**Physical Characteristics of Some Solar System Objects**

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  **Object**    **Physical Characteristics**

  Mercury       No appreciable atmosphere. No liquid water. Evidence for
                volcanic activity in distant past. Many impact craters.
                Surface temperature variations are the largest in the
                solar system, from --300°F to 800°F, depending on whether
                the surface is facing toward or away from the Sun. No
                protection from the Sun's ultraviolet light. Life not
                likely.

  Venus         Thick atmosphere, mostly carbon dioxide. No liquid water.
                Evidence of extensive volcanism in past; not clear if
                Venus is still volcanically active. Surface temperature
                close to 900°F. Atmospheric pressure at surface of Venus
                is about 90 times that at Earth's surface (similar to
                pressure 1 km under water on Earth). Life unlikely.

  Moon          No atmosphere. No liquid water. Many impact craters.
                Volcanoes active in distant past. Small amount of water
                ice may be found in craters near poles on far side. No
                protection from the Sun's ultraviolet light. Life not
                likely.

  Mars          Thin atmosphere, mostly carbon dioxide. Water ice in
                polar ice caps and possibly underground in permafrost. No
                liquid water now, but evidence of liquid water in past.
                Surface temperature ranges widely, from --207°F at the
                winter pole to almost 80°F on the day side during summer.
                Atmospheric pressure at surface is about 1/100^th^ that
                at the surface of the Earth. Evidence for active
                volcanism in the past, but now geologically inactive. No
                protection from the Sun's ultraviolet light at surface.
                Life possible especially underground or in polar ice
                caps.

  Jupiter       Thick atmosphere, mostly hydrogen. No distinct surface.
                No volcanism. No liquid water. Temperature at top of
                clouds is --240°F, but rises as you go deeper into
                atmosphere (reaching tens of thousands of degrees deep in
                interior). Atmospheric pressure increases from about the
                pressure at the surface of the Earth at top of Jupiter's
                clouds to hundreds of thousands of times the Earth's
                surface atmosphere deep in interior. Life not likely.

  Io            Moon of Jupiter. Very thin atmosphere (about a billion
                times less dense than Earth's atmosphere) of mostly
                sulfur dioxide gas. No liquid water. Very active
                volcanically, with geyser-like eruptions of sulfur
                dioxide and liquid sulfur flowing, like lava, from
                volcanoes. Surface temperature averages --220°F, although
                volcanic vents can be much hotter. Subject to bombardment
                by charged particles from Jupiter's magnetic field. No
                protection from Sun's ultraviolet light. Life not likely.

  Europa        Moon of Jupiter. Extremely thin atmosphere of oxygen
                (ozone). Surface covered in thin (ten kilometers or less)
                crust of water ice. Surface ice fractured by cracks, and
                jumbled crust resembling icebergs. Surface temperature
                --250°F. Very likely ocean of liquid water under ice,
                perhaps 100 km deep, heated by molten core. Lava
                eruptions possible on ocean floor. Subject to bombardment
                by charged particles from Jupiter's magnetic field. No
                protection from Sun's ultraviolet light at surface. Life
                possible in ocean.

  Ganymede      Moon of Jupiter. Extremely thin atmosphere of oxygen
                (ozone). No liquid water. Surface temperature --250°F.
                Surface riddled with grooves and ridges. Icy surface
                crust may overlay a mantle of rock, with a molten iron or
                iron/sulfur core. Subject to bombardment by charged
                particles from Jupiter's magnetic field. No protection
                from Sun's ultraviolet light at surface. Life not likely.
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**Physical Characteristics of Some Solar System Objects**

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  **Object**        **Physical Characteristics**

  Callisto          Moon of Jupiter. Extremely thin atmosphere of carbon
                    dioxide. Heavily cratered. No liquid water. Mixture of
                    ice and rock on surface and throughout the moon (not
                    differentiated into different layers like Earth and other
                    moons like Europa and Ganymede). Surface temperature
                    --250°F. Subject to bombardment by charged particles from
                    Jupiter's magnetic field. No protection from Sun's
                    ultraviolet light at surface. Life not likely.

