Asteroids and moons that lack a protective atmosphere are bombarded by micro-meteoroids: small but high-speed space rocks. These micro-meteoroids create tiny craters and impact residue. Energetic hydrogen and helium atoms from the sun also leave their marks on asteroids, irradiating surface and can creating visible blisters.
By building our understanding of these "space weathering" processes, we will be better able to make a connection between rocks that we can measure precisely in the lab on Earth, and the asteroids and moons studied by telescope.
Scientists have studied space weathering with soils from the moon returned by the Apollo astronauts, and with surface soil from asteroid Itokawa returned by Japan's Hayabusa robotic space mission.
But we need samples from more bodies (besides the moon and Itokawa) to understand how space weathering affects different types of bodies in the Solar System.
I have acquired samples of so-called "gas-rich" meteorites. The composition of this gas suggests that these rocks-from-space are samples of the surface soils of asteroids or small planets. But they are complex assortments of different stuff: the collision that created the meteorite mixed everything up into a "breccia", containing some surface soil, some deeper rock fragments, and maybe even some of the _impacting _asteroid.
I put these gas-rich meteorites in water and subjected them to more than 5,000 cycles of freezing and thawing. Water expands when it freezes, so the repeated freeze-thaw process broke up the meteorites into their constituent particles: a powder. I imaged this powder with an electron microscope, yielding thousands of secondary electron images of the surfaces of individual particles. Some of the particles are from the top surface of the asteroid soil, exposed to violent processing of space — but how do we know which ones?
We can use the lunar soils returned by the Apollo astronauts, as well as the asteroid Itokawa samples, to help us understand what we're looking for.
Craters are the direct result of impacts of high-speed projectiles, and they look somewhat similar on size scales from micrometers to kilometers. Microcraters (craters around one micrometer in diameter), are abundant in lunar soils and have been identified in samples from the surface of asteroid Itokawa. Craters can be differentiated from crevices and or holes in rocks called "vugs" by their raised rims (bright in secondary electron images) and surrounding impact residue (e.g. splash melt, see below).
Splash Melt (impact residue)
Secondary electron images of splash melt features on two Itokawa particles. Gray scale bars are one micrometer. Source
Splash melt is generated during a micro-meteoroid impact and can be flung far from the impact site on bodies, like asteroids, with low gravity. Splash melt can take a variety of shapes, but it looks like a liquid that impacted something and quickly froze solid.
Solar Wind Blistering
Secondary electron images of a solar-wind blistered surface (A) and a non-blistered surface (B). Scale bars are one micrometer. Source
The Sun continually spews out energetic atoms called the solar wind. Helium is the second most abundant atom after hydrogen. Helium impacts airless bodies like asteroids, embeds itself in the top surface, and diffuses so that little pockets of helium gas form just under the skin of the rock. The surface of the rock then becomes blistered. These blisters are full of helium gas.
Other Weird Stuff
There could be other products of space weathering in these samples that we're not yet aware of. Human eyes are excellent at picking out strange objects!
But how do we find these features in thousands and thousands of secondary electron images?
That's where you come in!
In the Asteroid Soil Search project, volunteers will look through secondary-electron images of disaggregated gas-rich meteorites to find any of the above features. If you see one of the above features — craters, splash melt, surface blisters, or just something really weird — just click "Yes". If you don't, click "No". If the image is out-of-focus, or has only substrate (carbon tape) with no visible meteorite grains, click "Bad image". It's as simple as that!
I have included calibration images to determine how well volunteers can locate impact craters of a given size. These are real images of disaggregated meteorites with impact craters (from foils returned by NASA's Stardust mission) overlayed and blended onto them. Some examples are below:
The initial images for this project are from the Fayetteville meteorite, a five-pound gas-rich ordinary chondrite (type H4) that fell the day after Christmas 1934 in Fayetteville, Arkansas.