Our paradigm of Mars has shifted several times, particularly during the past 40 years of spacecraft observations. Lowell's view of a world criss-crossed by canals built by a dying race of Martians was supplanted in the 1960s by Mariner views of a geologically dead world with an atmosphere too thin to support liquid water. Mariner 9's discovery of channels and volcanoes shifted the view to one of a planet where water existed in the past and led to renewed interest in the possibility that life might exist on our neighboring world. The Viking lander investigations quashed hopes of finding the martian soil teeming with microbial life, but refocused the interest in biological activity (past or present) to identifying localized oases. Recognition that we have samples of the planet's crust in the form of martian meteorites has greatly influenced our understanding of the planet's bulk chemistry and thermal evolution. The latest shift in our mental picture of Mars has revealed that water, either liquid or ice, has played a larger role throughout the planet's history than previously recognized. Our post-Viking view was that Mars had a thicker atmosphere and warmer, wetting surface conditions early in its history, but the post-Noachian period has been characterized by the cold, dry climate that we see today. Insights gained from MGS, Odyssey, MER, and MEx combined with increasing computational capabilities have revealed that short-lived climate excursions have occurred up through recent times, driven by changes in orbital parameters and the planet's obliquity.

The possibility of current (extant) or past (extinct) life on Mars has been of interest for centuries. Percival Lowell's early ideas of intelligent life forms who engineered an elaborate canal system have been disproved, but the question of whether microbial life exists or has existed on Mars remains an area of intense debate. The question of martian life is one of the driving forces behind the continuing exploration of the planet.

Martian conditions relevant to biology

Surface conditions on present-day Mars are extremely inhospitable to terrestrial life forms (Clark, 1998). The thin CO2 atmosphere is inefficient at retaining daytime solar heating, resulting in a temperature range of 130–250 K with an average temperature of 240 K. No evidence of biologic replication has been observed in terrestrial organisms at temperatures < 253 K (Beaty et al., 2006). The low temperatures and low atmospheric pressure at the surface prevent liquid water from existing for extended periods of time. Terrestrial life forms require water for survival, so the lack of liquid water on Mars is a major deterrent to life on the surface.

The thin martian atmosphere and the lack of a present-day magnetic field allow harmful radiation to penetrate to the surface (except perhaps in regions where rocks retain strong remnant magnetization [Alves and Baptista, 2004]). Odyssey's MARIE instrument (Badhwar, 2004) measured the radiation environment above the martian atmosphere until October 2003 when particles from a large solar flare caused it to cease functioning.

PA (p.a.) Abbreviation for ➤ position angle.

Paaliaq A small outer moon of Saturn, in a very elliptical orbit. It was discovered in 2000 and is about 19 km (12 miles) across.

2 Pallas A large asteroid discovered by Heinrich W. M. Olbers in 1802. With a diameter of 533 km (331 miles), it is the second largest asteroid. It is of the carbonaceous type, similar to the largest asteroid, ➤ Ceres. Its orbit is at the unusually steep inclination of 35° to the plane of the solar system.

Pallene A small moon of Saturn discovered in 2004 by the ➤ Cassini team. It orbits Saturn at a distance of 211 000 km (131 000 miles) between Mimas and Enceladus.

Palomar Observatory The observatory on Palomar Mountain in California where the 5-m (200-inch) ➤ Hale Telescope is sited. It is owned and operated by the California Institute of Technology (Caltech). The other principal instruments at the observatory are the 1.2-m (48-inch) Oschin Telescope (a ➤ Schmidt camera), a 46-cm (18-inch) Schmidt camera and the 1.5-m (60-inch) reflector owned jointly by Caltech and the Carnegie Institution of Washington.

Pan A small moon of Saturn orbiting in the ➤ Encke Division in the planet's ring system at a distance of 133 583 km (83 005 miles) from Saturn. It was found by Mark R. Showalter in 1990 from studies of images taken by the spacecraft ➤ Voyager 1 and ➤ Voyager 2.

Caelum (The Chisel) A small constellation with no star brighter than fourth magnitude. It was introduced into the southern sky in the mid-eighteenth century by Nicolas L. de Lacaille, who called it “Caela Sculptoris.”

caldera A large volcanic crater.

Caliban One of two small moons of Uranus discovered in 1997 by Brett Gladman and others, using the ➤ Hale Telescope. It is reddish in color and thought to be a captured ➤ Kuiper belt object. Its diameter is estimated to be 98 km (61 miles).