  Saturn            Thick atmosphere, mostly hydrogen. No distinct surface.
                    No volcanism. No liquid water. Temperature at top of
                    clouds is --220°F, but rises as you go deeper into
                    atmosphere. At the top of Saturn's clouds, the
                    atmospheric pressure is about that of Earth's at sea
                    level. The pressure is a thousand times greater deep in
                    the cloud interior. Life not likely.

  Titan             Moon of Saturn. Thick atmosphere, mostly nitrogen. No
                    liquid water. Atmosphere contains many hydrocarbons,
                    including methane and ethane. Atmospheric pressure at the
                    surface is 1.5 times that at the surface of the Earth.
                    Surface temperature is --290°F. Lakes or oceans of liquid
                    methane and ethane may exist on Titan's surface. Areas of
                    rock, like continents, may exist among the oceans or
                    lakes. Scientists think Titan's atmosphere may resemble
                    that on the Earth when life first began, and before life
                    put oxygen into Earth's atmosphere. Life possible, but
                    probably not water-based life.

  Uranus            Thick atmosphere, mostly hydrogen, some helium. No
                    distinct surface. No volcanism. No liquid water.
                    Temperature at top of clouds is --320°F, but rises as you
                    go deeper into atmosphere. Atmosphere may rest above an
                    icy or liquid mixture of water, ammonia, and methane,
                    which in turn surrounds a \"rocky\" core, about the size
                    of the Earth. Life not likely.

  Miranda           Moon of Uranus. No appreciable atmosphere. No liquid
                    water. Relatively few impact craters, but has huge cliffs
                    and canyons. Large impact at some time in the past may
                    have shattered Miranda, which then reassembled, leaving a
                    jumbled heap of odd-shaped fragments that we see as
                    cliffs and canyons. Surface temperature is --335°F, Life
                    not likely.

  Neptune           Thick atmosphere, mostly hydrogen and helium. No distinct
                    surface. No volcanism. No liquid water. Temperature at
                    top of clouds is --330°F, but rises as you go deeper into
                    atmosphere. Atmosphere may rest above an icy or liquid
                    mixture of water, ammonia, and methane, which in turn
                    surrounds a \"rocky\" core, about the size of the Earth.
                    Life not likely.

  Triton            Moon of Neptune. Extremely thin atmosphere, mostly
                    nitrogen. Surface temperature of --391°F is the coldest
                    of any planet or moon in solar system (average surface
                    temperature is --355°F). No liquid water. Surface
                    consists of ices of nitrogen, methane, carbon dioxide and
                    carbon monoxide. Active geyser-like eruptions spew
                    nitrogen gas and dark dust particles several kilometers
                    into the atmosphere. Life not likely.

                    

  **Physical        
  Characteristics   
  of Some Solar     
  System Objects**  

  **Object**        **Physical Characteristics**

  Pluto             Extremely thin atmosphere of nitrogen and methane present
                    when Pluto is closest to the Sun (atmospheric pressure
                    about 1/100,000^th^ that at surface of Earth). Atmosphere
                    freezes out onto surface when Pluto moves away from Sun.
                    Surface temperature of --373 °F. Surface ices of
                    nitrogen, with methane and traces of carbon monoxide. No
                    liquid water. Life not likely.

  Comet             Composed of water ice, frozen carbon dioxide (dry ice),
                    and rock. When close to Sun, ice turns into gas that
                    trails out from comet in a "tail" hundreds of thousands
                    of kilometers long. Tail contains gases of water vapor,
                    carbon dioxide, carbon monoxide, and other organic
                    molecules, plus dust particles. No liquid water. Contains
                    precursors of life and may have brought them to Earth in
                    collisions with young Earth. Life possible, but not
                    likely.

  Asteroid          No appreciable atmosphere. Composed of rock and/or iron
                    and nickel. Contain some organic molecules, including
                    some precursors of life and may have brought them to
                    Earth in collisions with young Earth. Life possible, but
                    not likely. No liquid water. Life not likely.
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