Conditions on Uranus

Shown in the upper color insert on page C-1, Uranus can no longer be thought of as a featureless blue-green sphere. A number of problems have inhibited scientists' understanding of the surface conditions of the planet: First, instruments have not been sensitive enough to detect subtle clouds. Second, Uranus's lack of surface heat flow has made scientists believe that violent weather is unlikely. Finally, Uranus's seasons are so long that humankind has only observed the planet closely during one season. As observational techniques have improved, Uranus's weather is revealing its complexity. Uranus's atmosphere is about 83 percent hydrogen, 15 percent helium, and 2 percent methane (though Uranus has relatively little hydrogen in bulk; it is simply concentrated in the atmosphere). Methane molecules absorb red light and reflect or emit only blue and green light, so the methane in the outer layers of the planet gives the planet its bright blue-green color.

The temperature at Uranus's tropopause (the point where temperature stops falling with increasing altitude and actually begins to rise) is about —364°F (—220°C) and is near the pressure level of 0.1 bar (see figure on page 24). The clouds are found about 30 miles (50 km) below the tropopause, at a pressure of about one bar and a temperature of —328°F (—200°C). Deeper in Uranus, where the pressure is about three bars, the temperature rises to —243°F (—153°C). Above the Uranian tropopause the concentrations of methane and hydrogen decrease dramatically. The region above the tropopause is called the stratosphere, a stable layer of atmosphere on all planets, because temperature increases with altitude. Increasing temperature creates stability because warmer material is less

Temperature Profile of Uranus's Atmosphere

The temperature profile of the Uranian atmosphere is similar to those of the other gas giant planets.

dense than colder material, so naturally it stays buoyantly above the colder and therefore denser material. In Uranus's stratosphere the temperature rises from the low of about —364°F (—220°C) at the tropopause to a high of perhaps 890°F (480°C) in the exosphere, the uppermost layer of its atmosphere. The extreme warmth of the Uranian upper atmosphere is not well understood; there is no explanation from the weak solar heating at the distance of Uranus or from the immeasurably small energy output from the planet's interior. The other gas giant planets have similarly hot exospheres, and on Jupiter the heat is thought to come from ionizing reactions tied to Jupiter's magnetic field and its auroras. Perhaps a similar process is at work on Uranus and Neptune.

What methane there is in the outer atmosphere freezes and settles back into the lower atmosphere with relatively little remixing. There are therefore few hydrocarbons for solar radiation to turn into smog, and the Uranian outer atmosphere remains clear. Because the outer atmosphere is both clear and warm, it has expanded much farther than the outer atmospheres of other gas giants. Even at the altitudes at which Uranus's rings orbit (1.64 to two times Uranus's radius) there is enough atmospheric gas to create drag on the rings. The smallest particles have the greatest surface-drag-to-mass ratio and so are most affected by the high atmosphere.The smallest particles are preferentially slowed by atmospheric drag, and they are the first to fall back into the tropopause, having been slowed to the point that Uranus's gravity pulls them in.This process tends to make Uranus's rings consist of larger particles.

From the images of Voyager 2 (see the lower color insert on page C-2) and later observations, such as the lower color insert on page C-1, it has long been thought that Uranus was almost featureless and completely lacked weather systems. It also stood to reason that if there was no measurable heat flux out of the planet and surface temperatures remained constant, then there was no driving force for winds. Recently, though, researchers have begun using large radar arrays on Earth to make high-resolution radar maps of Uranus (radar stands for radio detection and ranging, a technique that bounces radiation at radio wavelengths off a target and then measures the returned energy; for more information, see the sidebar "Remote Sensing" on page 26).

By bouncing radiation with wavelengths from 0.8 to 2.4 inches (2 to 6 cm) off Uranus, large-scale changes in the troposphere over a pressure range from about five to 50 bars can be imaged. Using these techniques, it has been shown that temperature gradients on Uranus change with latitude.The south pole appears to be hottest, and a distinct cooling in the troposphere occurs at a latitude of about —45 degrees.

