The Interior o f Nept une

Though Neptune is 1.5 times farther from the Sun than Uranus is, its surface temperature is about the same as that of Uranus. Unlike quiescent Uranus, Neptune does release heat from its interior. Neptune's internal heat makes up for the lower solar flux it receives at its great distance from the Sun. Neptune receives about 2.6 times less solar heat than does Uranus. Neptune must produce 1.6 times as much heat internally as it receives from the Sun to maintain its surface temperature and measured heat flux from its interior.This ratio between internal heat production and heat received from the Sun is the highest for any planet.

Heat flux is the rate of heat loss per unit area. Knowing the heat flux out of the surface of a planet allows the calculation of internal temperatures for the planet. Because Uranus has no heat flux, its internal temperatures are unknown. Neptune's, however, can be calculated. The shallow interior layers of Neptune consist of hydrogen (H2), helium (He), and methane (CH ), similar to Uranus's. Most of Neptune's interior is thought to consist of a deep layer of liquid, a sort of internal ocean. This ocean above the core consists mainly of water with some methane and ammonia at a temperature thought to be about 8,500°F (4,700°C). Pressure keeps the material in a liquid form rather than allowing it to evaporate.

For reasons that are not yet understood, Neptune is about 50 percent more dense than Uranus. Neptune is thought to have a small rocky core, but its core is not likely to be larger than Uranus's, perhaps about the mass of the Earth. Based on heat measurements at the surface, Neptune's core is thought to reach temperatures of 9,300°F (5,100°C), but the source of this heat is unknown. On terrestrial planets internal heat is partly produced by decay of radioactive elements, but the most common heat-producing radioisotopes (potassium, uranium, and thorium, among others; see the sidebar "Elements and Isotopes" on page 18) are not common on gas giant planets. Jupiter produces heat by the condensation and internal sinking of helium, but Neptune is thought to be too depleted in helium for this process to proceed. The source of its unusual internal heat remains unexplained.

Planetary magnetic fields are all thought to be caused in the most basic sense by the movement of heat from inside the planet toward its surface. Heat can be transported by conduction, simply by moving as a wave through a material and into adjoining materials.When the bottom of a pot on an electric stove heats up, it is heating through conduction from the electric element. Heat can also move by convection. If a piece of material is heated, it almost always expands slightly and becomes less dense that its unheated counterpart. If the material is less dense than its surroundings and is able to flow, gravity will compel it to rise and the more dense surroundings to sink into its place. In a planetary interior, heat moving away from the core can cause the planetary interior to convect. Hot material from nearer the core will rise. When rising hot material reaches shallower levels, it can lose its heat to space through conduction or radiation, and when its heat is lost, the material becomes denser and then sinks again.

The combination of vigorous convection in a material that can conduct electricity with rotation within a planet creates a dynamo effect leading to a planetary magnetic field. If the fluid motion is fast, large, and conductive enough, then a magnetic field can be not only created by but also carried and deformed by the moving fluid (this process creates the Sun's highly complex magnetic field). The inner core rotates faster than the outer core, pulling field lines into itself and twisting them. Fluid welling up from the boundary with the inner core is twisted by the Coriolis effect, and in turn twists the magnetic field. In these ways, it is thought, the field lines are made extremely complex.

Though the magnetic fields of all the gas planets differ from those of the terrestrial planets in form and strength, Neptune's is perhaps the most different. Neptune's magnetic dipole is tilted away from the planet's axis of rotation by 47 degrees and offset from the center of the planet by about 0.55 times the planet's radius, as shown in the figure on page 75. The field created is almost exactly as strong as Uranus's field at the level of one bar pressure, which is also the strength of Earth's field at its surface and about 10 times weaker than Jupiter's immense field.

On the terrestrial planets a dynamo is thought to form in the liquid metal outer core, convecting around the solid inner core. The gas giant planets lack any liquid metal layer; their magnetic fields are thought to form in circulating watery liquids. Jeremy Bloxham and Sabine Stanley at Harvard University have created models showing how the magnetic fields of Uranus and Neptune can be formed in thin convecting liquid shells in the planet's interiors, likely to consist, in a simple sense, of salty water (see the sidebar "Sabine Stanley and Planetary Magnetic Fields" on page 78). Their models demonstrate that the thin convecting shells in otherwise stable planetary interiors can create magnetic fields that are nonsymmetric and tipped away from the planet's axis of rotation.

People tend to think of magnetic fields just in terms of dipoles, meaning a two-poled system like a bar magnet. The Earth's magnetic field largely resembles a dipole, with magnetic field lines flowing out

Neptune's magnetic field is offset from both the center and rotation axis of the planet. Though it is not shown in this figure, all planetary magnetic fields are also pulled out into long magnetotails by the force of the solar wind.

Neptune's Magnetic Field

Magnetic and rotation axis

Rotation axis

Magnetic and rotation axis

Idealized dipolar magnetic field

Rotation axis

Neptune's magnetic field

Idealized dipolar magnetic field

Neptune's magnetic field

Sabine Stanley and Planetary Magnetic Fields ^Sabine Stanley began by studying astronomy as an undergraduate at the University of Toronto, but she found that, despite the beauty of the field, she wanted to work on something a little closer and more tangible. Her freshman-year physics professor mentioned to her the work he had done during his postdoctoral study in Boston, and, after her junior year, Stanley began a summer job at Harvard University with Jeremy Bloxham.

