Appearance and Conditions

As soon as spectroscopy was developed in the early 20th century, the technique was applied to Jupiter to try to determine of what its atmosphere consisted (for more, see the sidebar "Remote Sensing" on page 34). In 1863 Ernest Rutherford, the New Zealand—born geologist and physicist, had used more primitive methods of examining light reflected from Jupiter and discovered features he could not explain. Rupert Wildt, a researcher at Princeton University, was able to use spectroscopy in 1932 to identify Rutherford's anomalies as methane (CH ) and ammonia (NH ). These constituents were a bit of a surprise, because hydrogen and helium were known to be the most abundant gas species in the solar system, and so were naturally expected to make up the bulk of Jupiter. Hydrogen was difficult to detect on Jupiter and was not confirmed until the 1960s. Helium was not detected until missions began flying closer to Jupiter (for more on elements and molecules, see the sidebar "Elements and Isotopes" on page 20). Since then, techniques for infrared spectroscopy from Earth have been developed, and many more constituents of the Jovian atmosphere have been discovered. Jupiter's atmosphere consists of about 89 percent hydrogen (H ), 11 percent helium (He), less than a percent of methane, and still smaller amounts of other trace compounds, including phosphine (PH3), germane (GeH where Ge is germanium, a relatively rare element), water (HO), ethane (CH), and acetylene (CH).

The highest parts of the atmosphere, called the stratosphere, thermosphere, and exosphere, have no clouds in them on Jupiter. The boundary at

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

Atmospheric Opacity

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

Long-wavelength radio waves are absorbed by atmosphere

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

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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.

X-rays

When an X-ray strikes an atom, its energy can be transferred to the electrons or biting 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 Steffan-Boltzmann constant (equal to 5.670400 X 10-8 W m-2 K-4, and denoted with the Greek letter sigma, a):

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.

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Remote Sensing (continued) The hotter the body, the 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 parts, to

Planck Curves for Black Bodies

Planck Curves for Black Bodies

20 40 100

Wavelength (Micrometers)

20 40 100

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.

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

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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!

the bottom of the stratosphere is the tropopause, the altitude that marks the coldest temperature in the atmosphere. Above the tropopause, the temperature rises, and below the tropopause, the temperature also rises. Above the tropopause, temperature increases due to absorption of solar radiation by high-atmosphere molecules like hydrogen (H ) and methane (CH ). Below the tropopause, temperature increases due to heat conducted out of the depths of the planet.

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)

The highest clouds on Jupiter exist just below the tropopause. Jupiter's tropopause has a temperature of about —256°F (—160°C) and a pressure of about 0.6 bar. There are two or possibly three layers of clouds at different heights in the atmosphere. The first is an ammonia (NH) cloud layer at around 0.5 bars. Ammonia freezes in the cold temperatures of Jupiter's atmosphere at these pressures, forming the white cirrus clouds that are shaped into zones, ovals, and plumes.The second has its base at 1.3 bars and seems to be made of ammonium hydrosulfide (NH SH).The temperature at the tops of the ammonium hydrosulfide clouds is about —58°F (—50°C), and trace elements and compounds of sulfur (S) or phosphorus (P) are thought to give them their sandy or red colors. There is thought to be a thin water (H O) cloud below the ammonium hydrosulfide cloud. Jupiter seems to be depleted in water compared to solar abundances, and so it may not have the thick layer of water clouds predicted by thermodynamics to condense at about five bars, where the temperature is about 68°F (20°C). Since the Galileo probe entered the atmosphere in a dry, cloud-free area, and the five-bar level is not visible by remote sensing, scientists do not know if a planet-covering layer of water clouds exists. Galileo did detect numerous lightning flashes coming from anticyclonic zones in Jupiter's atmosphere.The lightning flashes originated in formations that seem to be immense thunderstorms, and they are at the level in Jupiter where water clouds are expected to exist. Water, because of its ability to carry a slight electric charge, is efficient at creating lightning. Shown in the upper color insert on page C-3, these lightning storms may be indirect evidence that water clouds do exist on Jupiter.

In the deeper tropopause there seem to be enrichments in carbon (C), sulfur (S), phosphorus (P), and nitrogen (N), to the point that they are more enriched there than in the Sun (the Sun is a convenient comparison because it represents the average bulk composition of the solar system, and so saying that carbon is more enriched than in the Sun means that carbon is more enriched than on average in the solar system). The Galileo probe also measured enrichments in the heavy elements krypton (K), xenon (Xe), and argon (Ar). All of these enrichments are on the order of two to three times the solar values.

The enrichment in nitrogen, and some of the other elements, is a problem in terms of planetary formation. Nitrogen is a volatile element and so should be gaseous and exist at solar values at all but the coldest temperatures. To condense a higher proportion of nitrogen than is found in the Sun, the temperature has to be about —400°F (—240°C). This is much colder than space in the vicinity of Jupiter's orbit, where the temperature is about —166°F (—110°C), and, in fact, much colder than temperatures past Uranus and Neptune, in the vicinity of the Oort cloud comets, where space is still about —355°F (—215°C).There are at least two theories that have been presented to explain this quandary. One is that the planetesimals that formed Jupiter came from

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