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.

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.

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.

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.