NEAR Shoemaker Science Update June 20, 2000
On May 30 NEAR Shoemaker announced the significant discovery that Eros is not a differentiated asteroid. In other words, Eros was never subjected to extensive melting and differentiation, which means segregation into layers of distinct compositions. How did NEAR Shoemaker make this inference, and what might it mean for the history of Eros?
NEAR Shoemaker has made the first detection of x-rays from an asteroid. These x-rays are not generated by Eros in the sense that the sun generates x-rays, or that x-ray tubes and synchrotrons on Earth generate x-rays (all of these produce x-rays by first creating high energy electrons, and then smashing them into a target or bending their paths in a magnetic field). Eros produces x-rays when illuminated with x-rays from the sun, but this emission process is fundamentally different from that which produces the reflected sunlight imaged by the camera. An x-ray photon from the sun can be absorbed by a single atom in the surface of Eros, if the photon has enough energy to eject an electron from the atom (the so-called photoelectric effect). We are interested in the case when the electron is ejected from the innermost shell. The atom is now missing an electron, and the vacancy in the innermost shell is quickly filled when another electron in a higher energy shell (usually, the next higher shell) drops in, emitting a new x-ray photon at one of the characteristic energies of the atom. This energy is usually close to, but less than, that of the original, incident x-ray photon. The filling of the first vacancy can, of course, create another vacancy, leading to emission of another characteristic photon (at lower energy) when the new vacancy is filled, and so on. This entire process, in which the absorbing material emits a spectrum of characteristic radiations after a photoelectric absorption, is called fluorescence. It is how ordinary fluorescent lamps work. Eros is acting as a fluorescent lamp, except that it glows in x-rays rather than visible light. The ordinary (diffuse) reflection process (meaning that by which Eros shines in visible light) also occurs for x-rays, but fluorescence is more important for x-rays.
The characteristic x-rays are the fingerprints of Fe, Mg, Si and other key elements at the surface of Eros. By measuring the strengths of these emissions, NEAR Shoemaker measures the numbers of the corresponding atoms in the surface. This measurement tells us about differentiation, because if the kind of differentiation we are interesed in for Eros has occurred, it would affect the relative proportions of these elements. Specifically, we are looking for metal-silicate differentiation, in which iron-nickel liquids settle down into the center of the body, leaving behind upper layers of silicate rock. This process naturally depletes the iron at the surface relative to silicon, for example, together with any species that would preferentially dissolve in the iron-rich melt. This type of differentiation occurred at Earth, together with other types — the silicate layers outside Earth’s core differentiated further into a mantle and a crust of distinct compositions. In the outer solar system, still more types of differentiation are found, such as between a rocky silicate core and an icy exterior for Ganymede at Jupiter.
NEAR Shoemaker’s x-ray measurements so far tell us that Eros is not differentiated, which means that iron-nickel grains on Eros are still intimately mingled with silicate grains, as they would be in primitive meteorites like the ordinary chondrites. There are complications, of course — some meteorites are interpreted as products of partial melting or differentiation, and NEAR Shoemaker will need more data to explore that possibility.
Why do we talk so much of differentiation? It is a milestone in the evolution of all the terrestrial planets (Mercury, Venus, Earth and its Moon, and Mars), which are differentiated bodies. However, the ~500 km asteroid 4 Vesta is also differentiated and perhaps deserves to be considered as another terrestrial planet (only historical accident truly disqualifies Vesta as a planet). It would be satisfying, but untrue, to say that planets must be differentiated bodies — Pluto is called a planet, but we do not know if it is differentiated. In any case, larger bodies tend to be more readily differentiated, because they trap heat and because they have stronger gravity, but we do not know how small a body can be and still become differentiated, nor do we know how large one can be and still avoid differentiation. There is at least one asteroid even larger than Vesta, 1 Ceres, that is believed to be undifferentiated.
Hence, we can say from the NEAR Shoemaker x-ray data that Eros is not completely differentiated, and that if Eros was once part of a much larger parent body, that parent was also undifferentiated. We cannot say whether this parent body ever was or was not larger than Vesta. There are many forks in the road to becoming a planet, and we have yet to see most of them.
Andrew Cheng NEAR Project Scientist
