September 22, 2008 is the first day of autumn for those of us in the Northern Hemisphere, and the first day of spring for those in the southern hemisphere. What this means is that the weather is going to start changing (if it hasn’t already) for everybody in the mid-to-upper latitudes (and those of you in the equatorial climates are probably wondering what the big deal is). The north pole is headed into a time of twilight and darkness, while the south pole will be enjoying more sunshine. In the north, our days start getting much shorter and in the south, the days start getting longer. Finally, on the equinox, the Sun is positioned directly over our equator (the Sun’s apparent position).
Fall colors in Ushuaia, Argentina (Copyright 2001, Carolyn Collins Petersen)
I always look forward to autumn because where I live, it’s got some of the nicest weather of the year (not too hot or muggy, not too cold). The leaves are starting to change colors, and anywhere that deciduous trees and ornamental bushes grow, you can see some gorgeous colors.
In New England (and the Rocky Mountains, where I grew up), fall color time is also prime tourist time. I suppose that’s true anywhere that colors change. I remember a few years ago, we went to South America (I was doing astronomy lectures on a cruise ship) in mid-March. When we got to the extreme southern tip of Argentina, the forests had already started to show colors like we see each year in New England. It was quite a treat to see autumn in March!
Stargazing-wise, autumn means increasingly longer nights, which is good. But, they’re colder nights, which means bundling up. For us in the northern hemisphere, autumn’s stars are lovely preview of what’s to come in winter. And, if I stay up late enough at night, I can start to see some of my favorite winter-time constellations (the early risers, anyway).
So, here’s to the change of seasons, no matter where we are!
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 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!
