SOLAR ORIGIN OF "PLANETARY" NEON? D. Heymann, Depts. of Geology and Space Science and Astronomy, Rice University, Houston, Texas 77001. "Planetary" gas contains Ne with 20~e/22~ew 8-10; the ratio in "solar" Ne is 12.5. The apparent 22~eexcess in "planetary" Ne has been assigned to 22~adecay, with 22~aformed by an early proton bombardment of grains in the solar system (1), proton bom- bardment of a gaseous solar nebula (2), or a nova or su ernova explosion (3). Because of the short mean lifetire of 2SNa of -3y. the discovery of live 22Na in the solar system would demand a local origin. Excess 22~e,calculated for bulk carbonaceous meteorites (4) yields the following average contents (in ~o-~c~~sTP/~):C I=0.93 ; C II=0.75; C III+IV=0.30. This trend parallels the average car- bon contents as it should, because nearly all of the "planetary" Ne is contained in a "carbon" phase (5) . ~utit also closely follows the proportions of the undeple ted low- temperature f rac- tion of Anders (6), which strongly favors a local ori in of 22~a. C 1's contain about 3 wt% "carbon". Hence 22Na/ ? 3~a= 3 .lo-7/~, where X = Na content of "carbon" in %, . The Na con- tent of "carbon" is not known. Ryter et al. (7) have shown that the energy requirements for generating L~/H 10'~in a full convective pre-main sequence sun are virtually prohibitivANa production in the sun dif- fers from Li production as follows: 1 the sum of target element abundances is only -0.05 that for LiT 2 threshold energy is -4 MeV as against 30 MeV for Li (7) ; 3 22Na is radioactive. The effective cross sections, however, are very similar, Calcula- tions of the energy expended to generate one 22Na atom show that this is about the same as that required to form one Li atom, when the proton spectrum is rather flat; i.e. y w 3(d~/dE= k~-Y). The basic reason for this is that the unfavorable target abun- dance is overcome by the utilization of the copious protons 4-30 MeV. The short mean lifetime of 22Na demands that freshly formed 22~amust be instantaneously ejected from the sun, which is very likely for the sun during a T-Tauri phase. Whether the ejected plasma could have been efficiently trapped around the sun, and whether any "carbon" could have formed quickly under these con- ditions needs further investigation. If both are possible, then 22~a/~in the flares could have been as small as 9.10'~~/~. This is a lower limit for a nebula of 1 Mo , accreting sole- ly from solar ejecta, because : 1 some 22~ais bound to decay be- fore it can be trapped in "carbon", 2 dilution with already extant, 22~a-freegas about the sun is possigle. However, if the "spi- king" with 22~aaffected only the region of the terrestrial pla- nets and asteroids, then the "nebula" is only -0.02 Ma , which would lower the ener y requirement proportionately. On balance, solar synthesis of 2qNa and subsequent formation of "planetary"

0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System SOLAR ORIGIN OF "PLANETARY" NEON?

D. Heymann

neon seems, energetically, possible .. The 22~acannot detectably change the composition of solar Ne b decay in an unfractionated solar gas. Since rough1 1/2 of the 32Ne in "planetary" gas is from 22Na decay, trapped 2 3Na/22Ne 3 -106.~in the "carbon". The composition of terrestrial Ne could mean that 23Na/22Ne trapped in ancestral terrestrial "car- bon" was N 10% smaller than in carbonaceous meteorite "carbon", or that the earth received a modest supply of solar neon in addi- tion to that brought in by its own "carbon". It is assumed here that 22~efrom 22Na associated with the bulk of the Na in the carbonaceous meteorites was lost. However, very small amounts may have survived in Na-bearing , Ne-retentive phases other than "carbon", which would repre sent the so-called Neon-E. The sun would also generate 36~1and I291 along with 22Na (cf. 2). Whether an effects should be detectable in 36~r and 129xe depends on : l36C1/35C1 and 1291/1271 ratios in the flares; 2 the might fractions Y and Z of 35~1and 1271 in the "carbon". In general, no isotopic anomalies are expected for nu- clides which have only very short-lived radioactive precursors as these decayed in the gasphase prior to chemical fractionations. This is true, irrespective of whether the precursors *re formed by proton or neutron-induced nuclear reactions. The flares would also have produced 3~ which decays to 3~e. However, hydrogen is probably a structural element of the "car- bon". The decay of 3~ bound to a peripheral -CH group for in- stance would almost certainly result in loss of 3He so formed. The decay of 3~ bound to "interior" carbon atoms (if this occurs) might cause sufficient local structural dislocations to whatever "cages" hold the inert gases so tightly in the "carbon" that, again, the 3He might have escaped. Thus, it is conceivable that the 3He/4He ratio in the "carbon" is still very close to the value in the pre-main sequence sun.

REFERENCES: (1) Jef ferey P.M., Anders E . (1470) Geochim. Cosmochim. Acta 34, 1175; (2) Reeves H., Adouze J. (1968)' Earth Planet. a. Lett. 4, 135; (3) Clayton D.D. (1975) Nature, 257, 36; (4) From Mazor E., Heymann D., Anders E. (1970) Geochim. Cosmochim. Acta -34, 781; (5) Phinney D., Frick U., Reynolds J.H. (1975) Lunar -Sci. -VII, vol. 11, pp. 691-693; (6) Anders E. (1965) NASA CR-299 Report; (7) Ryter C., Reeves H., Gradsztajn E., Adouze J. (1970) Astron. s.8, 389.

0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System