Polarized Light Provides Clues to Asteroid Composition
So what could the polarized light from an asteroid really tell us?
For starters, it can guide NASA scientists when it comes to selecting an asteroid to land on and to investigate its metal content.
Exploring the link between the polarization of light observed from an asteroid and its metal content is Dr. Sloane Wiktorowicz, a member of the technical staff, Remote Sensing Department, Engineering and Technology Group.
Every photon, he explains, is polarized. This refers to the orientation (vertical, horizontal, or in between) of the light wave’s vibration in space. Sunlight consists of an equal number of vertically- and horizontally-vibrating photons and is therefore “unpolarized.” Sunlight reflecting off a surface, such as an asteroid, will have an excess of photons vibrating with a certain orientation. This light is said to be “polarized,” and the excess of photons that are polarized (typically about one percent) depends on the material properties of the surface.
Using an Aerospace-developed sensor attached to a telescope at Lick Observatory near San Jose, California, he carefully and repeatedly observes and tracks the polarization properties of certain asteroids he has identified.
Through the telescope’s mirrors, he observes these asteroids and their bright and dark patches at different times during their rotation periods. This effort is being supported by a NASA Solar System Observations grant.
“Asteroids typically make a full rotation every seven or eight hours, which is pretty rapid,” he says. “By observing the changes in the polarized light during rotation, you’ll identify the pattern of bright and dark patches on a particular asteroid.”
He notes that the polarization fraction of visible light from the asteroid depends on whether there is a bright or dark patch in view. Because of the physics of how light scatters off rough surfaces, bright patches have weak polarization, while dark patches are strongly polarized. The fraction of polarized light, he says, changes depending on the number of light or dark spots on the asteroid.
To effectively study the asteroids he has selected, Wiktorowicz is developing a new sensor for Aerospace’s telescope mounted atop the corporation’s laboratories in El Segundo, California. He also plans to use more powerful telescopes such as Gemini North on the Big Island of Hawaii as well as continuing to use the Lick Observatory, which is owned and operated by the University of California.
In addition, the presence of surface metals may be detectable based on an asteroid’s circular polarization.
“Understanding the link between polarization and surface features, such as composition and metal content, is completely new. Only one asteroid was known to show polarization variations with rotation since the late 1970s, but we’ve discovered that two others, so far, show this effect,” he says.
“When considering whether to land on a particular asteroid, the defining factor may be whether observation indicates that an asteroid has a high metal content,” Wiktorowicz adds.
Studying asteroids and their various properties, and in particular their possible metal content, may play a deciding factor in which ones NASA will be inclined to land a rover on for future missions and further study. Another key characteristic in selecting an asteroid for future study, he says, is its size.
The planets and some large asteroids are thought to have formed by billions of years of accretion of rocky material onto their surfaces, according to Wiktorowicz. The high pressure at the core of large bodies causes intense heating, which melts rock and allows heavy metals to descend to the core, he says. Spherical planets and asteroids must have been molten at some point in the past in order to obtain their shape. Thus, metal in large asteroids is expected to be locked up in their cores instead of on their surfaces, he explains.
However, many asteroids are the debris left over from collisions of protoplanets, or planetary embryos still in the process of formation, billions of years ago. These asteroids, pieces of larger bodies, are expected to have their metals spread throughout the entire body. By measuring surface metal content, the formation history of the asteroid may be determined.
Circular polarization, which occurs when light is composed of both vertically and horizontally polarized photons differing in phase by 90 degrees, may highlight metal content and, in these instances, Wiktorowicz says there are three likely scenarios. When there is zero circular polarization during asteroid rotation, this suggests the entire surface has low metal content and that the asteroid may be a fragment originating near the surface of a larger body. When there is constant, high circular polarization during asteroid rotation, this suggests the entire surface has high metal content and that the asteroid may be a fragment originating from near the core of a larger body, according to Wiktorowicz.
Finally, when there is a strong change in the circular polarization during the asteroid’s rotation, he says this suggests there are metallic patches on the surface and that the asteroid may be a pristine body formed during the birth of the solar system.
Trying to determine the composition of an asteroid is something he finds both fascinating and fulfilling. “It’s what’s left over after the planets were formed,” says Wiktorowicz. “We are observing points of light in space and wondering if there was a bigger body of magnesium and other materials that were shattered.”
NASA is working to find ways of measuring different materials on asteroids, he says. “We’re planning to compare the data I’m collecting with in situ observations from space missions. We’re working to build confidence in knowing what our data means, which will lead us in a particular direction,” he adds.
The work being done at Aerospace will likely contribute to answering the question of which asteroid to visit first.