Remote Sensing

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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)

•5" £ Metric: 0.1 nm 1 nm 10 nm 100 nm 1 |im 10 |xm 100 [im 1 mm 1 cm 10 cm 1 m 10 m 100 m ■5 Inches: 4x10"9 4x10"8 4xKT7 4x10"6 4xKT5 4x10"4 0.004 0.04 0.4 4 40 400 4,000

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-Boltzman 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!

about 100 million years. Over time gardening from the solar wind dulls the brightness of ejecta rays.

Copernicus, about 60 miles (95 km) in diameter, is a large young crater visible just northwest of the center of the Moon's Earth-facing side.Though at about 1 billion years old it is not as bright and fresh as Tycho, Copernicus also has many bright ejecta rays surrounding it. The image of Copernicus shown on page 164 was taken by the Lunar

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)

Orbiter 2. Copernicus has no basalt filling and its prominent central peak is easily seen in the image.

If an impact is made by a small object, less than a few kilometers in diameter (but greater than a few meters, the minimum needed to make a crater), the resulting crater is shaped like a bowl, and referred to as a simple crater. Larger craters undergo more complicated rebounding during impact, and end with circular rims, terraced inner

Copernicus crater is relatively young, as shown by its bright ejecta rays not yet dimmed by the action of the solar wind and disturbance from other impacts. (NASA/JPL/ Lunar Orbiter 2)

wall slopes, well-developed ejecta deposits, and flat floors with a central peak or peak ring.These craters are called complex craters. On the Moon, where erosion does not wear down craters as it does on Earth and the acceleration of gravity is only about 5.2 feet per square seconds (1.6 m/sec2) compared to Earth's 32 feet per square seconds (9.8 m/sec2), the central peaks of craters can be several kilometers high. By comparison, Mount Everest is 5.5 miles (8.8 km) high. A large central peak can be seen in the photo on page 165 of the Taruntius crater on the Moon. In this image the large Taruntius crater (35 miles [56 km] in diameter) is in the upper left, and smaller simple craters can be seen in the lava plains below.

While Taruntius and its neighbors demonstrate the differences between simple and complex craters, the lunar craters Herschel and Ptolomaeus show that some lunar craters are flooded with basalt while others nearby are not. In the image on page 165 from the Apollo 12, Herschel crater, 25 miles (40 km) in diameter, is at the center of this frame. Herschel lies at 5.7 degrees south and 2.1 degrees west in the lunar highlands. To the right is the 102 miles (164 km) diameter crater Ptolemaeus.

Herschel's floor is well defined and covered with rubble, showing a clear central peak and terraced walls. Ptolomaeus's floor, on the other hand, is relatively smooth and flat, despite the fact that it is a larger crater. The flatness of Ptolomaeus's floor is due to a basalt filling, much as the large mare basins are filled. Ptolomaeus's basalt filling is pocked with small impacts that are thought to have been caused by

The lunar crater Herschel is bare of lava, while its neighbor Ptolomaeus has a smooth lava filling.

(NASA/Apollo (2/NSSDC)

The lunar crater Herschel is bare of lava, while its neighbor Ptolomaeus has a smooth lava filling.

(NASA/Apollo (2/NSSDC)

These ancient sinuous rilles east of Aristarchus Plateau are thought to have been created by flowing magma during the time of the mare basalt eruptions, over 3 billion years ago. (NASA/Apollo 15/NSSDC)

material excavated by Herschel (these are called Herschel's "secondaries ).

In the above section on basins the hypothesis is discussed that crater excavation allowed the basalt magmas to erupt onto the surface. In this case Ptolomaeus, being the larger crater, would have excavated more of the crust and therefore is more likely to have allowed basalt magma to flow onto the surface.This may seem to be a simple explanation for the basalt in Ptolomaeus and the lack of it in Herschel, but two other considerations must also be made: First, the Moon has been cooling over time, and at some point the interior became too cool for basalt lava to be available to flow into the craters. Perhaps Herschel is simply much younger, and by the time it was formed there was no more basalt to flow. The second question might then follow: At the time of the great basins, was the upper mantle of the Moon partly melted at all times? Why was there basalt available to flow into the craters?

It is possible that there was liquid basalt lying under the crust waiting for giant impacts to allow it to flow to the surface—but all

This large lunar rille was probably formed by the collapse of a lava tube. (NASA/Apollo (o/GRIN)

the time that the basalt is waiting it is cooling and crystallizing. Another possibility is that the impact craters themselves caused the lunar mantle to melt. When the giant impacts that created the basins excavated crustal material, the bottom of the crust arched upward and made a dome in its bottom, as described in the section on basins above. The impact and the dome in the crust can influence the mantle beneath to start convecting, moving up into the dome and down off the rim. It is possible that the basalt formed by melting in these convection currents, and that is why basalt was available to erupt into the basins:The basin formation itself caused the mantle to melt.

The lunar surface is also marked with rilles, linear or curving features with the appearance of channels or perhaps collapsed lava tubes. They are thought to represent features created by moving magma at the time of the mare basalts, particularly because many originate at craters.The image on page 166 was taken on the Apollo 15 east of the Aristarchus Plateau. The largest rille in this southward-looking view

This large lunar rille was probably formed by the collapse of a lava tube. (NASA/Apollo (o/GRIN)

is Rima Prinz, which starts at the center of the image at the crater Prinz (about 28 miles, or 46 km, in diameter).

This image on page 167, from Apollo 10, shows a more perplexing rille, which appears to run up and over a small ridge. This rille may well be a collapsed lava tube. If the ceiling of a lava tube collapsed along its length, it would create the appearance of a channel running over all the topography above it.

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