In 1999 the Hubble Space Telescope (see the lower color insert on page C-1) took near-infrared images of Uranus that show giant cloud systems 1,250 miles (2,000 km) in diameter.These clouds appear to circle Uranus at speeds above 310 miles per hour (500 km/hr).The Hubble Space Telescope has seen as many as 20 bright clouds at different

Remote Sensing

Remote sensing is the name given to a wide variety of techniques that allow observers to make measurements of a place they are physically far from. The most familiar type of remote sensing is the photograph taken by spacecraft or by giant telescopes on Earth. These photos can tell scientists a lot about a planet; by looking at surface topography and coloration photo geologists can locate faults, craters, lava flows, chasms, and other features that indicate the weather, volcanism, and tectonics of the body being studied. There are, however, critical questions about planets and moons that cannot be answered with visible-light photographs, such as the composition and temperature of the surface or atmosphere. Some planets, such as Venus, have clouds covering their faces, and so even photography of the surface is impossible.

For remote sensing of solar system objects, each wavelength of radiation can yield different information. Scientists frequently find it necessary to send detectors into space rather than making measurements from Earth, first because not all types of electromagnetic radiation can pass through the Earth's atmosphere (see figure, opposite page), and second, because some electromagnetic emissions must be measured close to their sources, because they are weak, or in order to make detailed maps of the surface being measured.

Spectrometers are instruments that spread light out into spectra, in which the energy being emitted at each wavelength is measured separately. The spectrum often ends up looking like a bar graph, in which the height of each bar shows how strongly that wavelength is present in the light. These bars are called spectral lines. Each type of atom can only absorb or emit light at certain wavelengths, so the location and spacing of the spectral lines indicate which atoms are present in the object absorbing and emitting the light. In this way, scientists can determine the composition of something simply from the light shining from it.

Below are examples of the uses of a number of types of electromagnetic radiation in remote sensing.

Gamma rays

Gamma rays are a form of electromagnetic radiation; they have the shortest wavelength and highest energy. High-energy radiation such as X-rays and gamma rays are absorbed to a great degree by the Earth's atmosphere, so it is not possible to measure their production by solar system bodies without sending measuring devices into space. These high-energy radiations are created only by high-energy events, such as matter heated to millions of degrees, high-speed collisions, or cosmic explosions. These wavelengths, then, are used to investigate the hottest regions of the Sun. The effects of gamma rays on other

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Gamma, X-ray, and ultraviolet light is absorbed by atmosphere and does not reach the surface

Infrared: largely absorbed by atmosphere


Visible light: blue to red

Long-wavelength radio waves are absorbed by atmosphere

Short-wavelength radio waves reach Earth's surface (used for communication with spacecraft)

Visible light: blue to red

Short-wavelength radio waves reach Earth's surface (used for communication with spacecraft)

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The Earth's atmosphere is opaque to many wavelengths of radiation but allows the visible and short radio wavelengths through to the surface.

solar systems bodies, those without protective atmospheres, can be measured and used to infer compositions. This technique searches for radioactivity induced by the gamma rays.

Though in the solar system gamma rays are produced mainly by the hottest regions of the Sun, they can also be produced by colder bodies through a chain reaction of events, starting with high-energy cosmic rays. Space objects are continuously bombarded with cosmic rays, mostly high-energy protons. These high-energy protons strike the surface materials, such as dust and rocks, causing nuclear reactions in the atoms of the surface material. The reactions produce neutrons, which collide with surrounding nuclei. The nuclei become excited by the added energy of neutron impacts, and reemit gamma rays as they return to their original, lower-energy state. The energy of the resultant gamma rays is characteristic of specific nuclear interactions in

