Astronomy
Characterizing the Surface Composition of Asteroids through Spectroscopy
Quick fact
Asteroids are identified as C-type (carbon-rich) or S-type (silicate-rich) primarily by the shapes of their light spectra, which show distinct absorption bands caused by iron in minerals.
Why this is interesting
We can't land on almost every asteroid, yet we know what they are made of. How do we do it?
Read the full explanation
Understanding Characterizing the Surface Composition of Asteroids through Spectroscopy
Spectroscopy is like reading a fingerprint of light. When sunlight hits an asteroid, it reflects off the surface minerals. That reflected light is imprinted with the mineral's signature: certain wavelengths are absorbed by the mineral's atoms and molecules. By splitting the light into a rainbow (spectrum), we can see dark bands or dips where the asteroid soaked up specific colors. Each mineral has a unique pattern of absorption, so matching the pattern tells us what the surface is made of. For example, the mineral pyroxene produces strong absorption near 1 and 2 micrometers, while clay minerals show features near 3 micrometers.
A deeper explanation
The mechanism relies on physics: when light interacts with a mineral, electrons in transition metals (like iron) absorb light at specific energies, moving between energy levels. Silicates like olivine and pyroxene contain iron, which gives distinct absorption bands in the visible to near-infrared. Water or hydroxyl in hydrated minerals produces a characteristic absorption near 3 micrometers. Thus, the spectrum acts as a diagnostic of mineral composition. Additionally, the overall slope and color of the spectrum indicate space weathering, which alters the surface over time. By comparing asteroid spectra to laboratory spectra of meteorites, we can infer the asteroid's makeup and even link it to the type of meteorite that fell to Earth. This technique is central to understanding the building blocks of planets and the history of the early solar system.