In January of this year, Dr. DANIEL GOLDIN1 ordered the release of mass spectrometer data from the 1995 entry of the Galileo probe into Jupiter. One can find the data on the internet at the web address: http://webserver.gsfc.nasa.gov/code915/gpms/dat asets/gpmsdata.html The Galileo Probe Mass Spectrometer took readings every 0.50 seconds (NIEMANN et al.2) producing signals for mass/charge (m/q) = 2-150. This includes all stable noble gas isotopes.
Signals for noble gas isotopes at mass/charge (m/q) = 21, 40, 78, 124 and 126 are difficult to distinguish from background and those at m/q = 20, 22, 36, 38, 80, 82, 84 and 86 displayed significant variations. Contamination may have resulted from incomplete adsorption of hydrocarbons by Carbosieve, the material used in the enrichment cells (NIEMANN et al.2). Thus, the most reliable results lie in the high mass region that includes xenon, the noble gas of choice.
What makes xenon the noble gas of choice? Besides the fact that it has more stable isotopes than any other noble gas and lies in the region of the mass spectrum that has the least contamination, it has also provided the most information about the ea rly history of the solar system and the origin of its elements. Xenon isotopes contain decay products of the first two extinct radionuclides3,4 discovered in the solar system in the 1960s. In 1960, xenon provided the first hint that isotopic ratios of primordial elements might vary within the solar system5, and xenon isotopes first carried the message in 1972 that one form of xenon, Xe-X, might have been "... added to our solar system from a nearby supernova, although no evidence for the addition of products from a separate nucleosynthesis event has been found in other elements." (MANUEL et al.6, p. 100)
Soon after confirmation7 of excess 124,126Xe and 134,136Xe in the Allende meteorite from the p- and r-processes of nucleosynthesis8, xenon isotopes in the Murchison meteorite revealed a complementary component9, Xe-S, characterized by excess 128-132Xe from the s-process of nucleosynthesis8. More important for the present study are the finding10 and confirmation11 that primordial He is always closely coupled with isotopically strange Xe-X in meteorites.
Mineral separates of the Allende carbonaceous chondrite illustrate this close coupling10 of primordial He with xenon of strange isotopic composition (Fig.1). The isotopic ratio on the vertical axis, 136Xe/134Xe, consists of isotopes that are only produced in the r-process8. The elemental ratio on the horizontal axis consists of the nuclide widely rated as second most abundant in the solar system, 4He, and the reference xenon isotope, 134Xe. Linkage of primordial 4He with Xe-X is seen in all classe s of meteorites11, and in each case isotopic ratios extrapolate to xenon of "normal" isotopic composition as elemental abundances of He vanish.
At least two r-processes created the heavy xenon isotopes shown in Fig. 1. One r-process made 136Xe/134Xe 0.8 in a region with little or no He. Since fusion depletes light elements like He from the stellar interior, abundant Fe there might s erve as seed nuclei for this r-process. The other r-process made 136Xe/134Xe 1.0 in a region with abundant He. Since spontaneous and high energy fission typically produce 136Xe/134Xe 1.0, this xenon component may be from the process that a lso created actinide elements like Th and U. Burbidge et al.8 (p. 582) note that "The r-process path is terminated by neutron-induced fission at A~260 and the nuclear matter is fed back into the process at A~108 and A~146."Enrichments of specific isotopes of one element, like 136Xe, and their close association with chemical abundances of another element, like He, call into question conventional ideas about the origin of the solar system and its elements. Currently, two theories on the origin of the solar system and its elements compete for public attention.
The first and most popular theory contends that the solar system began as a well mixed cloud of gas and dust, and our Sun formed as a fully convective star from Jupiter-like material. All planets initially had a composition like Jupiter, but the inner part of the solar system was hotter. This caused the terrestrial planets to lose the bulk of their mass as volatile elements and to segregate more dense elements like iron into their cores. According to this model, Xe-X and other isotopically anomalous elements represent only a tiny amount of exotic nucleogenetic material that was added to the solar system12,13, either injected into the solar system from nearby stars or carried into the solar system in interstellar dust grains that somehow became embedded in meteorites14.