Remote Sensing (continued)

the surface, so measuring their intensity and wavelength allow a measurement of the abundance of several elements. One of these is hydrogen, which has a prominent gamma-ray emission at 2.223 million electron volts (a measure of the energy of the gamma ray). This can be measured from orbit, as it has been in the Mars Odyssey mission using a Gamma-Ray Spectrometer. The neutrons produced by the cosmic ray interactions discussed earlier start out with high energies, so they are called fast neutrons. As they interact with the nuclei of other atoms, the neutrons begin to slow down, reaching an intermediate range called epithermal neutrons. The slowing-down process is not too efficient because the neutrons bounce off large nuclei without losing much energy (hence speed). However, when neutrons interact with hydrogen nuclei, which are about the same mass as neutrons, they lose considerable energy, becoming thermal, or slow, neutrons. (The thermal neutrons can be captured by other atomic nuclei, which then can emit additional gamma rays.) The more hydrogen there is in the surface, the more thermal neutrons relative to epithermal neutrons. Many neutrons escape from the surface, flying up into space where they can be detected by the neutron detector on Mars Odyssey. The same technique was used to identify hydrogen enrichments, interpreted as water ice, in the polar regions of the Moon.


When an X-ray strikes an atom, its energy can be transferred to the electrons orbiting the atom. This addition of energy to the electrons makes one or more electrons leap from their normal orbital shells around the nucleus of the atom to higher orbital shells, leaving vacant shells at lower energy values. Having vacant, lower-energy orbital shells is an unstable state for an atom, and so in a short period of time the electrons fall back into their original orbital shells, and in the process emit another X-ray. This X-ray has energy equivalent to the difference in energies between the higher and lower orbital shells that the electron moved between. Because each element has a unique set of energy levels between electron orbitals, each element produces X-rays with energies that are characteristic of itself and no other element. This method can be used remotely from a satellite, and it can also be used directly on tiny samples of material placed in a laboratory instrument called an electron microprobe, which measures the composition of the material based on the X-rays the atoms emit when struck with electrons.

Visible and near-infrared

The most commonly seen type of remote sensing is, of course, visible light photography. Even visible light, when measured and analyzed according to wavelength and intensity, can be used to learn more about the body reflecting it.

Visible and near-infrared reflectance spectroscopy can help identify minerals that are crystals made of many elements, while other types of spectrometry identify individual types of atoms. When light shines on a mineral, some wavelengths are absorbed by the mineral, while other wavelengths are reflected back or transmitted through the mineral. This is why things have color to the eye: Eyes see and brains decode the wavelengths, or colors, that are not absorbed. The wavelengths of light that are absorbed are effectively a fingerprint of each mineral, so an analysis of absorbed versus reflected light can be used to identify minerals. This is not commonly used in laboratories to identify minerals, but it is used in remote sensing observations of planets.

The primary association of infrared radiation is heat, also called thermal radiation. Any material made of atoms and molecules at a temperature above absolute zero produces infrared radiation, which is produced by the motion of its atoms and molecules. At absolute zero, -459.67°F (-273.15°C), all atomic and molecular motion ceases. The higher the temperature, the more they move, and the more infrared radiation they produce. Therefore, even extremely cold objects, like the surface of Pluto, emit infrared radiation. Hot objects, like metal heated by a welder's torch, emit radiation in the visible spectrum as well as in the infrared.

In 1879 Josef Stefan, an Austrian scientist, deduced the relation between temperature and infrared emissions from empirical measurements. In 1884 his student, Ludwig Boltzmann derived the same law from thermodynamic theory. The relation gives the total energy emitted by an object (E) in terms of its absolute temperature in Kelvin (T), and a constant called the Stefan-Boltzmann constant (equal to 5.670400 x 10-8 W m-2 K-4, and denoted with the Greek letter sigma, ct):

This total energy E is spread out at various wavelengths of radiation, but the energy peaks at a wavelength characteristic of the temperature of the body emitting the energy. The relation between wavelength and total energy, Planck's Law, allows scientists to determine the temperature of a body by measuring the energy it emits. The hotter the body, the

Remote Sensing (continued)

more energy it emits at shorter wavelengths. The surface temperature of the Sun is 9,900°F (5,500°C), and its Planck curve peaks in the visible wavelength range. For bodies cooler than the Sun, the peak of the Planck curve shifts to longer wavelengths, until a temperature is reached such that very little radiant energy is emitted in the visible range.

