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Fast flying bugs, Slow cooling Earth

Q: How fast can a bug fly? (Sidney, Conway, Arkansas)

A deer fly, Chrysops sincticornis, laying eggs (the lower cream-colored mass). [Jerry Butler, University of Florida]

A:

How fast can a bug fly? Fast. "A tabanid fly (such as a deer or horse fly) has been clocked at 90 miles per hour (145 km/h)," says Rudy Scheibner, entomologist emeritus at the University of Kentucky in Lexington.

Other fast-flying insects include hawk moths (timed at 33 mph [53 km/h]), sphinx moths (33 mph), and dragonflies (species Anax parthenope clocked at almost 18 mph [29 km/h]).

Still others work hard but go slow. The honeybee flaps her wings about 190 times per second but flies at 7 mph (11 km/h). A midge flails his wings more than 1000 times per second — the record — but just putters along.

Insects evolved flight about 300 million years ago yet many still lack the right stuff. Puny thin wings work hard to lift big bulky bodies. The fruit fly burns as much fuel (proportionately) as a jetliner that zooms at 600 mph (970 km/h). For the same relative energy, the fruit fly flounders at 2 to 3 mph (3 to 5 km/h).

"Faster is not necessarily better, though," says John Capinera, entomology professor at the University of Florida. Insects must be doing it right, considering how long they’ve been at it. "Humans have a ways to go to catch that record!"

Further Surfing:

University of Florida: Deer flies, yellow flies, and horse flies

University of Kentucky: Insect Info

Time Magazine: How insects fly

Smithsonian Institute: Insect flight

Q: Why is the center of the earth hot? Why does it remain hot? (Eric, Someplace, Earth)

A: Oddly enough, most of the heat deep within the center of Earth originated with Earth — 4.6 billion years ago.

Earth’s core stays hot because heat escapes slowly through insulating rock layers. But, perhaps more importantly, the molten metal at the center of Earth releases heat as it changes from a liquid to a crystalline solid, and, therefore, adds heat to the core.

In the beginning, gravity gradually pulled rubble orbiting the proto-Sun together to form a proto-Earth. The debris collided as it drifted closer together and the hits generated heat. Eventually, as the gathering rocks compressed into a planetesimal, the heat melted iron and nickel and, being heavy, these elements dropped to Earth’s center and pushed lighter rocks up.

Over ensuing eons, the core cooled. As it cooled the liquid metal at the very center of the core solidified, says Gary Glatzmaier, professor of earth sciences at the University of California at Santa Cruz. Iron becomes a solid under high pressure as it cools and the pressure is greatest at the core’s center.

The solid inner core and liquid outer cores deep within Earth — still enormously hot. Click for the rest of the figure.  [USGS]

"So, the core solidified, starting at the center, and, as it cooled further, the solid inner core grew as it still is today — but very slowly," says Glatzmaier. See figure.

In addition to primeval heat, Earth’s core also gets heat from radioactive decay of uranium, thorium, and potassium. These elements, however, mostly occur in the mantle (above the core) and so contribute little to core heat.

(Potassium may also occur in the core — contributing to core heat — Glatzmaier says, but the issue is still unresolved.)

Some significant heat comes from heat release as the molten core metal solidifies. Much as water gives off heat as it freezes, the liquid iron of the outer core releases heat when it crystallizes into a solid.

The core is still hot although its temperature is not known precisely — but our best estimate is about 10,000F (5000C). That’s almost the Sun’s surface temperature.

Further Reading:

Tarbuck, Edward J., and Frederick K. Lutgens. Earth. Upper Saddle River, New Jersey: Prentice Hall, 1999.

USGS: Inside Earth

MadSci Network: How hot is the Earth’s center by Nick Hoffman

University of Tennessee: the Sun’s surface, the Photosphere

(Answered Feb. 11, 2005)