How much of Earth is Composed of Dark Matter?
Dark Matter in our Backyard
A mysterious invisible matter, called “dark matter” is theorized by Science to make up almost 85% of the matter in the universe. Dark matter is considered important to scientific theories to explain the structure of the universe, how stars behaved at the edge of galaxies and how galaxies interacted. The general notion is that dark matter is “out there” somewhere beyond the Earth. However, is there dark matter in our backyard — in the Solar System and even on Earth?
If you asked your accountant, she would say “of course!” She would explain to you that there are multi-million-dollar scientific projects now being undertaken, underground and in our skies, to detect dark matter particles, passing through our planet and our bodies. Why would anyone spend so much money if dark matter could not be found on Earth? But are there any scientific reasons to believe that Earth (and even the Moon) may be composed of not only ordinary matter, but also dark matter?
Sensing by Spacecrafts and Satellites
Data from several GPS (Global Positioning System) satellites suggest that a halo of dark matter around Earth is causing our planet to be heavier than first believed. Using data on the satellites in the U.S. GPS, Russian GLONASS and European Galileo groups, Professor Ben Harris from Texas University in 2014 measured Earth’s mass as sensed by each satellite. From the calculations, Professor Harris concludes that Earth has more mass than that established by the International Astronomical Union.
Before this, in 2009, researchers at the Institute of Advanced Studies in Princeton, argued that changes in the speeds of space probes as they flew past the Earth could be explained by dark matter bound by Earth’s gravity.
Even earlier, there were observed deviations from predicted accelerations of the Pioneer 10 and 11 spacecrafts after they passed about 20 astronomical units on their trajectories out of the Solar System. The two spacecraft were launched in 1972 and 1973. Marcus Chown, an award-winning Science writer, journalist and broadcaster maintains that the Pioneer anomaly was caused by the presence of clumps of dark matter in the Solar System. Just when you expect gravity to peter out at the edge of the Solar System, it seems that the spacecraft experienced accelerations which could not be explained by the ordinary matter in the Solar System we knew. So, what were the origins of this dark matter in the Solar System?
Dark Matter Clouds
Jürg Diemand, a physicist at the University of California in Santa Cruz, US, and colleagues say that computations suggest that small clouds of dark matter pass through Earth on a regular basis. Perhaps a million billion of them drift around our galaxy’s dark matter halo. These clouds float through Earth every 10,000 years in an encounter lasting about 50 years, according to Diemand.
Priscilla Frisch of the University of Chicago says that the Solar System is currently sitting inside an interstellar cloud of dark matter. The existence of the cloud and its geometry can be deduced from its effect on the spectra of nearby stars and cosmic rays. She calculates that our Solar System first encountered the cloud (moving at right angles to it) between 2,000 and 8,000 years ago, i.e. at the dawn of human civilization.
Dark Matter Rain
About 90% of the Milky Way is composed of dark matter (higher than the 85% for the universe as a whole). Our Solar System and the Earth sits inside this dark matter halo of the Milky Way. As it orbits the galaxy at a speed of almost 220km per second, it sweeps through the invisible sea of dark matter particles. Every kilogram of ordinary matter on Earth encounters and scatters as many as a thousand WIMPs (a type of dark matter particle) per day.
Dark Matter Wind
According to researchers from the University of Oxford, the Sun is harboring a vast reservoir of dark matter. Astrophysicists Ilidio Lopez and Joseph Silk reasoned that passing dark matter particles would be captured by the gravity of heavy bodies like the Sun. We know that, in addition to heat and light, the Sun constantly emits low density plasma of charged electrons and protons called the ‘solar wind,’ which blasts out from the Sun in all directions at very high speeds to fill the entire Solar System and beyond.
The composition of this solar wind has been largely analyzed by Science, up to now, to consist of only ordinary matter in the form of plasma. If there is a large dark matter reservoir in the Sun, it is a logical next step to expect dark matter particles captured by the Sun from various sources to be also blown out from the Sun in its solar wind. Dark matter winds from the Sun would be sending trillions of dark matter particles hurtling to the tiny Earth every second.
What about Newton’s Laws?
The numerous sources of dark matter particles: from the galaxy’s halo, from the Sun’s solar wind, and from passing clouds of dark matter; suggest that there could be a local excess of dark matter in our Solar System over and above the galactic background. However, since the orbits of the planets comply with Newton’s gravity laws very closely, on initial analysis, can we conclude that the dark matter content on Earth, in the planets and in the Sun cannot be significant?
