Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe

by Lisa Randall

Isaac Newton made another important deduction when he realized that comets move in oblique orbits. He demonstrated by using his gravitational inverse square law, which says the strength of gravity is four times smaller for an object twice as distant—that objects in the sky have to follow elliptical, parabolic, or hyperbolic orbits.

The familiar phases of matter are solid, liquid, and gas—which for water amounts to ice, water, and steam. Atoms are arranged differently in each of the phases, with solid ice the most structured and gaseous vapor the most random. When a phase transition converts liquid into gas—as happens when water boils, for example—or solid into liquid—as occurs when ice cubes melt—the material stays the same since all the same atoms and molecules are present.

(Note that temperature and physics both play a role because melting and boiling points are different at different pressures, as anyone who has tried to make pasta in Aspen, Colorado, at eight thousand feet above sea level, would know.)

Planetary scientists call those elements with melting point below 100 degrees kelvin gases—independent of the actual phase the matter is in. Those with low melting point, but not so low as gases, are referred to—again by planetary scientists—as ices, though whether or not the material is actually an ice also depends on the actual temperature. That is why Jupiter and Saturn are called gas giants, while Uranus and Neptune are sometimes called ice giants. In both cases, the interior is actually a hot, dense fluid.

Gases (in the sense used by planetary scientists) are a subset of volatiles, which are elements and compounds with low boiling points—such as nitrogen, hydrogen, carbon dioxide, ammonia, methane, sulfur dioxide, and water—that might be present in a planet or atmosphere.

The coma can be much bigger than the nucleus—thousands or even millions of kilometers across, sometimes even growing to the size of the Sun. Bigger dust particles remain in the coma whereas lighter ones are pushed into the tail by the Sun’s radiation and charged particle emissions. A comet consists of the coma, the nucleus it surrounds, and the tail that streams away.

The nuclei don’t have sufficient gravity to round out their structure, so they have irregular shapes, varying in size from a few hundred meters to tens of kilometers across.

The inner regions, which give rise to short period comets, are called the Kuiper belt and the scattered disk, while much farther out is the hypothesized Oort cloud, which produces long-period comets and to which I’ll soon (figuratively) return.

Jupiter is the biggest known relatively local perturber since its mass is more than double the total mass of all the other planets combined.

The Kuiper belt lies in a region greater than 30 times as far from the Sun—between about 30 and 55 AU away.

The presence of ice rather than gas is due to the location of the belt and its consequent low temperature of about 50 degrees kelvin—more than 200 degrees colder than the freezing point of water.

The greater eccentricity, range of locations, and degree of inclinations—up to about 30 degrees—distinguish the scattered disk population from that of Kuiper belt objects, as does their instability.

Jupiter and Saturn used each other to stabilize their orbits—Jupiter orbits the Sun exactly twice as fast as Saturn.

But these planets destabilized Uranus and Neptune—putting them into different orbits, with Neptune becoming more eccentric and orbiting farther out. En route to its final destination, Neptune likely scattered many planetesimals into more eccentric orbits and many others into more inner orbits where they would rescatter or get ejected by Jupiter’s influence.

The Voyager spacecraft and its measurements were directly accessing reaches of space that no mission has ever entered before. It’s true that the data collection system based on eight-track tapes had to be tinkered with along the way, that it no longer had a working camera, and that its equipment has about a million times less memory than a smartphone does today. But the spacecraft is still operational and is currently the man-made object most distant from the Earth and the Sun.

The case in England for meteorites had been further solidified even earlier as well when a 56-pound stone fell on December 13, 1795 at Wold Cottage in Yorkshire. With an increased appreciation of the methods of chemistry—just recently separated from alchemy—and with so much firsthand evidence, meteorites were finally recognized in the nineteenth century for what they were.

The Earth is pretty small and moves pretty quickly around the Sun—at about 30 km/sec.

Most meteoroid hits—including all the big ones—happened well before people were around to watch, never mind record, them.

Merle Shoemaker—a key player in scientific understanding of impacts—found rare forms of silica at the crater that could only have arisen from rocks containing quartz that were severely shocked by impact pressure. Aside from a nuclear explosion—unlikely 50,000 years in the past—a meteoroid impact is the only possible known cause.

Impact craters are the result of extraterrestrial objects hitting the Earth with sufficient energy to create a shock wave that excavates a circular crater—which is awesome indeed. The shock wave—not the direct impact—is responsible for the circular shape of impact craters.

The expanding supersonic wave creates stress levels far in excess of the local material’s strength. This creates rare crystalline structures, such as shocked quartz, that are found only in impact craters—and in the blast region of nuclear explosions.

But impact craters generally have a different chemical composition, including metals and other materials—such as nickel, platinum, iridium, and cobalt—that are rare on the Earth’s surface.

Also useful in distinguishing craters are impact breccias, which consist of fragments of rock held together by a fine-grained matrix of material—again indicating an impact that shattered what was initially there. Shocked fused glasses are also interesting in that their formation requires both high pressure and elevated temperature. Their unusually high density helps identify them.

These distinctive shock and melt features are critical to confirming impact events since there is no other way for them to form.

Impact craters also have raised rims—again not typical for volcanic craters.

Seafloor evidence is largely eliminated every 200 million years, since plate tectonics changes the ocean floor in a conveyer-belt-like process of spreading and subduction that covers up any preexisting evidence on this time scale.

The oldest rocks on the planet’s surface contain evidence of life in fossils that date from about 3.5 billion years ago—about a billion years after the Earth’s formation and rather soon after it stopped being bombarded by asteroids and comets from space.

Oxygenic photosynthesis emerged about a billion years later—and with it an atmosphere that most likely triggered many extinctions, but which also precipitated the emergence of multicellular algae.

Only in the early 1800s did the French naturalist and nobleman Georges Cuvier recognize the evidence that some species had entirely disappeared from the planet.

Surprisingly, we currently live in a region—300 light-years across—called the Local Bubble, which is a vacuum-like domain with very low hydrogen density within the interstellar medium in the Orion Arm of the Milky Way. Only recently –perhaps in the last few million years—did we enter this warm, low-density, partially-ionized region, with its relatively sparse interstellar environment. During this time, the region enclosed by the heliosphere boundary—where the solar wind dominates over the interstellar medium—has been exceptionally large.

We know dark matter’s average energy density in the cosmos (from the microwave background), its density nearby (from the rotation velocities of stars in the galaxy), that it is “cold”—which is to say, it moves at only a fraction of the speed of light (because we observe structure on small scales in the cosmos), that it interacts at most extremely weakly—both with ordinary matter and with itself (from the lack of discovery in direct searches and from measurements such as that of the shape of the Bullet Cluster), and that it doesn’t carry electric charge.