Venus 1
Data
Name – The planet Venus was named for the Roman goddess of love and beauty
because of its radiant appearance in the sky. It is the only planet named after a
female. In fact, with few exceptions, the topographic features of Venus have been
named after real or mythical women.
Location – Venus is the second planet from the Sun, with an average distance of
approximately 67 million miles.
Size – Venus measures 7505 miles in diameter, making it only slightly smaller than
the Earth.
Orbital Period – 224.7 days.
Rotational Period – 243.08 days. Venus has the slowest rotation rate. Moreover, it
rotates retrograde (from east to west). On Venus, if an observer could see it through
the clouds, the Sun would rise in the west and set in the east. At present, there is no
solid explanation why this is so.
Number of Satellites – 0.
Observations
Venus is outshone only by the Sun and Moon. It owes its brilliance both to its
proximity to the Earth (Venus is the closest planet to Earth) and the Sun and to its
layers of clouds, which serves as an excellent reflector of sunlight.
Like Mercury, Venus is an inferior planet, restricting observations to a few hours
after sunset and before sunrise. The early Greeks thought it was two objects and
called it “Phosphorus” when it was in the morning sky and “Hesperus” when it was in
the evening sky.
Venus has also been called the Earth’s “sister” or “twin” planet; terms derived from
Venus’ similarity to Earth’s size, mass, density, and being the nearest planet to Earth.
Moreover, it was once speculated that Venus might have an Earth-like environment,
hospitable to life. However, the advancements of observational techniques and
spacecraft exploration have shown Venus to be a world quite different from the Earth.
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Atmosphere
Composition – Venus’ atmosphere is composed of 96.5% carbon dioxide (CO2) and
3.5% nitrogen (N2). Also, trace constituents of oxygen and water vapor have been
detected.
Cloud Composition – Initially, the yellowish-white clouds shrouding Venus were
believed to be composed of water, like those on Earth. However, observational
data in the 1970s showed these clouds are made of sulfuric acid (H2SO4). Venus’
clouds are formed by photochemistry – chemical reactions driven by the energy of
ultraviolet sunlight. Specifically, in Venus’ upper atmosphere, ultraviolet
radiation breaks down water vapor (H2O) into hydrogen (H) atoms and hydroxyl
(OH-) radicals. Sulfur dioxide (SO2), thought to be derived from volcanic activity
on Venus’ surface, chemically reacts with the hydroxyl radicals to produce the
sulfuric acid clouds.
Pressure – The atmosphere of Venus is much more massive than the Earth’s. The
pressure exerted at the surface of Venus by its atmosphere is 92 bars (92 times the
sea-level pressure on Earth). This is equivalent to the pressure a deep-sea diver deals
with at a depth of approximately 3000 feet.
Temperature – Based on Venus’ proximity to the Sun and the amount of reflected
solar radiation, its surface temperature was predicted to be approximately 140 F.
However, infrared measurements conducted in the 1960s suggested a temperature of
about 900 F. Subsequent studies by spacecraft have confirmed this high temperature
and have shown it to be nearly uniform over the entire planetary surface.
Why So Hot? – Incoming solar radiation is absorbed by a planet’s surface, heating
it up. The heated surface reradiates the energy as longer wavelength radiation
(infrared and radio). When an atmosphere is present, a portion of the reradiated
energy is absorbed causing a warmer temperature. The surface temperature
increases until a balance is established between the incoming solar radiation and
the outgoing planetary radiation. This process is called the greenhouse effect.
Because of Venus’ massive carbon dioxide-enriched atmosphere, it is very
effective in trapping the outgoing radiation, causing a very intense greenhouse
effect, and therefore the observed high surface temperature.
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Characteristics 2 – The atmospheric conditions near the surface of Venus are related
to the atmosphere’s extreme mass and the planet’s slow rotation. Circulation is very
slow and stable, with surface wind speeds rarely exceeding 4 miles/hour. Changes in
temperature are minimal from noon to midnight and equator to poles. Furthermore,
because of the high temperature of the lower atmosphere, sulfuric acid cannot
condense to form droplets, and therefore the Venus’ atmosphere is very clear. Some
sunlight does get through the cloud layer, providing an illumination about equivalent
to a heavy overcast on Earth. At an altitude of about 30 miles, the temperature of the
atmosphere decreases to about 85 F, and becomes significantly less dense (about 0.5
bars). A haze forms, gradually turning into sulfuric acid clouds at higher elevations.
Also, these high altitude conditions generate increased wind speeds, similar to Earth’s
jet streams. These winds exceed speeds of 200 miles/hour, producing what is referred
to as a super-rotating atmosphere.
Lightning 3 – In 2007, the European Space Agency’s Venus Express confirmed
that the Venus’ atmosphere generates lightning. Scientists currently know of only
three other planetary bodies in the solar system that generate lightning: Earth,
Jupiter and Saturn. However, unlike the water-associated lightning found on
Earth, Jupiter, and Saturn, Venusian lightning is associated with clouds of sulfuric
acid.
