Image courtesy of Pete Mouginis-Mark, Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa

Hawaii is well known to have been created from volcanic activity, and the geologic hotspot below the islands is the most studied in the world. In this image, we see a type of lava flow known as pahoehoe at Kilauea. The image was used to illustrate a December Science article titled “Mantle Shear-Wave Velocity Structure Beneath the Hawaiian Hot Spot.”

The lava flow originates from well below the surface of the Earth to the layer known as the mantle. The mantle, a cross-section between our planet’s crust and outer core, is an extremely hot layer of the planet, where temperatures range between 500 to 900 °C (932 to 1,652 °F) at the upper boundary with the crust to over 4,000 °C (7,230 °F) at the lower boundary with the core.

Image courtesy of Pete Mouginis-Mark, Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa.

The universe originated about 13.7 billion years ago with the Big Bang, so light cannot have been traveling for more than 13.7 billion years. “Since we know how old the universe is,” Halzen says, “and the speed of light, we can calculate the size of space we can see.”

Halzen, who directs a giant telescope in Antarctica that is designed to see neutrinos spewed by titanic explosions in the distant universe, says we can theoretically see about 13.7 billion light years in any direction. Because light travels almost 6 trillion miles in a year, it can “only” have traveled about 80 billion trillion miles since the Big Bang.

Practically, astronomers struggle mightily to see objects at these astounding distances. It’s likely that the extremely distant objects that created this ancient light have since blown themselves to bits, but the light from those explosions has not yet reached Earth.

Solar eclipse from space

You’ve seen photos of lunar and solar eclipses, or maybe you’ve even been present for one yourself, but have you ever seen an eclipse of the Earth? Astronaut Bill McArthur and flight engineer Valery Tokarey snapped this photo from aboard the International Space Station on March 29, 2006 during a total eclipse of the sun. That day the shadow of the moon’s umbra crept from Brazil to central Asia, where the eclipse’s path of totality finally ended with a fully-blackened sun sinking below the horizon of western Mongolia.

In this image, the southern shores of the Mediterranean Sea are subsumed by a lunar shadow that extended more than 2,200 miles into North Africa’s Sahara desert. Framing the dark sphere in a halo of progressively lighter shades is the penumbral shadow. Within the shadow observers witnessed a partial eclipse of the sun, whereas those in the darkest center circle experienced a total eclipse.

Image credit: NASA

The late 18th century English economist Thomas Malthus – one of the first to express concern about overpopulation – observed that there should be no more people in a country than can “daily enjoy a glass of wine and piece of beef for dinner.” But what if people choose tofu and beer instead?

The contemporary “ecological footprint” model offers another way to think about the Earth’s carrying capacity.

“If everyone on the planet enjoyed the lifestyle of an average U.S. citizen, the Earth could support only one to two billion people,” says Naughton. “But if everyone used resources at the rate of the average African citizen, far more people could be supported.”

Joel Cohen, a prominent population researcher at Rockefeller and Columbia universities, offers a more nuanced perspective: “He points out that there is no ultimate answer to this question,” says Naughton. “Rather, we need to think about broader issues of equity and sustainability when we consider human numbers and environmental impacts.”

This deep, dark region retains heat from the hot gas and dust that formed Earth about 4.5 billion years ago.

The middle layer, the mantle, is solid rock, but it is hot enough to flow slowly, like Silly Putty. The movement, called convection, brings hot rock from the lower mantle to the surface. Cooler rock at the top of the mantle sinks.

The overall effect of convection is to create “conveyor belts” that transport the giant plates that form Earth’s crust. Mantle rock rises close to Earth’s surface along the mid-oceanic ridges. Some of the mantle rock melts, rises further, and, and where melt forms, rises, warms rock above it, which cools crystallizes to and to forms new ocean crust. As ocean the new crust moves away from a ridge, it cools and become denser, eventually sinking back into the mantle.

“As the continental plates are carried along on this conveyer belt, they may crash together (the Himalayas), slide past one another (California), or separate (Baja California),” says Goodwin. Over hundreds of millions of years, the continents have merged and re-separated in their continual movement around the globe. This movement explains why fossils of tropical animals are found in Antarctica, she says.