, “including abundant pre-solar grains, true stardust that has formed around other earlier stars, some during supernova explosions, associated with extremely pristine organic matter that must pre-date the formation of our planets.”

You can see a sample of the dust particles here. They are extremely tiny — only a few thousands of a millimeter in diameter.   Two grains appear to have materials that scientists predict match the solar system’s birth nebula. One dust particle contained four pre-solar silicate grains (meaning grains that existed well before the solar system’s birth nebula formed) with an unusual chemical composition that matches the kinds of silicate grains that might form in supernova explosions. This is pretty good evidence that our birth nebula was seeded by the death throes of older, massive stars that once existed near our part of the galaxy.

More closeups of comet dust grains from the pre-solar-system neighborhood, more than 4.5 billion years ago. (Click to embiggenate.)
More closeups of comet dust grains from the pre-solar-system neighborhood, more than 4.5 billion years ago. (Click to embiggenate.)

One of these grains is a fragment of olivine and was found next to a hollow globule of carbon, most likely of interstellar origin. Carbon is an interesting element to find because it is intimately bound up in the structures that ultimately build life.

Organic coatings are suspected to be the shells of time capsules that protected and secured the survival of some of these fragile stellar silicate grains as they made their way through the interstellar environment and, later on, the high radiation environment of the newly forming Sun.

Detecting the Chemistry of Life

This isn’t the only big news coming from the WASS meeting.  Two researchers are also presenting a paper about the detection of two of the most complex molecules yet discovered in interstellar space: ethyl formate and n-propyl cyanide. Their computational models of interstellar chemistry also indicate that yet larger organic molecules may be present — including the so-far elusive amino acids, which are essential for life. The scientists used the IRAM 30-meter telescope in Spain to look at a region of the sky near the star-forming region Sagittarius B2.  The molecules were found in a hot, dense cloud of gas that also contains a newly formed star.

Large, organic molecules of many different sorts have been detected in this cloud in the past, including alcohols, aldehydes, and acids. The new molecules ethyl formate (C2H5OCHO) and n-propyl cyanide (C3H7CN) represent two different classes of molecule — esters and alkyl cyanides — and they are the most complex of their kind yet detected in interstellar space.

This is pretty cool news on both fronts. These findings by separate groups of scientists tell us that we (our planet and our star) came from some of the same processes we see happening throughout the galaxy.  The precursors of life are out there floating around in interstellar space, and scientists are finding more and more of them. It’s one thing to know and suspect these facts, but quite exciting to find evidence of our origins as part of the normal evolution of the universe and its stars and galaxies.

astronomy news, comet, comet dust, solar system formation, stardust

Comet Dust and the History of the Solar System

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Comet Wild 2 Dust Studies

The history of the solar system is written on the surfaces of planets and moons, but can also be read in dust particles found in the clouds surrounding comet nuclei. How does this work?  Think back a few billion years, to when the solar system was first forming. We had a cloud of raw materials-gases, ices, and dust. You could (if you were around back then) take samples of that cloud material and do a chemical analysis on them. You’d determine the mix of elements and also the isotopes of those elements. (Think of isotopes as different forms of the same element. Chemists call them different “species.” So, you could have helium-3 or carbon-12 or carbon-13.) Study those isotopes and they can give you a lot of information about the timeline of history that our solar system experienced.

Comets formed pretty early in the history of the solar system, making them treasure troves of information about the chemical makeup of the gas and dust cloud that eventually birthed the rest of the solar system. So, it’s obvious why scientists send spacecraft (like the Stardust mission) to gather up comet dust: they can use it to fill in the gaps of our knowledge about how the solar system formed and what those early materials were. We know the big picture: that the rocky worlds formed close to the Sun, and that the volatile gases and ices that existed there were melted or driven off to the outer parts of the solar system (an icy deep-freeze that made a great home for gases and icy particles). Now scientists are examining the bits of dust that come flying off comets as they come close to the Sun in their orbits. And, those “bits” have interesting tales to tell.

Tiny crystals from the Wild 2 comet, captured by NASA’s Stardust mission, resemble fragments of the molten mineral droplets called chondrules, shown here, found in primitive meteorites. That similar flash-heated particles were found in Wild 2, a comet formed in the icy fringes of outer space, suggests that solid materials may have been transported outward in the young solar system. Photo by: Noriko Kita
Tiny crystals from Comet Wild 2 were captured by NASA’s Stardust mission. They resemble these fragments of molten mineral droplets called chondrules. found in primitive meteorites. That similar flash-heated particles were found in Wild 2, a comet formed in the icy fringes of outer space, suggests that solid materials may have been transported outward in the young solar system. Photo by Noriko Kita/Courtesy University of Wisconsin-Madison.

This week, a group of scientists led by Tomoki Nakamura, a professor at Kyushu University in Japan, publicized their analysis of oxygen isotope compositions of three crystals from the halo of Comet Wild 2. Their goal is to the origins of comet materials. Nakamura and University of Wisconsin-Madison scientist Takayuki Ushikubo analyzed the tiny grains – the largest of which is about one-thousandth of an inch across – using a unique ion microprobe in the Wisconsin Secondary Ion Mass Spectrometer (Wisc-SIMS) laboratory.This spectrometer is the most advanced instrument of its kind in the world.

The researchers were surprised to find oxygen isotope ratios in the comet crystals that are similar to asteroids and even the Sun itself. You have to ask yourself: how can this be, if comets formed well away from the Sun (and asteroids)?

Since these samples more closely resemble meteorites than the primitive, low-temperature materials expected in the outer reaches of the solar system, its entirely possible that heat-processed particles may have been transported outward in the young solar system, and eventually embedded in the icy nuclei of comets.

As you might imagine, this is stirring interest among planetary scientists. The findings complicate what used to be a simple view of solar system formation (that I described above).  What are these minerals doing in a comet that came to the inner solar system from out past the orbit of Pluto?  What sort of migratory patterns did early solar system materials follow? The answers will come from more studies of comet dust, and when they do, these little bits of ancient “stuff” will help revise and clarify the details in the theory of how the solar system grew and evolved. Stay tuned!