There Is More To The Story

The Moon itself is thought to have formed about 50–80 million years after the formation of the Solar System, possibly well after the rearrangement of the giant planets’ orbits. So it may not, after all, be the best place to look if we want to understand the earliest evolution of the Solar System. Where else can we look? Meteorites provide a possible answer. Some are very old, even older than the Moon. Lacking this information, it is not possible to piece together into a coherent picture what these extraterrestrial rocks tell us. If we knew where meteorites originate, we could send spacecraft to the parent asteroids to observe their surfaces and provide us with the necessary compositional and geological data of their source terrains. The first step in this endeavor is to establish how many distinct parent asteroids are sampled by meteorites. Some isotopes may be rare and difficult to measure. For instance, about 99.98 percent of hydrogen has one neutron, while only 0.02 percent is in the form of deuterium, with two neutrons. Also, depending on the degree of chemical evolution of the minerals, solid, liquid, and gaseous phases may have been present at some point in time, as typically happens on Earth.

Where Is the Love?

Oxygen is an element that fits the bill. On Earth, samples exhibit a variable concentration of 16O, 17O, and 18O, but their abundances are not independent of each other. Oxygen isotopes in terrestrial planets and asteroids. The dashed line indicates terrestrial samples. The Moon is very similar to Earth, while other extraterrestrial rocks depart significantly from Earth’s oxygen ratios. When researchers analyzed oxygen isotopes in meteorites, they stumbled on something unexpected. Meteorites, much to their surprise, do not follow the same terrestrial trend. For the same reasons, oxygen isotopes provide the most direct way to assess whether different meteorites are genetically related, or whether they derive from different parent bodies. In a few cases, this method has also been used to assess whether a rock of doubtful origin is truly extraterrestrial. Such oxygen isotope analyses have shown that meteorites originate from some 120 different asteroids. These could in principle allow scientists to multiply the kind of detailed investigations carried out on lunar rocks. The problem is how to find these parent asteroids among the hundreds of thousands of known asteroids.

Doing Nothing

Because of the great distance that separates us from the main belt, they appear through the lens of a telescope as faint specks of light, and we know very little about the individual objects. Chances are that their formation, via violent collisions, could have wiped out the prior evolution recorded in these rocks. To minimize all these complicating factors, the scientist’s dream is to link specific classes of meteorites with large and very old asteroids. Finding the right parent body and meteorite connection seems a hopeless task. They come in two main types. Howardites are roughly cobbled together mixtures, known as conglomerates, of both types of rocks and formed near the surface, probably by impact mixing. This implies that eucrites and diogenites are from the same parent body, and that is also indicated by similar oxygen isotopic ratios. Mineral and chemical analyses of these rocks have revealed a complex and fascinating evolution. But unlike Virgil’s Vesta, with her everlasting fire, once the internal heating of her namesake faded, with the exhaustion of 26Al, the asteroid solidified throughout, perhaps in a few tens of millions of years. And it has remained in the main belt, silently carrying testimony of its early formation until, several billion years later, we humans were able to reconstruct Vesta’s story from a few rock splinters collected on Earth. And it is that composition that first pointed the finger at Vesta. Scientists have a powerful tool to infer the surface properties of asteroids from millions of kilometers away by looking at the reflected Sun’s light.

What A Shame

The trick to inferring composition is to measure the intensity of reflected light at different wavelengths, using a spectrograph. On Earth, for instance, tree leaves appear green because they reflect the wavelength that we perceive as green, from 0.5 to 0.57 µm, while they absorb the red component. Spectroscopy is a very powerful technique because different minerals can be identified by the distinct patterns of absorption features in their reflected light, from which we can observe a dip in intensity at particular wavelengths because they have been absorbed. Spectroscopy has been widely used in the lab since the early 1800s to study the nature of the Sun’s light, but it only became possible to apply it to study asteroids in the early 1970s. And the largest of them is Vesta. Scientists needed to travel to Vesta to find out more. The clock was set in motion. It took Dawn just under four years to reach Vesta and perform the critical maneuver of orbit insertion. This allowed the spacecraft to leave its heliocentric orbit and become bound to Vesta. This was the first time in history that a spacecraft had orbited a main belt asteroid, the first of a long list of firsts in space navigation. I had the privilege of becoming a member of the Dawn science team weeks before the start of the Vesta exploration. I remember the mounting excitement as we waited for the first sequence of images to be beamed back to Earth as Dawn approached Vesta. A blurred speckle of light gradually came into focus, as each image provided more detail of the surface than the last. Finally, Vesta’s surface was revealed. Asteroid Vesta as seen by Dawn. Its surface is battered with impact craters, including the Rheasilvia Basin in the southern hemisphere with its central mound near the south pole. The ragged southern hemisphere is wildly irregular. At the center of a huge cavity, a massive mount towers some 15 km above a deep, circular valley. This landform, named the Rheasilvia Basin, is an impact structure 500 km in diameter. And Matronalia Rupes slopes down on the other side into a second roundish depression, which appears to be an older impact basin, 400 km wide, which has been named Veneneia.