Evolution – Although it is likely that the processes leading to the formation of Venus’
and Earth’s present-day atmospheres were similar, the loss of water on Venus is the
key to understanding the divergence in atmospheric evolution between these two
worlds. The Earth’s early atmosphere was composed mainly of nitrogen, carbon
dioxide, and water vapor. While, volcanic outgassing is likely the principal source of
these gases, the water vapor may also have been acquired through impacts of comets.
Subsequently, the temperature was low enough to allow the water vapor to condense,
and over time, fall as rain. This rain, bringing most of the carbon dioxide with it,
formed the Earth’s oceans. A portion of the carbon dioxide precipitated out of the
newly formed oceans to form sediments that eventually turned into carbon-bearing
rocks, such as limestone. A younger Venus is believed to have possessed Earth-like
oceans, but these completely evaporated as the temperature rose due to the increasing
luminosity of the Sun. Solar ultraviolet light separated (dissociated) water molecules
into their constituent atoms, hydrogen and oxygen. The light mass hydrogen atoms
easily escaped from Venus because of their larger velocities, while the more massive
oxygen atoms with smaller velocities remained to combine with rocks on the surface
and other gases in the atmosphere. Without water (rain) to wash out the carbon
dioxide, the atmosphere remained rich in this gas producing the present atmospheric
conditions.
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Surface
Study – Until fairly recently, Venus' dense cloud cover has prevented scientists from
uncovering the geological nature of the surface. Developments in radar telescopes 4
and radar imaging systems orbiting the planet have made it possible to see through
the cloud deck to the surface below. Four of the most successful missions in
revealing Venus’ surface are: NASA's Pioneer Venus mission (1978), the Soviet
Union's Venera 15 and 16 missions (1983-1984), and NASA's Magellan radar
mapping mission (1990-1994). As these spacecraft began mapping the planet, a new
picture of Venus emerged.
Radar Images – The interpretation of radar images is very different from
photographs. The differences in contrast and apparent brightness are caused by a
variety of factors that are all based on how strongly the surface reflects the beam
of radio energy back to the receiver. More specifically, a radar-dark image is
produced by a surface that does not reflect much energy and a radar-bright image
is formed by a surface that reflects a strong signal. The reflectivity is influenced
primarily by the: (1) surface composition, (2) surface orientation (angle of
reflection) 5, and (3) degree of surface roughness 6.
Affect of External Processes – Because of Venus’ low surface wind speeds and lack
of water, the external processes of weathering, erosion, and deposition have a limited
affect on modifying the planet’s surface.
Affect of Internal Processes – Tectonic activity on Venus appears to be associated
with mantle plumes similar to hot spot activity that formed the Hawaiian Island chain,
and regional contraction and extension of the crust caused by mantle downward
movement and upward movement respectively. The term blob tectonics is sometimes
used to describe the mechanism associated with Venus’ surface deformation.
Rate of Tectonic Activity – The rate tectonic activity, and the relative age of a
planet’s surface, can be estimated by counting the number of craters per unit area
of surface. Less cratering suggests a younger surface, and therefore a more
geologically active planet that tends to erase crater scars. An analysis of the
number of craters on the surface of Venus implies an age of about 500 million
years. Furthermore, data from the Magellan probe, suggests that tectonic activity
has virtually ceased altogether, with only the occasional small eruption of lava
and sporadic heating at the base of the lithosphere to change Venus’ surface.
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Equilibrium Hypothesis Versus Global-Catastrophic Hypothesis – As mapping of
Venus’ surface proceeded, the craters revealed to be scattered randomly. Unlike
the Moon, Mercury, and Mars, there were no highly cratered (ancient) regions
intermixed with lightly cratered (younger) regions. The entire surface of Venus
appeared to be about the same age. The finding of a randomly distributed crater
population led to a scientific debate. One explanation is that volcanic eruptions
destroy craters about as fast as they form, so the numbers of craters remains about
the same. This is called the “equilibrium hypothesis” or what some geologists
called the uniformitarian model. Another interpretation is the “global-
catastrophic hypothesis”, according to which massive volcanic eruptions
approximately 500 million years ago resurfaced the entire globe and thus erased
all the craters that had formed earlier. Having been wiped clean, the surface again
began to collect craters, and nothing much else happened geologically except for
some faulting and a few large volcanoes.
Topography 7 – Venus’ surface is divided into two principal regions: (1) the lowlands,
comprising about 90% of the surface and (2) the highlands, making up the remaining
10% (compared with 45% continental surface on the Earth). The lowlands are
basaltic in composition and result from volcanic lava deposits. The highlands are
mainly located in two continent-like features called Aphrodite Terra and Ishtar Terra,
and are similar in size to Africa and Australia respectively. They were likely formed
by low-density crustal material piling up to great thickness. Located within Ishtar
Terra, the Maxwell Mountains (named after the 19th century Scottish scientist who
first formulated the laws of electromagnetic radiation) represent the highest summit
on Venus, rising to an elevation of about 7 miles.
Features – Venus exhibits some surface features varying and unique to the planet.
