Mysterious NEPTUNE.

For centuries, Neptune, the eighth planet from the sun, has been viewed--when visible at all--as the drab stepsister to other planets in our solar system. Then in 1989, NASA's Voyager 2 flew by the fuzzy nobody and sent snapshots to Earth. Those startling images forever changed the way scientists look at Neptune. Now more than a decade later, Neptune continues to excite and mystify astronomers. This giant ball of gas may even be raining diamonds!

Voyager 2 nabbed close-ups of a brilliant blue orb with swirling white clouds and a turbulent storm system with 2,000 kilometer (1,200 mile) per hour winds--by comparison, gale-force winds on Earth blow a 150 miles per hour.

Changeable weather is no surprise to earthlings, because the sun heats our planet's surface and sends water vapor and air rising to create clouds and wind. But farther out in the solar system, the sun's energy is so reduced its effect ought to be minimal. That's not the case with Neptune. "Neptune defies all models of what's expected," says space scientist Heidi Hammel at the Space Science Institute in Boulder, Colo. "Since its energy isn't coming from the sun, the question is, where's it coming from?"

Neptune's complex atmosphere (see diagram) may hold the key. Scientists calculated the planet's radiation balance (amount of energy emitted compared with the amount of sunlight received) and found Neptune radiates 2.6 times as much energy as it absorbs from the sun. "You've got great heat coming from the inside and very little heat from the outside, so you're not in a stable situation at all," says Hammel.

STRANGE ATMOSPHERE

Unlike Earth, which has a very thin layer of atmosphere, Neptune is a thick swath of atmosphere surrounding a small solid core. In the upper third of the planet is a mixture of heated gases--hydrogen, helium, methane, and water. Methane gives Neptune its vibrant blue coloring. And frozen methane high up in the atmosphere produces wispy clouds. The pressure in Neptune's outer atmosphere would probably be similar to that on Earth, except with temperatures as low as -350 [degrees] F. However, deeper into the planet, heat and pressure increase dramatically. The inner two-thirds of Neptune is an ocean, probably with the consistency of pudding, composed of molten rock, water, liquid ammonia, and methane.

RAINING DIAMONDS

Scientist think that one-third of the way inside Neptune's frenzied atmosphere churns an ocean of methane, water, and ammonia under extreme heat and pressure. And physicist Robin Benedetti at the University of California at Berkeley believes diamonds as big as phones or cars may be raining there. How come? Last year, she re-created Neptune's violent interior in her lab by blasting methane with high pressure and heat.

For pressure, Benedetti used a contraption called a diamond anvil cell. The apparatus consists of two modest engagement ring-size diamonds. Diamond, made of carbon, is the hardest mineral on Earth. "You can press on it with a lot of force and create really high pressure [force divided by area]," Benedetti explains.

Imagine diamonds that are flat on top and narrow to a point. "I squeezed a teeny drop of liquid methane between the two points," she says. "Given the points' tiny area, when we apply a lot of force the pressure on the methane becomes huge [100,000 to 500,000 times Earth's atmospheric pressure]."

To simulate Neptune's extreme heat, the physicist flash a laser beam at the squeezed methane. "When they interact it gets really hot [3,600-5,000 [degrees] F]," she says. "The methane would suddenly turn black." The black spot? Diamond dust!

Methane gas is composed of four hydrogen atoms around a carbon atom. But under such extreme heat and pressure, the bonds that hold the atoms dissolve. Carbon atoms then bind to one another to form diamond crystals. "We're not sure how big diamonds might be inside Neptune," says Benedetti. "That depends on factors like temperature and environment. But conceivably you could make diamonds as big as boulders," says Benedetti.

How does diamond-making explains Neptune's internal energy? "We think methane in the ocean of water and ammonia breaks down to make diamonds," she explains. The diamonds might be hailing down for thousands of kilometers to the ocean bottom--the planet's center--releasing heat as a result of friction (rubbing of one object surface to another). "This could generate a huge amount of energy."

Even if Neptune hails diamonds, forget about jewelry. "Nothing we could build would withstand the conditions to bring the diamonds back to Earth," says Benedetti.

WHAT A TRIP

Astronomers dream of collecting firsthand information to learn more about mysterious Neptune. But how do you get a spacecraft to make a 2.6 billion-mile trip quickly?

Voyager 2, bogged down with thousands of tons of fuel, took over 12 years. It even got a boost from a rare alignment of the outer planets that created a "gravitational super-highway" --which won't reoccur for 200 years. If launched today, a similar spacecraft wouldn't reach Neptune until 2030! "Nobody's interested in launching something that's so slow it won't arrive until after everybody who sent it up is either retired or dead," says John Brophy of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. But JPL may have found a solution.

The plan involves rigging a spacecraft with a lightweight aluminum-coated plastic sail one-twentieth the thickness of a human hair. The energy source: the sun. "This is basically a mirror in space," says JPL engineer Charles Garner. As photons (light particles) from the sun bombard the sail they bounce off--imparting up to double their momentum to the spacecraft. Blown by photons, the "sailcraft" could journey to Neptune in three years! "It would open up a whole new era of deep-space and interstellar exploration," says Gardner. Until then, however, Neptune remains a deep dark mystery.

ODD BALL

Triton, Neptune's largest satellite, or moon, is the only satellite in the solar system that revolves in the opposite direction of its parent planet. Why? Scientists believe Triton formed independently of Neptune but was later "captured" by the planet's strong gravitational pull. Neptune's strong grasp (blue) may also skew its neighbors' movements. Next door, Pluto (purple) and icy rocks in the Kuiper belt (gray) travel in oval, not circular, orbits.

Cross-Curricular Connection

English: Write a fictional account of your explorations of Neptune.

Did You Know?

* Neptune orbits the sun once every 165 years. Although it's usually the eighth most distant planet from the sun, it switches places with Pluto for 20 years of each Plutonian orbit (248 years) to be the ninth, or most distant planet in our solar system.

* Triton, Neptune's large icy moon, features active water geysers. It also has the coldest temperatures in the solar system.

* Neptune has four rings and they have names! The outermost is Adams (which has three prominent arcs named Liberty, Equality and Fraternity); next is an unnamed ring; then Leverrier (whose outer extensions are called Lassell and Arago), and finally Galle.

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National Science Education Standards 

Grades 5-8: properties and changes of properties in matter * motions and forces * transfer of energy

Grades 9-12: structure properties of matter * motions and forces * interactions of energy and matter

Resources

"Neptune Rising," Discover, September 2000. p. 55

"Hello, Neptune," Science World, November 17, 1989, p. 7

"Do Diamonds Hail Down Upon Neptune and Uranus," UniSci: unisci.com/stories/19994/1001993.htm

Directions: Read "Mysterious Neptune" and then fill in the blanks.

1. Neptune is the -- planet from the sun.

2. Scientists believe -- may be raining down in Neptune's atmosphere.

3. A satellite is also known as a --.

4. -- is the only satellite that evolves in the -- direction of its parent planet.

5. Invention that may allow a spacecraft to reach Neptune in as little as three years: --

ANSWERS

1. eighth 2. diamonds 3. moon 4. Triton, opposite 5. solar sail