Brownleeite
Published on Jun 15, 2008 at 2:21 pm.
4 Comments.
Filed under comets.
(Updated June 17)
It isn’t every day that a new mineral is discovered. But, I recently read a NASA press release about a new mineral discovered in some comet dust. The mineral is being named brownleeite in honor of University of Washington astronomer Donald Brownlee. He is a pioneer in the study of comet dust. But, he is best known to many people as one of the authors of the book Rare Earth: Why complex life is uncommon in the universe, in which the authors make a case for Earth being pretty special. They argue that things have to be just right for higher life forms to evolve, so technologically advanced lifeforms such as ourselves are likely to be very rare in the universe.
The new mineral is believed to be from comet 26P/Grigg-Skjellerup. As comets swing around the Sun, they shed material. Some of the material is composed of ions, and it is blown away by the solar wind. But, some of the material is particulate in nature. These dust grains then often continue to orbit the Sun for a long time. In 2003, as Earth was passing through a swarm of these particles from comet Grigg-Skjellerup, NASA had a high altitude research aircraft flying. On board that aircraft was a device scooping up air and dust for a team of researchers led by Keiko Nakamura-Messenger. At that altitude, most of the dust comes from above (micrometeoroids) rather than below (volcanoes). The dust grains were analyzed, and a hitherto unknown mineral was found. 
This mineral was a manganese silicide (containing manganese and silicon). Brownlee had long ago proposed that some method such as this would be useful to capture cometary samples. Interestingly, he was also the principle investigator of the Stardust Mission, which sent a spacecraft to comet 81P/Wild 2. The team proposed honoring Donald Brownlee by naming the new mineral after him.
Brownleeite is particularly interesting in that it was found in space. There are fewer than 4400 minerals known. From the earliest days of space exploration, astronomers have wondered if there were minerals found elsewhere that were unlike anything on Earth. That idea was seized upon by the makers of Star Trek. In the Star Trek universe, there exists a mineral called dilithium that has special properties. Dilithium is portrayed as naturally occurring on select worlds. But, Star Trek introduced dilithium several years bore Apollo 11 returned the very first samples of rocks from another world (the Moon). But, the Moon rocks turned out to have few new minerals in them, and none with such esoteric properties as dilithium. The proportions of minerals were unlike any rocks on Earth, but almost every mineral found on the Moon had a counterpart on Earth. The unmanned spacecraft sent to Mars have similarly found minerals there that have Earth analogs. Once again, the proportions are unlike Earth rocks, but the minerals are mostly familiar. But, brownleeite is different. It is a new mineral. That makes this find exciting. Since minerals form in the way that they do as a result of their environment, this tells us that the environment under which comets form is unlike that on Earth. Of course, we were sure of that, but this is confirmation. And, since we have not found this mineral in meteorites, then it may suggest that comets even form under different conditions than the asteroids. Now, I may be reading an awful lot into just an announcement of a new mineral, but I want to underscore the significance of this find.
What makes brownleeite especially interesting is that it is not something that was expected. The press release did not go into great detail, so I don’t know much about it. But, the report did say that this mineral had not been expected to have formed in the comet or in the solar nebula. Since the mineral is obviously there, that means that scientists will have to figure out where it came from and how it got there. This suggests that we still do not understand the early solar nebula, planet formation, or the role of comets and asteroids yet.
One thing that the news report said, though, was that the mineral is a semiconductor. Being composed of nothing esoteric, the mineral can be synthesized on Earth, but it has not been found naturally occurring. Now, right off, I want to head off a few questions and people jumping to wild conclusions. The term “semiconductor” is not synonymous with the term “artificial.” In other words, semiconductivity is a material property of some materials. Everyone is well aware that some semiconductors can be manufactured, but that is about the same as saying that conductors and insulators can be manufactured. Simply because something is a semiconductor doesn’t mean that somebody made it. So, a semiconducting mineral in a comet does not imply that there were aliens messing around with the comet. Semiconductors are naturally occurring here on Earth, too. Diamonds and sapphires can also be manufactured, and they occur naturally, too. By saying that brownleeite is a semiconductor, all the mineralogists are saying is that it has the electrical property that it can be either a conductor or an insulator based upon external influences. That is what a semiconductor is. And, of course, being able to control those external influences is what semiconductor electronics is all about. Remember, one of the best known semiconductors is silicon, and that is naturally occurring on Earth (and in space).
So, brownleeite is a mystery only in that it is unknown on Earth and it is the first new mineral found in a comet, and one of the few minerals found in space that are not known here on Earth. That we didn’t expect to find it in space means that we still need work to fully understand the formation of planetary systems.
-Astroprof
Image Credit: Mary Levin, University of Washington
L. Keller on June 17, 2008 at 9:57 am: 1
There are a number of minerals that occur in extraterrestrial samples that have not been found on earth. A few examples…from the Moon: hapkeite, tranquilityite, armalcolite (the latter is interesting because the mineral name is a combination of the Apollo 11 astronauts last names).
Astroprof on June 17, 2008 at 10:30 am: 2
Thanks for the correction. I am editing my previous statements.
star trek planet system | 82123 on June 21, 2008 at 4:37 pm: 3
[…] 51 Minuten gefunden uslm http://addison-tw.blogspot.com/Ohne Titel - http://andie-3.blogspot.com/|||Brownleeite15. Juni 2008 Astroprof That idea was seized upon by the makers of Star Trek. In the Star Trek […]
MsLaurel on June 21, 2008 at 7:13 pm: 4
Cool.