November 20, 2019
Meteorites: Pieces of the cosmos delivered to our doorstep
By Steven Dibb
What rocks from space teach us about our solar system and ourselves
When I was 15 years old, I saw the Perseid meteor shower for the first time. Having grown up in urban, light-polluted Los Angeles, I had never witnessed the true majesty of a meteor shower. That summer, my dad and I were on a road trip across the American Southwest before I started school in September. For a few nights in mid-August, we were camped in western Colorado, far from the obscuring lights of a major city.
As the sun went down on our first night — the sky fading slowly from blue to velvet black — some of the more familiar denizens of the night sky became apparent. These were the giant planets Jupiter and Saturn, and a few of the brightest stars — all visible even in LA. After a short dinner of hot dogs and potato salad, we turned our attention back to the night sky, now entering full celestial darkness. This time, with my eyes more adjusted to the night, I was struck by what I saw.
The night sky was so loaded with stars that it seemed to have a texture, like a heavy fabric, glaring with points of light of every color and brightness. Before I could blink, I saw the first “shooting star” of the night — a white streak flashing from the constellation Perseus (hence, “Perseids”). Within only a few seconds, more followed, with colors varying between white to lime green, some breaking into multiple, smaller streaks and continuing on a slightly different path. It was an aesthetic experience that elicited feelings of awe and curiosity. I remember thinking, what are these things? Where do they come from? What are they made of?
In some ways, the story of “shooting stars” is more exciting than watching them. These brilliant phenomena are caused by small pieces of dust and rock hitting the top of our planet’s atmosphere at incredibly high speeds — often clocked at miles per second. Most of these small grains do make it to the ground, but are too small to be meaningfully recovered. Those rocks that are about the size of a marble or larger actually make it to the ground for us to more easily find, study, and admire.
The intact pieces of these rocks are called meteorites. They offer a way to learn about our solar system in a direct, physical way. Encoded within the structure, mineralogy, and chemistry of meteorites is information on the formation and evolution of our solar system. For the Psyche mission, which will visit one of the largest metallic asteroids in the solar system, the study of meteorites is critical to the design, development, and operation of the spacecraft.
Types of meteorites
There are many different types of meteorites and they come from different places in the solar system, including the Moon, Mars, asteroids, and (possibly) comets. Understanding the relationships between the different groups of meteorites is an ongoing field of study.
Meteorites are generally broken up into three separate families based on structural properties and chemical composition. The three groups are 1) the chondritic meteorites, 2) the primitive achondrites, and 3) the achondrites.
While these classes refer to the presence or absence of a mineralogical structure called a chondrule, in fact not all chondrites have observable chondrules. Chondrules are spherical — or nearly spherical — droplets of minerals that reflect a history of melting, evaporation, and recondensation of preexisting solids in the young solar nebula before the formation of planets.
So, a better metric may be dividing the meteorite families into separate groups based on their whole rock composition.
Chondrites are meteorites that have approximately the same composition as the Sun (except for highly volatile elements — those that vaporize and escape at relatively low temperatures) and are thought to be sourced from primitive bodies that never underwent planetary differentiation.
Achondrites, on the other hand, are igneous rocks (melts, partial melts, and melt residues) that have been delivered to Earth from planetary bodies that were subject to some degree of differentiation, and thus can have a bulk chemical composition quite different from the Sun.
The primitive achondrites are somewhere in between, lacking structural chondrules but being closer in chemical composition to traditional chondrites. Meteorites are further divided into classes, clans, and groups based on their oxygen isotope abundances and other, more specific chemical properties.
How can we use meteorites to study other places in the solar system?
A simple visual inspection of some meteorites can reveal a specimen unlike anything commonly found on the surface of our planet. For example, iron meteorites, which are mostly made of an iron-nickel alloy with occasional inclusions of other minerals, are very different from most rocks found in day-to-day life. Their recognition as meteorites gives us an understanding that other planetary bodies in our solar system may be made of materials not regularly found in our daily experience.
After a visual inspection to begin the classification process, the study of meteorites can be remarkably detailed. A meteorite’s bulk mineralogy and texture can be further classified from slices of meteorites called thin sections. More precise studies make use of powerful scientific techniques, some of which overlap with how we study the other planets in our solar system. For example, reflectance data of asteroids collected at telescopic observatories allows us to understand how much sunlight is reflected by an asteroid’s surface. We can compare this data to the reflectance properties of meteorites, which then allows us to make interpretations about how the surface compositions of asteroids are related to the minerals and structural properties of meteorites.
What are some connections between (16) Psyche, the mission, and meteorites?
The Psyche mission will visit the largest M-type asteroid, called (16) Psyche. The name “(16) Psyche” refers to the order in which asteroids were discovered in recent centuries — with the asteroid we call Psyche being the 16th asteroid to be discovered. The asteroid is classified as an M-type asteroid because its reflectance properties are consistent with metal, especially iron meteorites. So even the classification of the asteroid is based on the study of meteorites!
The Psyche spacecraft will be equipped with a set of science instruments that will test hypotheses of the formation of (16) Psyche. The three instruments are 1) a multispectral imaging system, 2) a gamma ray and neutron spectrometer, and 3) a magnetometer. They have been carefully selected and designed to make robust measurements of the properties of (16) Psyche. Their designs are strongly informed by our knowledge of meteorites.
From the perspective of the spacecraft, (16) Psyche will become more than a single pixel when the spacecraft is about 4 million kilometers away. Until this point, we will continue to observe and admire the asteroid as an enigmatic point of light, much as I admired the shooting stars on that warm summer night in Colorado. As (16) Psyche fills more of the field of view of the spacecraft’s cameras, some questions may be answered, but many more will be raised. I greatly look forward to seeing the first images returned by the spacecraft and working to understand how this object influences our understanding of not only our own planet, but the solar system as a whole.