Let us take a step back and analyze this question. When we say that dark matter is causing stars to revolve around a galaxy at a much faster rate, we are talking about the impact of the galaxy’s dark matter halo’s gravitational interaction with these stars. If there was a substantive halo of dark matter around the Solar System, we would expect this extra mass from the halo to accelerate planets at the edge of the Solar System much faster than what would be allowed by Newton’s laws. We do not observe any significant deviation of Newton’s laws in the Solar System (although there are some minor acceleration anomalies discussed here). Hence, we can conclude that only a very low-density halo of dark matter exists around the Solar System.
However, this does not disallow planets and the Sun to harbor vast amounts of dark matter within their bodies. If all the planets and the Sun had significant amounts of dark matter, they would still be complying with Newton’s laws, because their individual masses would simply include the mass of the dark matter. To make the analysis more dramatic, one could imagine (hypothetically) that if all the planets and the Sun were composed wholly of dark matter (and had the same mass that is currently ascribed to the Sun and the planets), they would still comply with Newton’s gravitational laws with no significant anomalies in their rotational curves (assuming that the Solar System had only a very low-density halo of dark matter around it). So, the absence of any significant anomalies (and deviation from Newton’s laws) does not necessarily rule out the possibility that there could be dark matter cores and/or halos in and around the planets and in the Sun.
The mass of the Earth is computed based on the gravitational acceleration measured at different places on Earth. This assumes from the start that the acceleration is due only to ordinary matter. The contribution of any dark matter in or around Earth to its current mass has been ignored. A similar assumption is made when computing the mass of the Sun and the other planets.
Estimates of Dark Matter Content and Distribution between the Moon and Earth
Dark Matter Around Earth
Xu and Siegel, from the University of Arizona, have computed dark matter densities for all the planets in the Solar System. In 2009 Stephen Adler of the Institute for Advanced Study in Princeton, N.J. noted that if the mass of Earth and the Moon when measured together seems greater than their masses separately, it would mean that the difference could be attributed to a halo of dark matter in between.
Adler examined research that gauged the distance from the Earth to the Moon with lasers reflecting off lunar mirrors planted by 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, this could be attributed to the effects of a dark matter halo between the Moon and the artificial satellites.
Adler estimates that about 24 trillion metric tons of dark matter lies between Moon and the Earth. 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 estimated in 2009 that the dark matter density peaked at about 70,000 km from the surface and petered out nearer to the surface of the Earth.
Dark Matter in Earth’s Core
On the other hand, physicist, David Peat, says that the best calculations suggest that Earth’s core could contain as much as 10 per cent shadow matter, which is generally considered to be the same as dark matter. If there was any acceleration caused by dark matter in Earth’s core, it was historically simply attributed to ordinary matter. Although dark matter is generally assumed to pass through the Earth, more recent analysis suggests that they may lose energy as they interacted with ordinary matter particles on Earth. They could therefore be trapped in Earth’s core and have been accumulating over 4.5 billion years. Dark matter could also be already present during the formation of the Solar System — so that ordinary and dark matter worked together to form our Solar System. This embryonic dark matter could be locked inside the core of the Earth.
Dark Matter in the Moon’s Core
Alan Lowey, a former Royal Aircraft Establishment scientist, said that he has proposed an alternative to Newton’s idea of the theory of the ocean tides. Lowey proposes that the dark matter in the Moon’s core could be the reason behind the creation of ocean tides on the earth.
Putting all of the above together, it appears that there is a high density of dark matter in Earth’s core, which peters out as it approaches the surface and then increases in density again to peak at about 70,000 kilometers from the Earth’s surface. The density then peters out (again) to the surface of the Moon, and then increases and peaks in the core of the Moon.
S. L. Adler, (2008) arXiv: 0805.2895v3 [astro-ph].
Xu and E. R. Siegel, (2008) arXiv: 0806.3767v1 [astro-ph].
Ben Harris, University of Texas, GPS satellites suggest Earth is heavy with dark matter, New Scientist, 2013.
Tech2, Newton’s theory of gravity ‘out of date’, Dark Matter can explain ocean tides, UK scientist proposes, 2020.
Jay Alfred, Our Invisible Bodies, 2005.