The second idea proposes that the solar system began as heterogenous debris of a spinning supernova (SN) that exploded axially15 to produce a bipolar nebula and a heterogeneous accretion disk in the equatorial plane surrounding the collapsed SN core. According to this model, our Sun formed on the collapsed SN core and terrestrial planets accreted heterogeneously. Their iron cores formed first in the central Fe rich region. Giant planets like Jupiter formed from outer layers of the SN, the source of Xe-X and abundant light elements like H, He and C. Intrasolar diffusion enriches light elements and lighter isotopes of individual elements at the solar surface to produce its high abundance of light elements and mass fractionated16 isotopes of solar wind xenon (SW-Xe).
Jupiter acts as a test for both theories. If the solar system began as homogenous material and diffusion has not enriched lighter nuclides at the solar surface, Jupiter would contain xenon that looks very much like that found in the solar wind (SW-Xe). On the opposite end of the spectrum, if the solar system began as heterogenous SN debris, Jupiter would contain Xe-X. Our analysis of the xenon isotopes for the testing of these theories began with separation of the xenon isotopes into three groups.
Separation of the xenon peaks for analysis occurred according to the relative abundances of each. This grouped the isotopes into three mass regions and reduced any problems from mass fractionation or preferential transmission in the instrument based on abundance. This also separated the isotopes on basis of nuclear origin. Xenon's three most abundant isotopes, 129Xe, 131Xe and 132Xe, are from a combination of the (r+s) processes of synthesis8. The isotopes of medium abundance, 134Xe and 136Xe, are from the r-process of synthesis8. Isotopes from the p-process, 124Xe and 126Xe, were too low in abundance for measurement. Thus, the two isotopes of lowest measured abundance in the Jovian atmosphere, 128Xe and 130Xe, are primarily from the s-process of synthesis8.
Of these three groups of isotopes, only the r-products are able to distinguish between SW-Xe and Xe-X. Specifically the 136Xe/134Xe ratio that was shown earlier in Fig. 1 most clearly separates Xe-X and SW-Xe in the atmosphere of Jupiter.
For the Galileo probe measurements in Jupiter, the possibility of contamination had not completely disappeared even in the high mass region of xenon. Because of this, a known high mass hydrocarbon contaminant (Ct) that appeared in every sweep (m/q=77) proved useful for extracting the actual ratios from the contaminated numbers. Fig. 2 shows this on a graph of 136Xe/134Xe vs 77Ct/134Xe for the Jupiter data.
Xenon in both enrichment cells have values of 136Xe/134Xe higher than those in the solar wind. Extrapolation of the isotopic ratios to 77Ct/ref.Xe-> 0 gave the values of uncontaminated xenon isotopic ratios shown in Table 1 for Jupiter. In the case r-products, the reference isotope is 134Xe and the 136Xe/134Xe ratio extrapolates to a value of 1.03 (Fig. 2). Besides Xe-X, no other type of primordial xenon has 136Xe/134Xe > 1.
This value of 136Xe/134Xe = 1.03 also occurs in He-rich separates of Allende, as shown in Fig. 1. The presence of Xe-X in the He-rich atmosphere of Jupiter suggests that the linkage of Xe-X with primordial He spanned planetary distances in the primitive solar nebula.
Our finding that isotopic abundances of xenon in Jupiter are closer to those of Xe-X than to SW-Xe and the implied linkage of elemental and isotopic heterogeneities over planetary distances in the primitive solar nebula lends credence to the hypothesis that the solar system began as chemically and isotopically heterogenous stellar debris15, rather than as a well-mixed cloud of gas and dust to which a small amount of exotic nucleogenetic material was added12,13,14.Acknowledgement- We are grateful to Dr. Daniel S. Goldin, NASA Administrator, for releasing the mass spectrometric data from the Galileo Probe entry into Jupiter and to the Galileo Probe Mass Spectrometer Team for the beautiful planning, designing and execution of the experiment that made this pioneering measurement of isotopic species in Jupiter possible.
TABLE 1. Xenon isotopic ratios in Jupiter, in "strange" Xe, and in the solar wind.
|Solar Wind Xe15|
|128Xe/130Xe||0.566 ± 0.070||0.569 ± 0.099||0.56||0.55||0.51|
|129Xe/132Xe||1.054 ± 0.091||1.080 ± 0.101||1.04||1.06||1.06|
|131Xe/132Xe||0.803 ± 0.042||0.657 ± 0.065||0.89||0.84||0.82|
|136Xe/134Xe||0.985 ± 0.124||0.896 ± 0.079||1.03||1.04||0.80|