Category Archives: comet

Comet Dust and the History of the Solar System

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!

Flying Over Mars on the Way to a Comet

Image of Mars taken with the camera onboard the Philae lander.
Image of Mars taken with the camera onboard the Philae lander.

The European Space Agency’s Rosetta spacecraft did a flyby of Mars this weekend and sent back some way cool images. Rosetta is really headed toward a rendezvous with a comet 67/P Churyumov-Gerasimenko early in the year 2014. But, along the way, it has to pass by Mars and Earth to get a couple of much-needed velocity boosts so that it will be able to make the journey to the comet, some 800 million kilometers from the Sun. Since the spacecraft was going to be in the neighborhood of Mars, the mission teams decided to test out some of the instruments. The Philae lander, which will be settling onto the surface of the comet, was switched on and used in what they call “autonomous” mode (in which it relied on power from its batteries to run itself). The Rosetta Philae lander imaging system snapped the image of Mars, below.

Along with visual imagery, the spacecraft’s other instruments measured the planet’s atmosphere, studying it in ultraviolet light (which lets them see structures in the atmosphere, such as high-altitude clouds).

Rosetta’s next flyby is past Earth, which should be very interesting to see! I’m fascinated with this mission since its final destination is a comet. Back when I was in grad school, I spent a lot of time studying comets. They may be frigid balls of ice and dust and rock, but they do put on a good show when they get close to the Sun. For example, as the comet nears the Sun, radiation pressure strips dust particles from the comet as the surface ices melt. Those dust particles stream away from the comet’s nucleus and form the dust tail, that long, curving tail that makes a comet look the way it does.

Structures in the Mars atmosphere, as seen by the OSIRIS wide-angle camera on Rosetta. Credit: ESA ? 2007 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA You can read more about this flyby here.
Structures in the Mars atmosphere, as seen by the OSIRIS wide-angle camera on Rosetta. Credit: ESA © 2007 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA You can read more about this flyby here.

But, there’s another structure in that tail, the plasma tail. If the comet has enough volatiles (chemical elements or compounds) in its ices, it sends a stream of gas molecules out behind it. They encounter the solar wind, which is a stream of positive ions and electrons streaming out from the Sun and entrained in a magnetic field—making the plasma tail. The encounter causes heating, and the plasma tail will glow, especially in the ultraviolet. Like the dust tail of a comet, the plasma tail also points away from the Sun as the comet moves around it in its orbit.

Our interest was in the plasma tails because they actually act as “solar wind socks.” Study a plasma tail and what you find out will tell you about conditions in the solar wind. How does this work? As a comet moves through the solar wind, the plasma tail can grow, break off, and then regrow. We wanted to measure the growth and figure out what was happening in the solar wind to cause the plasma tail appear to break away. To do that, we needed a lot of images of comets in different parts of the solar wind.

We also wanted to measure the solar wind, and for that, we used data from the Ulysses spacecraft, which is looping around the Sun, over and over again, giving astronomers a long-term look at the Sun and the solar wind.

The Ulysses spacecraft and orbit information
The Ulysses spacecraft and orbit information

It turns out that the solar wind is not the same all over. In other words, the solar wind that emanates from regions near the mid-section of the Sun has different particle “loads” and speeds than the solar wind from mid-latitudes and polar regions. Those “loads” and speeds are what affect a comet’s plasma tail. As a comet rounds the Sun, and particularly if it’s changing latitude, it will encounter different loads and velocities. It will also pass through areas where the direction of the magnetic field in the solar wind changes quite abruptly. When it does, the plasma tail breaks off and starts to regrow. You can actually see this happen over time, if you take a series of images of a plasma tail during a comet’s closest approach to the Sun. (You can read more about our work here.)

Now, the Rosetta mission is interested in the surface of the comet, which will tell us much more about the ices on its crust, as well as the dust component. It will get at them in a form we don’t often get to see: in their “natural” unheated state.

Cometary ices (and dust) are usually considered to be the oldest, and often the most primordial, bits of solar system material. Study those and you learn something about the conditions in the solar system at its birth. But, it’s good to do that study on materials that haven’t been heated by sunlight or otherwise partially destroyed or decomposed.

Like the Rosetta Stone, which was the key that helped people understand the language and cultures of ancient Egypt, the Rosetta spacecraft will provide the key that will unlock the mysteries behind the building blocks of the solar system: the chemical elements and compounds hidden away in comets for the past 4.6 billion years.

I always thought it was kind of ironic that each time we studied a planet’s plasma tail, we were watching bits of solar system history flash before our eyes. Now, it’s exciting to know that a spacecraft will land on one and sample materials that haven’t changed much since they first formed the comet.