Humans radiate most strongly at an infrared wavelength of 10 microns (micron is another word for micrometer, one millionth of a meter). This infrared radiation is what makes night vision goggles possible: Humans are usually at a different temperature than their surroundings, and so their shapes can be seen in the infrared.

Only a few narrow bands of infrared light make it through the Earth's atmosphere without being absorbed, and can be measured by devices on Earth. To measure infrared emissions, the detectors themselves must be cooled to very low temperatures, or their own infrared emissions will swamp those they are trying to measure from elsewhere.

In thermal emission spectroscopy, a technique for remote sensing, the detector takes photos using infrared wavelengths and records how much of the light at each wavelength the material reflects from its surface. This technique can identify minerals and also estimate some physical properties, such as grain size. Minerals at temperatures above absolute zero emit radiation in the infrared, with characteristic peaks and valleys on plots of emission intensity versus wavelength. Though overall emission intensity is determined by temperature, the relationships between wavelength and emission intensity are determined by composition. The imager for Mars Pathfinder, a camera of this type, went to Mars in July 1997 to take measurements of light reflecting off the surfaces of Martian rocks (called reflectance spectra), and this data was used to infer what minerals the rocks contain.

When imaging in the optical or near-infrared wavelengths, the image gains information about only the upper microns of the surface. The thermal infrared gives information about the upper few centimeters, but to get information about deeper materials, even longer wavelengths must be used.

Radio waves

Radio waves from outside the Earth do reach through the atmosphere and can be detected both day and night, cloudy or clear, from Earth-based observatories using huge metal dishes. In this way, astronomers observe the universe as it appears in radio waves. Images like photographs can be made from any wavelength of radiation coming from a body: Bright regions on the image can correspond to more intense radiation, and dark

Planck Curves for Black Bodies

Wavelength (Micrometers)

The infrared radiation emitted by a body allows its temperature to be determined by remote sensing; The curves showing the relationship between infrared and temperature are known as Planck curves.

parts, to less intense regions. It is as if observers are looking at the object through eyes that "see" in the radio, or ultraviolet, or any other wavelength, rather than just visible. Because of a lingering feeling that humankind still observes the universe exclusively through our own eyes and ears, scientists still often refer to "seeing" a body in visible wavelengths and to "listening" to it in radio wavelengths.

Radio waves can also be used to examine planets' surfaces, using the technique called radar (radio detection and ranging). Radar measures the strength and round-trip time of

Remote Sensing (continued)

microwave or radio waves that are emitted by a radar antenna and bounced off a distant surface or object, thereby gaining information about the material of the target. The radar antenna alternately transmits and receives pulses at particular wavelengths (in the range 1 cm to 1 m) and polarizations (waves polarized in a single vertical or horizontal plane). For an imaging radar system, about 1,500 high-power pulses per second are transmitted toward the target or imaging area. At the Earth's surface, the energy in the radar pulse is scattered in all directions, with some reflected back toward the antenna. This backscatter returns to the radar as a weaker radar echo and is received by the antenna in a specific polarization (horizontal or vertical, not necessarily the same as the transmitted pulse). Given that the radar pulse travels at the speed of light, the measured time for the round trip of a particular pulse can be used to calculate the distance to the target.

Radar can be used to examine the composition, size, shape, and surface roughness of the target. The antenna measures the ratio of horizontally polarized radio waves sent to the surface to the horizontally polarized waves reflected back, and the same for vertically polarized waves. The difference between these ratios helps to measure the roughness of the surface. The composition of the target helps determine the amount of energy that is returned to the antenna: Ice is "low loss" to radar, in other words, the radio waves pass straight through it the way light passes through window glass. Water, on the other hand, is reflective. Therefore, by measuring the intensity of the returned signal and its polarization, information about the composition and roughness of the surface can be obtained. Radar can even penetrate surfaces and give information about material deeper in the target: By using wavelengths of 3, 12.6, and 70 centimeters, scientists can examine the Moon's surface to a depth of 32 feet (10 m), at a resolution of 330 to 985 feet (100 to 300 m), from the Earth-based U.S. National Astronomy and Ionosphere Center's Arecibo Observatory!