California Nebula (NGC 1499) A bright ➤ emission nebula in the constellation Perseus, named for it resemblance to the shape of the US state. It forms the rim of a dark nebula of gas and dust illuminated by the star Xi Persei.

Callirrhoe A small outer moon of Jupiter discovered in 1999. Its diameter is about 9 km (6 miles).

Callisto The second largest moon of Jupiter and one of the four discovered in 1610 by Galileo. With a diameter of 4821 km (2996 miles), it is also the third largest moon in the solar system. Callisto is the darkest of the Galilean satellites and also the least dense, which suggests that it contains a high proportion of water, though detailed images returned by the ➤ Galileo spacecraft indicate that the surface has more rock and dust than previously supposed. Under its thick icy crust, Callisto may have a liquid ocean several kilometers deep. ➤ Voyager and Galileo images show a heavily cratered surface with no high hills or mountains.

aberration An effect that makes the observed position of a star slightly different from its true position. It results from a combination of the finite speed of the starlight and the motion through space of the observer on Earth. Most aberration is due to Earth's yearly motion in orbit around the Sun and is called annual aberration. A much smaller contribution from Earth's daily rotation is called diurnal aberration.

absolute magnitude A number that gives the true, relative brightness of an astronomical body, ignoring the dimming effect of distance. The absolute magnitude of a star is the ➤ magnitude it would appear to be if it were 10 ➤ parsecs away. The absolute magnitude of a planet, asteroid or comet is the ➤ apparent magnitude it would have if it were at a distance of 1 AU from both the Sun and Earth, with its disk fully illuminated.

absolute zero The lowest possible temperature. It is the zero point of the Kelvin temperature scale used in science. Its equivalent on the Celsius scale is −273.16°C.

absorption line A sharp dip in a continuous ➤ spectrum. Absorption lines look like narrow gaps in a spectrum. They are seen in the spectra of the majority of stars. In the case of the Sun, they are known as ➤ Fraunhofer lines.

Atoms create these dark lines by absorbing radiation. Each chemical element creates a unique pattern of lines. By measuring the strengths of absorption lines it is possible to deduce the abundance of the various elements, though the lines are also affected by temperature, density and other factors.

z The symbol normally used for ➤ redshift.

zenith The point directly overhead.

zenithal hourly rate (ZHR) The hypothetical rate at which meteors belonging to a particular ➤ meteor shower would be observed by an experienced observer, watching a clear sky with limiting magnitude 6.5, if the radiant were located in the zenith. In practice, observed rates are always lower, because fewer meteors are detected when the radiant is lower and skies are rarely so ideally clear.

zodiac A belt of 12 constellations through which the Sun's path in the sky – the ➤ ecliptic – passes. They are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius and Pisces. Formerly, the ecliptic went through only these 12 constellations, but the effects of ➤ precession and the precise definitions of constellation boundaries mean that it now also goes through a thirteenth, Ophiuchus. Since the orbits of the planets lie very nearly in a plane, their apparent paths remain in or close to the zodiacal constellations.

In traditional astrology, the zodiac is divided into 12 equal 30° portions, each of which is allocated to a “sign,” but these do not correspond exactly to the astronomical constellations, which are of varying sizes. The effect of precession has also contributed to increasing disparity between the astrological signs and the astronomical constellations.

zodiacal light A faint cone of light in the sky extending along the ecliptic. It is visible on clear moonless nights in the west following sunset, and in the east just before sunrise.

UBV photometry A method of measuring the colors of stars introduced in the 1950s. It is based on measuring the ➤ magnitudes of stars in three regions of the spectrum called U (ultraviolet), B (blue) and V (visual), which are centered on wavelengths 350, 430 and 550 nm, respectively. More bands were added later to extend the scheme into the infrared. These are called R, I, J, H, K, L, M and N, ranging from 0.7 to 10.2 µm.

UFO Abbreviation for ➤ unidentified flying object.

UKIRT Abbreviation for ➤ United Kingdom Infrared Telescope.

UKST Abbreviation for ➤ United Kingdom Schmidt Telescope.

ultra-luminous infrared galaxy (ULIRG) A galaxy emitting exceptionally strongly in the infrared. ULIRGs were first detected by the ➤ Infrared Astronomical Satellite in 1983. They appear to be the results of collisions between two or more galaxies, which trigger immense bursts of star formation. The infrared radiation is emitted by dust, which absorbs energy from hot, newborn stars. Observations suggest that many ULIRGs are the mergers of three or more galaxies.

ultraviolet astronomy The study of electromagnetic radiation from astronomical sources in the wavelength band 10–320 nm. Ultraviolet (UV) radiation is strongly absorbed by Earth's atmosphere, so all UV observations have to be carried out from satellites. The earliest observations were made during brief rocket flights in the 1940s and 1950s. The first satellite to make systematic ultraviolet observations was the first Orbiting Solar Observatory (OSO-1) in 1962.