Craters – Tectonic activity has erased many craters on the surface of Venus.
However, due to the limited affect of external processes, the few craters
remaining are in pristine condition. The Magellan probe discovered almost 1000
craters on Venus, ranging in size from the large double-ringed crater named
Mead 8, with a diameter of nearly 175 miles, down to as small as 2 miles across.
Although, Venus’ craters display many of the same features of craters observed
on the Moon, Mercury, and Mars, Venus’ dense atmosphere has caused a number
of unusual affects relating to their formation:
1. Cut-Off Size – Venus’ surface does not exhibit craters less than approximately
2 miles across. Apparently, the planet’s massive atmosphere destroys most
incoming projectiles that would produce craters of these sizes.
2. Splotches 9 – Incoming projectiles slightly smaller than the cut-off size do not
leave craters, rather marks called splotches. They are interpreted as locations
where Venus’ surface has been disrupted by the shock waves from projectiles
that did not survive passing through the dense atmosphere.
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3. Irregular Craters and Crater Clusters 10 – Craters on Venus less than about 20
miles in diameter are usually irregular or multiple excavations. Apparently,
the incoming projectiles that produced these craters broke apart during
passage through the planet’s dense atmosphere.
4. Butterfly-Shaped Ejecta Blanket 11 – The ejecta blankets of many Venus’
craters display a “butterfly-shaped” pattern. A crater formed from an oblique
(i.e., not vertical) impact generates a trailing wake through the dense
atmosphere that prevents the ejecta from moving in the direction from which
the projectile came.
Volcanoes – Volcanoes are quite prevalent on the surface of Venus. It is not
known if any volcanoes are currently active, but certainly such activity is recent
on a geological timescale. Many of the planet’s volcanic structures appear very
similar to their terrestrial counterparts, but there are some variations as well.
Shield Volcanoes 12 – Like Earth and Mars, the largest volcanic structures on
Venus are shield volcanoes. They are characterized by shallow slopes and a
summit caldera (the remnant of the vent through which an eruption takes
place), and can be several hundred miles across and a few miles high.
Pancake Domes 13, 14 – Formed by the eruption of thick, viscous lava, pancake
domes are circular, flat-topped volcanoes that can be nearly 30 miles in
diameter and 2 miles high. Apparently, all of the lava comprising a pancake
dome is erupted at once from a single vent.
Tick Volcanoes – Like the pancake domes, tick volcanoes are broad,
mostly flat features, and they often have a central pit or vent structure.
The difference is that they are surrounded by an array of short, radial
ridges. (In this image 15, the “head” of the tick is defined by a set of small
collapse pits.) The origin of the “leg” ridges is unknown, but two options
have been suggested. First, the ridges may outline avalanche scars. In this
case, the tick is simply an old pancake dome with a heavily eroded rim.
The second option is that the ridges mark dikes (bodies of rock that cut
across surrounding layers of rock) running out from the central “body”.
Coronae 16 – A corona is a unique volcanic structure of Venus, characterized
by large concentric rings of fractures, within which large eruptions of lava
have occurred repeatedly. Apparently, each corona is the result of a mantle
hot spot (a plume of rising magma). The rising mantle plume causes the
overlying crust to bulge and fracture, which leads to frequent episodes of
volcanism. When the upwelling subsides, the bulge deflates, producing a
collapsed dome 17. Alternatively, coronae may represent a stage in plume
development that will lead to later surface eruptions and the construction of
shield volcanoes.
Arachnoids 18 – Arachnoids are similar in form, but generally smaller than
coronae. One theory concerning their origin is that they are a precursor to
coronae formation.
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Lava Channels – Venus also experiences eruptions of very low viscosity lava
that can flow across the surface for great distances before solidifying 19. The
result is the formation of lava channels of remarkable length. For example,
Hildr 20 is over 4,000 miles long. In contrast, the longest lava channels on
Earth extend only a few tens of miles.
Wind Streaks 21 – Although wind speeds near Venus’ surface are too slow to
make sand and dust effective as cutting tools and agents of erosion, they are fast
enough to move these particles. In Magellan’s images, wind-related streaks are
observed in the lee of obstacles. These streaks are not deposits of sand and dust;
instead the wind has changed the surface in ways that make it rougher (and thus
bright in radar images) or smoother (radar-dark).
Interior
Composition and Structure – At present, no seismic data is available concerning
Venus, and therefore the planet’s interior is poorly understood. The internal
composition of Venus is probably similar to that of the Earth, since their sizes,
masses, and average densities (5.20 g/cm3 as compared to Earth’s average density of
5.52 g/cm3) are nearly identical. Observations of Venus’ surface have shown rocks of
an igneous nature 22, supporting the proposed model of an internal structure that is
differentiated into a crust, mantle, and core 23.
Magnetic Field – Spacecraft studies have repeatedly demonstrated that Venus has no
magnetic field. It is likely that Venus has a metal core and a portion of it is in a liquid
state, but the rotation rate is too slow to maintain the necessary convective currents to
produce a magnetic field.
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