levels in the atmosphere. Most of these are visible only in the near infrared, but some can be seen in radio and visible wavelengths, as well. The image of Uranus shown in the lower color insert on page C-1 demonstrates the large number of visible clouds and also shows 10 of Uranus's moons. Recently images from the Keck II telescope (completed in 1996 on Mauna Kea in Hawaii) have also shown large cloud features in the Uranian troposphere above the methane clouds. Uranus

Venus is imaged almost exclusively in radar because of its dense, complete, permanent cloud cover. Radar images of Venus have been taken by several spacecraft and can also be taken from Arecibo Observatory on Earth. The image below makes a comparison between the resolution possible from Earth using Arecibo (left), and the resolution from the Magellan spacecraft (right). Arecibo's image is 560 miles (900 km) across and has a resolution of 1.9 miles (3 km). The Magellan image corresponds to the small white rectangle in the Arecibo image, 12 x 94 miles (20 x 120 km) in area. Magellan's resolution is a mere 400 feet (120 m) per pixel.

The far greater resolution obtained by the Magellan craft (right) shows the relative disadvantage of taking images of Venus from the Earth (left) using the Arecibo Observatory. (NASA/Magellan/JPL)

was long thought to have only mild winds, but the better resolution of the Hubble Space Telescope and of the Keck II telescope show that Uranus has immense winds, some as fast as 360 miles per hour (580 km/hr).

The clouds on Uranus are thought to consist mainly of methane because the other constituents of the atmosphere, hydrogen and helium, will not condense into fluid droplets or freeze into crystals at the temperature and pressure conditions of the Uranian atmosphere. The other trace molecules thought to contribute to cloud formation are ammonia (NH ), ammonium hydrosulfide (NH SH), and hydrogen sulfide (H S).The location of the clouds is predicted based upon the temperature at which methane vapor will condense. Methane ice clouds are expected to form at pressures less than about one bar. Between about five bars and one bar, ammonia and hydrogen sulfide ice clouds should form. At pressures greater than five bars, clouds of ammonium hydrosulfide, water, ammonia, and hydrogen sulfide form, both alone and in solution with one another.

Both the Hubble Space Telescope and the Keck Observatory were recently able to resolve about 20 clouds on Uranus, nearly as many clouds on Uranus as the previous total in the history of modern observations.The cloud features seen in Keck and Hubble images are thousands of kilometers in diameter and exist at pressures from one to one-half bars. Improved imaging technology may be partly responsible, but more likely, cloud patterns on Uranus are highly dependent upon seasons, and the planet is moving into its second since humans have had the technology to see the planet.

Beyond the requirement of the most modern, high-resolution imaging techniques to see details in Uranian clouds, a second barrier to understanding Uranus's weather has simply been the length of its year, more than 84 Earth years. Although Uranus has been observed since its discovery more than 200 years ago, no one has ever seen this view of the planet in the modern era of astronomy. Because Uranus is tilted completely onto its side and orbits the Sun once every 84 years, it has 20-year-long seasons.The northern hemisphere of Uranus is at the end of its decades-long winter.

Early visual observers reported Jupiter-like cloud belts on the planet, but when NASA's Voyager 2 flew by in 1986, Uranus appeared as featureless as a cue ball. This may have been due to where Uranus was in its year, locked in northern-hemisphere winter. Since then, the planet has moved far enough along its orbit for the Sun to shine at mid-latitudes in its northern hemisphere. Observations over a period of five years have shown changes in the temperature scales and patterns, and this initial data will allow better modeling of the Uranian atmosphere. It is apparent from these images that seasonal changes caused by Uranus's orbit do change wind and cloud patterns on the planet.

By 2007 the Sun will be shining directly over Uranus's equator. During these periods, scientists have a better chance to see weather caused by seasonal changes. As imaging systems improve and Uranus can be observed through more of its seasons, scientists may find that Uranus has a much more active atmosphere than had previously been thought.

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