Bloxham specializes in the study of planetary magnetic fields. This field can be a difficult one for a young student to begin with because it requires advanced understanding of the physics of magnetism and fluid mechanics, as well as a great facility with computer codes. Among other techniques, Bloxham creates models of dynamic planetary interiors to see how a magnetic field might be initiated, how it develops, and what it might look like today and in the future. Stanley's summer work went so well that after her graduation she returned to Harvard to do her doctoral work with Bloxham.

Stanley works in the general area of planetary magnetic fields, trying to answer questions about how different planets create their fields, how the shapes and strengths of the fields relate to the internal structure of the planets, and how magnetic field generation affects the planet itself. Does magnetic field generation speed the cooling of the planet's interior? Are there ways scientists can learn about a planet's past by studying its current magnetic field? She models the physical forces and interactions inside the fluid region of a planet using computer models. The models start with assumptions about the compositions and temperatures of the material inside the planet, and then they track the resulting movements of the material, its temperature changes, and the production of the magnetic field. These models have to solve a number of related equations (heat transport, conservation of mass, fluid motion in response to stress, and magnetic induction) for many individual locations over the planet for each of many time periods as the field develops. This scope of computing requires supercomputers; using a supercomputer (an SGI Origin 2000) with 16 processors, a single model must run continuously for several weeks to produce enough data to understand part of the development of a planetary field.

Stanley is trying specifically to understand what makes planets' magnetic fields different from one another. Though Uranus and Neptune are unusually similar planets, their fields are quite different from each other in orientation and placement. The magnetic fields of the Earth, Jupiter, and Saturn are dipolar, similar to a bar magnet, but Uranus's and Neptune's fields are not. Their fields have strong quadrupole and octupole components and are not symmetric around their axes.

The Earth's magnetic field is generated in its fluid outer core, which is a thick shell of liquid metal with a small solid core consisting mainly of iron in its center. Uranus and Neptune, in contrast, may have small iron cores, but these cannot be the sources of their magnetic fields. Stanley and her colleagues think that the magnetic fields of Uranus and Neptune are generated in a shell of convecting fluid in the planets' interiors. Simple dynamo production in a thick fluid shell will create a dipolar magnetic field, according to theory and the models that Stanley runs. To create the complex magnetic fields of Uranus and Neptune, Stanley finds, a planet requires a thin shell of convecting fluid with nonconvecting fluid in its interior. The interior fluid may be nonconvecting because its density resists movements caused by heat transfer (this is called a stably stratified fluid). A thin shell of fluid can produce more complex magnetic field patterns because different areas of the shell have more trouble communicating with each other. Convective patterns are forced to be on a small scale because the shell is thin, and so more convection cells form over the area of the shell, and convection on one side of the shell is much more weakly related to convection on the other side than it would be in a deep fluid region with larger convection cells. A strong magnetic field, they find, can itself begin to move the stably stratified fluids beneath the convecting shell, creating even more complex magnetic field patterns.

On Uranus and Neptune, Stanley and her colleagues believe, the thin convecting shell that creates the magnetic dynamo consists of some combination of water, ammonia, and methane. Under high pressure the molecules of these materials break apart and form smaller molecules and atoms with electric charges. This ionic soup can act as electric currents as it moves because of convection. To test the predictions that their computer models make, though, Stanley and her colleagues require data from another mission to the distant gas giant planets. There is at least a discussion at NASA about a potential mission to Neptune that would measure its magnetic field in more detail.

Stanley finished her Ph.D. at Harvard in 2004 and moved on to a postdoctoral research position at the Massachusetts Institute of Technology (MIT), working with Maria Zuber. Zuber is a mission scientist for NASA as well as a professor of geophysics and the department head of the Earth, Atmospheric, and Planetary Science Department at MIT. With Zuber, Stanley focused on the magnetic field of Mercury to determine whether its anomalous field might be generated in a very thin shell of liquid metal around an otherwise solid core. In 2005 Stanley accepted faculty position at the University of Toronto. Stanley's work is at the forefront of the understanding of magnetic fields, and her work will continue to unravel some of the unanswered questions about planetary dynamics and evolution.

of the south magnetic pole and into the north magnetic pole, but there are other, more complex configurations possible for magnetic fields. The next most complex after the dipole is the quadrupole, in which the field has four poles equally spaced around the sphere of the planet. After the quadrupole comes the octupole, which has eight poles. Earth's magnetic field is thought to degenerate into quadrupole and octupole fields as it reverses, and then to reform into the reversed dipole field. Neptune's field is largely a dipole but has strong highorder components, creating a complex field.

Electrical currents in shallow liquids and ices, rather than any generated by a core dynamo, may create Neptune's magnetic field. The strongest electric currents in Neptune are thought to exist at about half the radius of the planet. The tilt of the magnetic field relative to the rotation axis of the planet means that the shape of the magnetosphere (the region around Neptune devoid of solar wind because of magnetic field protection) changes dynamically as the planet rotates. This continuous complex change may contribute to heating of the highest portions of Neptune's atmosphere, described below. Both continuous and irregular, bursting radio emission come from Neptune, again very similar to Uranus's emissions.The rotation of the magnetic field is also thought to be a controlling producer of these emissions.

Thanks to the sophisticated computer modeling of the Harvard University team and others, some insight can be gained into the interior of this remote planet. With only density, heat flow, and magnetic field measurements, along with some compositional information from spectrometry, reasonable inferences can be gained without direct data.The understanding of Neptune is ahead of the understanding of Uranus simply because Neptune has measurable surface heat flow. All the scientists studying these distant planets, though, wish for a new space mission to take new measurements and allow testing of the hypotheses that have been made.

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