See Inside January 2009

Does Dark Matter Encircle Earth?

Dark matter might exert measurable effects on Earth, the moon and gas giants

Dark matter is five times as abundant as normal matter in the universe. But it continues to be an enigma because it is invisible and nearly always passes right through normal matter. Astronomers only found out about dark matter by inferring its presence from the gravity it exerts—notably, it keeps spinning galaxies from flying apart. Rather than peering at distant galaxies to study it, though, astronomers might want to look closer to home: dark matter could be exerting measurable effects in our own solar system.

Specifically, investigators should target Earth and the moon, insists theoretical physicist Stephen Adler of the Institute for Advanced Study in Princeton, N.J. If the mass of Earth and the moon when measured together seems greater than their masses separately, he explains, the difference could be attributed to a halo of dark matter in between.

Adler reaches this conclusion in part after examining studies that measured the mass of the moon with lunar orbiters and that of Earth with the LAGEOS geodetic survey satellites—
laser-beam-reflecting spheres that have been in orbit for many years now. Lasers fired at the satellites reveal the radius of each satellite’s orbit and how long each takes to complete that orbit. From such measurements, scientists can calculate the gravitational pull on the satellites and, hence, the amount of mass exerting that pull.

Next Adler examined research that gauged the distance from Earth to the moon with lasers reflecting off lunar mirrors planted by the Apollo missions. If Earth exerts an unusually stronger pull on the moon, which lies roughly 384,000 kilometers out, than on the LAGEOS satellites, about 12,300 kilometers away, the added pull could be attributed to a dark matter halo between the moon and the artificial satellites. Based on current data, Adler estimates in the October 17 Journal of Physics A that at most some 24 trillion metric tons of dark matter lies between Earth and the moon. Such a dark matter halo might explain the anomalies seen in the orbits of the Pioneer, Galileo, Cassini, Rosetta and NEAR mission spacecraft, he adds.

Adler also speculates that dark matter could exert dramatic effects on the four gas giants in our solar system—Jupiter, Saturn, Uranus and Neptune. If these massive worlds have gravitationally captured dark matter, then dark matter particles could smash into them—rare events but enough to heat up the gas giants and account for why the insides of these planets (and even Earth) seem hotter than known mechanisms can explain. It might also account for why Uranus seems anomalously cold—the planet is bizarrely tilted, perhaps because of a colossal impact, and Adler surmises that this collision might have knocked away most of the dark matter cloud that might typically have heated Uranus.

The possible planetary heating by dark matter may also hold clues to the substance’s unknown properties—how often it collides with normal matter, say, or whether dark matter clumps around stars and planets as opposed to spreading evenly across the galaxy, remarks theoretical astrophysicist Ethan Siegel of the University of Portland. For example, if dark matter particles are their own antiparticles, as some researchers theorize, the energy released when they annihilate themselves would heat up the planets far more than mere collisions with atoms. Such a scenario would imply that dark matter cannot clump much in our solar system, or else the solar system would be much hotter.

Astrophysicist Annika Peter of the California Institute of Technology is skeptical that dark matter is altering the heat of the planets, saying that it would take “a seriously unrealistic amount of dark matter.” And astronomer Andrew Gould of Ohio State University doubts that much dark matter clumps in the solar system—he argues that gravitational interactions with the planets should mostly eject it, just as they cleared out much of the solar system’s original normal matter. Still, Siegel thinks, as the solar system plows through the galaxy, it could be accreting additional dark matter.

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