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Most remote sensing of planets and moons is by passive sensors, which simply detect and record natural radiation emitted by the atmosphere or surface (see Chapters 6, 9, and 13). However, in recent years, a number of spacecraft have been equipped to carry out active remote sensing with lidars (laser imaging detection and ranging systems) or radar.

One of the main uses of radar is altimetry. The distance between a spacecraft and the planet below can be calculated by precise measurement of the time it takes for a microwave signal to return to the spacecraft from the surface, irrespective of whether it is an ocean, ice, or land. Over time, such data can provide topographic maps of an entire planet.

Figure 3.13 Spacecraft such as ESA's CryoSat use radar to map surface topography. The first radar echo comes from the nearest point to the satellite. CryoSat can measure the angle from which this echo originates, so that the source point can be located on the ground. This, in turn, allows the height of that point to be determined.

(ESA)

The characteristics of the echoes also provide further information about the roughness of the surface, wave heights or wind speeds over the ocean. Such measurements can be made 24/7, regardless of cloud cover or night conditions.

One of the major uses of radar altimetry has been the investigation of variability in sea surface height and its impact on the general circulation of the oceans. Such measurements highlight the importance of eddies in shaping and controlling major ocean currents. They also reveal the growth and development of major climatic events, such as El Niño.

The accuracy of altimetry measurements depends on knowing the spacecraft's precise orbital position. Many Earth observation satellites carry a radio receiver and laser reflectors for precise orbit determination.

By combining two or more radar images of the same area, spaceborne systems also make it possible to produce maps showing surface change associated with both earthquakes and subsurface volcanic activity which are accurate to within a few millimeters.

Another area of interest is the determination of changes in the surface height and area of global ice cover, particularly in the Arctic Ocean, Greenland, and Antarctica. Measurements made over a number of years can reveal whether the ice sheets are losing or gaining mass and thickness – an important clue with regard to climate change.

Ground‐based radar has been used for many years to reveal rain and snowfall from storm systems. Radar instruments installed in spacecraft are now being used to give a wider perspective on major storms. For example, the US‐Japanese Tropical Rainfall Measuring Mission (TRMM) satellite carried the first radar flown in space to measure precipitation. The instrument worked by measuring the echoes backscattered from rain. Since the strength of the echo is roughly proportional to the square of the volume of falling water, the instrument produced very accurate estimates of rainfall.

Figure 3.14 A multi‐temporal radar image of the Bay of Naples in Italy as seen on three separate occasions by ESA's ERS‐2 satellite. With its numerous buildings, the city of Naples (top center) is very radar reflective and bright, as are mountain ridges facing the radar. To the east of Naples is Vesuvius, one of the most explosive volcanoes in Europe. To the west is a much older volcanic region, the Phlegrean Fields. The colored patches north of Naples are fields of crops. The colors offshore indicate sea surface roughness on each viewing date.

(ESA)

Other spacecraft such as CloudSat carry a millimeter‐wavelength, cloud‐profiling radar which is over 1,000 times more sensitive than typical weather radar. It can not only take a vertical slice through clouds and storm systems, even in the polar winter, but also distinguish between cloud particles and precipitation.

Major glaciations are known to have taken place during the late Proterozoic (between about 800 and 600 million years ago), the Pennsylvanian and Permian (between about 350 and 250 million years ago), and the Quaternary (the last 4 million years).⁹ There is evidence that at least two dozen warm‐cold cycles have occurred during the past 1.6 million years, with the most recent glacial advance peaking about 20,000 years ago and ending about 10,000 years ago. Today, most of Earth's fresh water is locked up in the ice sheets that persist over Antarctica and Greenland.

Various theories to explain the ice ages have been proposed, including the emergence of supercontinents due to continental drift, reductions in solar activity, asteroid impacts, enormous volcanic eruptions, and cosmic dust clouds obscuring the Sun. However, the most widely accepted explanation for the recent ice advances is the astronomical theory popularized by the Serbian scientist, Milutin Milankovitch, between 1920 and 1941.

According to Milankovitch, Earth's current glacial‐interglacial cycles are mainly the result of slow, but significant, changes in its orbit (see Orbit and Rotation). Three orbital parameters are especially important in causing the waxing and waning of ice sheets:

 Changes in the eccentricity of Earth's orbit over a period of 100,000 years;

 Changes in the tilt of Earth's axis over a period of 41,000 years;

 Precession of the orbit over a period of 22,000 years.

Milankovitch developed a mathematical model that was able to calculate how these variables would influence the amount of insolation reaching the planet's surface at different latitudes. He showed that Earth's changing orbital geometry can reduce the amount of solar radiation reaching high latitudes (around 65°N) by 10–15%. These periods of lower summer heating (and melting) coincide with advances of the polar ice sheets and mountain glaciers. Nevertheless, the Milankovitch theory does not explain all of the fluctuations in ice cover during the Quaternary or in the distant past, and other factors almost certainly played a part.

Figure 3.15 Earth's ocean currents are driven by the prevailing winds. Many of the currents circulate around the subtropical high pressure zones – so they flow clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. Currents moving away from the equator are regarded as “warm”; those moving away from polar regions are “cold.” Huge amounts of heat energy are transferred from the tropics to higher latitudes by warm currents. This helps to keep the northwestern coasts of Europe and North America warmer than would otherwise be the case.

(NASA)

Figure 3.16 The oceanic conveyor belt. Warm water is carried by ocean currents to higher latitudes, where the stored heat is released into the atmosphere. As the water cools it becomes denser. In regions where the water is also very salty, such as the North Atlantic, the water becomes dense enough to sink to the bottom. This travels at depth around Antarctica and into the Indian and Pacific basins before returning to the surface. Carbon dioxide is also transported during this circulation. Cold water absorbs carbon dioxide from the atmosphere, and some is carried to great depth. When deep water returns to the surface in the tropics and is warmed, the carbon dioxide is released back into the atmosphere.

(NASA)

Figure 3.17 (Top) Normal conditions in the Pacific Ocean. The trade winds blow steadily toward the west, piling up warm ocean water. Moist, warm air rises, causing heavy convectional rain. In contrast, conditions are dry in the eastern Pacific, where cold water rises from depth and cools the air above. (Bottom) During El Niño, the trade winds weaken and warm water moves eastward, causing wet conditions in the Americas and dry conditions in the western Pacific. The red areas of warmer water correspond to higher sea surface.

(ESA‐ATG medialab)

What we do know is that, at the height of the last major advance, ice sheets about 2 km thick covered much of Europe and North America. Enough water was locked up in these ice sheets to cause sea levels to drop more than 100 m below present levels. Such dramatic changes in ice cover inevitably led to major shifts in surface drainage and vegetation cover. The subsequent melting of the massive ice sheets has caused the crust to rebound – a process called isostatic uplift which is still going on today.