Solar System Surprise

During two years in interplanetary space, a NASA spacecraft called Genesis collected particles from the solar wind. Its purpose was to capture these atoms from the Sun and return them to Earth, where scientists would determine the solar abundances of various stable isotopes of nitrogen and oxygen, as well as other elements. These solar abundances can also be thought of as solar system abundances, since the Sun and planets all formed from the same cloud of matter and most of their combined mass resides in the Sun. The mission to determine these abundances might have been a complete success if the spacecraft's parachutes hadn't failed during re-entry, causing the solar wind collectors to shatter upon impact with the ground.

Fortunately, in addition to the passive collectors, the Genesis capsule contained an instrument, designed by Los Alamos's Jane Nordholt, Roger Wiens, Ronald Moses, and Steven Storms, that concentrated solar wind particles onto a small target. The target managed to survive the crash, thanks to its strong mechanical design. (Wiens describes Genesis as "the biggest comeback mission since Apollo 13.") The surviving target was analyzed for the abundances of three isotopes of oxygen—16O, 17O, and 18O—and two isotopes of nitrogen—14N and 15N—in order to compare the solar system concentrations of these isotopes to those found here on Earth. As it turns out, relative to the bulk of the solar system, our planet appears to be an anomaly.

solar system photo

Early in the history of our solar system, the matter that would eventually form planets and other bodies acquired slight differences from the solar system's original chemical composition.

In terms of mass, oxygen is by far the most abundant element in the inner solar system planets, Mercury through Mars, and 16O is by far its most abundant isotope. Samples from the Earth, Moon, Mars, and meteorites generally share the same abundances of all the stable oxygen isotopes, but the Genesis solar wind samples revealed 7 percent less 17O and 18O (relative to 16O) than what has been found in these inner solar system samples. That is, the abundance ratios of both 17O/16O and 18O/16O were 7 percent smaller in the solar wind. It follows that whatever caused the 7-percent enrichment of these two isotopes in the inner solar system bodies relative to the Sun operated on both isotopes in the same way, even though their masses differ. One possibility, known as photochemical self-shielding, would have operated when the solar system was very young, as molecules of carbon monoxide (CO, the most abundant oxygen-bearing gas at the time) were broken up by intense ultraviolet radiation from the young Sun. Wavelengths most efficient at breaking up C16O were consumed relatively close to the Sun, due to the much greater abundance of the 16O isotope. Wavelengths efficient at breaking up C17O and C18O traveled farther out into the planet-forming region, freeing up an excess of the heavier oxygen isotopes for incorporation into planets.

The Genesis results for nitrogen were similarly enlightening. The team found 38 percent less 15N (relative to the much more common 14N) in the solar wind than is found in Earth's atmosphere. The same photochemical self-shielding effect could be responsible for this deficiency, with solar ultraviolet light selectively breaking up the molecule N2 rather than CO in this case. However, with only two stable isotopes of nitrogen, evidence for its self-shielding is less conclusive than it is for oxygen. Other nitrogen samples from the solar system include meteorites, which come from relatively nearby in the inner solar system and have a similar isotope composition to the Earth, and comets, which come from the outer solar system, beyond the planets, and have more than double the 15N/14N ratio found on Earth. Taken together with the Genesis data, these results suggest that rocky, inner solar system bodies like the Earth had multiple sources of nitrogen: Some was primordial, like the solar wind composition, and some was enriched in 15N, like cometary composition. The mixture of these two sources led to an isotope ratio in between the two. Indeed, it is known that the planets were bombarded by comets in the past. However, Jupiter's nitrogen ratio matches that of the Sun. Thus, the Genesis results imply a mystery as to why the terrestrial planets' nitrogen was strongly influenced, either by comets or photochemical self-shielding, while Jupiter remained largely unaffected.

Genesis crash site in Utah photo

The Genesis crash site in Utah.

The measured abundances from Genesis validate the predictions of the photochemical self-shielding theory, at least for oxygen, but they also point researchers toward new mysteries to investigate, such as the cometary enrichment history of Earth, Jupiter, and other solar system bodies. In this sense, the Genesis experiment provided the best of both worlds, answering some questions and raising others—not too shabby for a spacecraft that suffered a terminal-velocity crash in the desert.

—Craig Tyler

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