FEATURE ARTICLE
The Galactic Environment of the Sun
The heliosphere appears to protect the inner solar system from the vagaries of the interstellar medium
Priscilla Frisch
A Changing Galactic Environment
We do not know whether the interstellar cloud complex flowing past the sun is a homogeneous structure. On the basis of more distant interstellar clouds, it's quite possible that the Local Interstellar Cloud contains relatively small structures (perhaps 100 to 10,000 AU across) with very high densities (more than 1,000 particles per cubic centimeter). If our solar system should pass through such a dense cloud fragment, the dimensions of the heliosphere would change dramatically.
My colleague Gary Zank at the University of Delaware and I have recently modeled the changes that might take place should the heliosphere encounter a dense interstellar cloud. If the density of the Local Interstellar Cloud increased to 10 particles per cubic centimeter, the heliosphere would contract to a radius of about 15 AU and the heliopause would become unstable (oscillating in and out of existence). The density of interstellar hydrogen at 1 AU would increase to about 2 atoms per cubic centimeter and dramatically alter the interplanetary environment of the earth. (By comparison, virtually all of the interstellar hydrogen is ionized before it gets to the earth's orbit under current conditions.) A more severe scenario—say a cloud with a density of 1,000 atoms per cubic centimeter—would alter heliosphere physics entirely and probably contract the heliosphere to within a few AU of the sun. Planets such as Saturn, Uranus, Neptune and Pluto (all of which are outside 9 AU) would be fully exposed to the flux of interstellar neutrals. Interstellar gas would overwhelm the solar wind at 1 AU. These simulations suggest that, to a certain extent, the solar wind acts to "protect" the inner planets from certain types of changes in the local galactic environment.
Changes in the sun's galactic environment, moderate or otherwise, must have taken place in the past. Indeed there is evidence on earth suggesting that the local galactic environment has not been stable. Ice-core samples from the Antarctic show spikes in the concentration of beryllium-10 (which has a half-life of 1.5 million years) during two events, one about 60,000 years ago and another about 33,000 years ago. What events could have caused these sudden increases in beryllium? One possibility is a sudden increase in the cosmic-ray flux on the earth's atmosphere, which would increase the precipitation of radioactive beryllium onto the planet's surface. A couple of mechanisms have been proposed to explain such an increase in the cosmic-ray flux near the earth, including a supernova shock and an encounter with a small, but dense cloud fragment in the Local Interstellar Cloud. The supernova proposal is consistent with the observation that interstellar dust grains within 30 light-years of the sun show indications of destruction by a shock wave traveling 100 to 200 kilometers per second. Still the causes of the beryllium spikes remain uncertain.
There is also evidence for older supernova events: Enhanced levels of iron-60 in deep-sea sediments have been interpreted as indications that a supernova explosion occurred within 90 light-years of the sun about 5 million years ago. Iron-60 is a radioactive isotope of iron, formed in supernova explosions, which decays with a half life of 1.5 million years. An enhanced presence of this isotope in a geologic layer indicates the recent nucleosynthesis of elements nearby in space and their subsequent transport to the earth (perhaps as part of dust grains).
There is an avenue for further research along these lines. In principle, the deposition of interstellar matter onto geologically inert surfaces within the solar system should provide evidence for changes in the local galactic environment. Since the outer planets would have experienced the raw interstellar medium more often than the inner planets, it might be especially illuminating to compare the deposition of interstellar dust on bodies of the inner solar system relative to those of the outer solar system. Since the size of the dust grains that can penetrate the heliosphere depends on the strength of the solar wind, such a record would allow space scientists to disentangle the relative effects of the solar-activity cycle from variations in the galactic environment.
The planets may hold a record of past changes in the sun's galactic environment, but future changes can only be anticipated by mapping the galaxy around us. Indeed, studies of ultraviolet absorption lines suggest that there may be two interstellar clouds between the sun and the star Alpha Centauri (the sun's nearest neighbor, about 4 light-years away). Could one of these clouds be a small, dense structure embedded inside the local cloud complex? We don't know since we do not have an adequate understanding of the velocity structure of the local interstellar medium. The "cloud fragment" may be nothing more than turbulence in interstellar gas. Whatever it is, if it is real, it should sweep past the sun within the next 3,000 years.
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