RA Abbreviation for ➤ right ascension.

radar astronomy The use of radar for astronomy. Radar can be used to detect ➤ meteor showers, to measure distances to bodies in the solar system and determine their size and shape, and to map the surfaces of planets and moons. Radar signals transmitted by the 305-m (1000-foot) radio telescope at ➤ Arecibo Observatory have been used to map Venus and to characterize the size, shape and structure of ➤ asteroids. The ➤ Magellan spacecraft, placed in orbit around Venus, used ➤ synthetic aperture radar to map the planet's surface, which is concealed by opaque cloud, and the ➤ Cassini spacecraft used radar to study the surface of ➤ Titan. Radar is of fundamental importance for making precise measurements of distance within the solar system.

radial velocity The velocity of an object relative to an observer along the line of sight.

radiant The point on the celestial sphere from which the trails of meteors belonging to a particular ➤ meteor shower appear to radiate. Meteors entering Earth's atmosphere from a stream create trails that are almost parallel but perspective makes them seem to diverge from a point in the sky.

radiation belt A ring-shaped region around a planet where electrically charged particles (electrons and protons) are trapped and spiral around the direction of the magnetic field of the planet. The radiation belts surrounding the Earth are known as the ➤ Van Allen belts. Similar regions exist around other planets with magnetic fields, such as Jupiter.

Sagan, Carl Edward (1934–1996) The American planetary scientist Carl Sagan was one of the most well-known and influential figures in astronomy in the USA during his lifetime. He was particularly known for his interest in extraterrestial life and he strongly advocated searching for extraterrestrial intelligence. As an advisor to NASA, he contributed to many of NASA's planetary exploration space missions. A gifted popularizer, he wrote several bestselling books. His television series Cosmos, first shown in 1980, was an enormous success. In it Sagan examined a wide range of issues in science, particularly the origin of life in the universe. From 1968 he worked at Cornell University and directed the Laboratory for Planetary Studies there.

Sagitta (The Arrow) The third-smallest constellation, but nevertheless a distinctive little group of stars. The two brightest stars are third magnitude. It lies in a rich part of the Milky Way, next to Aquila.

Sagittarius (The Archer) The southernmost constellation of the zodiac. The center of the Galaxy (the Milky Way) lies behind the star clouds in Sagittarius. It is a large constellation, with many bright stars. It also contains a large number of star clusters and nebulae. The ➤ Messier Catalogue lists 15 objects in Sagittarius, more than in any other individual constellation. They include the ➤ Lagoon Nebula, the ➤ Trifid Nebula, the ➤ Omega Nebula and the third brightest ➤ globular cluster in the sky, M22.

Sagittarius A The strongest radio source in the constellation Sagittarius.

Baade, (Wilhelm Heinrich) Walter (1893–1960) Walter Baade was born in Germany and began his astronomical career at the Bergedorf Observatory of the University of Hamburg. While there, he discovered the unusual asteroid ➤ Hidalgo. In 1931 he moved to the ➤ Mount Wilson Observatory in California where he worked until his retirement. He discovered a total of 10 asteroids, including ➤ Icarus.

In the 1940s, he used the 100-inch telescope at Mount Wilson to resolve individual stars in the ➤ Andromeda Galaxy and two of its small companion galaxies. This led him to divide stars into two broad groups, ➤ Population I and ➤ Population II. He realized that Population I had the characteristics of young stars while population II was older. He also found that each population had its own kind of ➤ Cepheid variable star. Cepheids had been used incorrectly to judge the distance of the Andromeda Galaxy and Baade showed that it was twice as far away as previously thought. In the 1950s he worked on identifying radio sources, including ➤ Cygnus A.

Baade's Window An area of sky around the globular cluster NGC 6522 in the constellation Sagittarius, which is particularly rich in stars. The astronomer Walter ➤ Baade drew attention to it. He realized that very distant stars are visible in that direction because there is relatively little interstellar material to hide them.

Baikonur The Russian manned space-flight center, established by the former Soviet Union. It is situated north-east of the Aral Sea in Kazakhstan.