It is the second brightest object in the night sky and is our closest planetary neighbor. Here Venus can be seen in conjunction with the Moon.
Both Venus and Earth are rocky planets….
with a similar size, mass, density…
and force of gravitational attraction.
It is amazing to consider that, 4 billion years ago, the atmospheres of Earth and Venus were not so very different.
At that time both planets experienced major tectonic activity; volcanic eruptions ejected vast amounts of carbon dioxide, among other compounds, into both planets’ atmospheres.
This image shows Earth as it might have looked like 4 billion years ago.
The atmospheres of both planets were composed of carbon dioxide with smaller proportions of ammonia, methane and water vapour. The water vapour condensed to form clouds and liquid water.
Since that time 4 billion years ago the atmospheres of both planets have taken different evolutionary paths. Today their atmospheres could not be more different.
The atmosphere of Venus today
The stand out feature of Venus’s atmosphere is that there is a huge amount of it! Because there is so much atmosphere, the weight pushing down onto the surface of the planet from above is simply enormous!
(Read this article for an explanation of atmospheric pressure)
To withstand such enormous pressure, which reaches 90 bars on the surface, any astronaut walking on the planet would have to wear special protection in the form of an incredibly tough exosuit.
By way of comparison most submarines on Earth do not dive to depths anywhere near the 90 bars as found on the surface of Venus.
Submarines do not generally descend to depths greater than 450 meters; at this depth the pressure only reaches 45 bars.
Atmospheric pressure on Venus decreases rapidly with increased altitude. As this graph shows, the atmospheric pressure on Venus reaches 1 bar, the surface pressure at sea level on Earth, 5o kms above the surface of the planet.
The atmosphere above Venus extends much further into space than the atmosphere above Earth. The troposphere of Venus, where all the weather happens, extends for an altitude of 100 kilometers; contrast this with Earth’s troposphere which extends to an altitude of only 14 kilometers.
The atmosphere on Venus is mostly composed of carbon dioxide gas (96.5%), with some nitrogen (about 3.5%) and tiny amounts of other gases. In contrast Earth’s atmosphere is composed of 78% nitrogen, 21% oxygen with only 0.03% carbon dioxide gas.
Venus has 300,000 times more carbon dioxide in its atmosphere than Earth!
Mean average temperatures on the surface of Venus reach a sweltering 465 °C; compare this with the mean average temperature on Earth of only 15 °C.
This image was captured by the Venera 13 spacecraft which landed on the planetary surface in October 1981.
The 20 kilometer deep cloud blanket surrounding the planet….
prevents spacecraft taking optical photos of the surface.
As a consequence the only details known about the planet’s surface come from the use of ‘radar imaging’; radio waves are bounced off the planet’s hard surface allowing us to ‘see’ through the clouds.
The amount of sunlight reaching the surface of Venus through the thick clouds would be equivalent to standing on Earth on a dull, overcast day in the middle of winter.
The upper part of the planet’s cloud blanket is composed of tiny droplets of sulfuric acid which form sulfuric acid clouds. These sulfuric acid clouds produce sulfuric acid rain. This rain never reaches the ground; it evaporates before it can reach the planetary surface.
The lower atmosphere generates lightning which can strike the surface.
Average wind speeds on Venus regularly reach velocities higher than four hundred kilometers per hour.
In contrast average wind speeds on Earth are much slower; the fastest winds in the Southern Ocean on Earth only average around 32 kilometers per hour.
Rotation around its axis
Venus rotates around its axis incredibly slowly taking 243 earth days to make just one full revolution.
Venus rotates in a retrograde motion; it rotates in the opposite direction to the Earth which rotates in a prograde or direct motion.
An observer standing on the surface of Venus would see the sun rising in the west and setting in the east 243 earth days later!
The reason for Venus’s slow retrograde rotation around its axis is not known with any certainty.
One plausible explanation is that a collision billions of years ago with another small planet….
knocked Venus upside down.
Orbit around Sun
Venus orbits the Sun in the same way as all other planets in the Solar System, following the rotational direction of the Sun.
A venusian year takes the equivalent of 225 earth days; this makes a venusian year 18 earth days shorter than a venusian day!
Venus orbits the Sun at a distance of 108 million kilometers; Earth orbits the Sun at a distance of 150 million kilometers.
In this image Venus can be seen in transit across the face of the Sun ‘only’ 42 million miles from the Earth.
As a result of extensive lava flows the surface of Venus is surprisingly smooth; the smoothness of the planetary surface can be seen in this image showing the extensive lava flows of the Eistla Regio region of the planet. Also visible is the mountain Gula Mons ( 3 km high) and the impact crater Cunitz which is 48.5 kilometers (30 miles) in diameter.
In fact 80% of the surface of Venus is covered in volcanic lava.
This satellite image shows the dark, smooth lava flows of the Plains of Lakhsmi bordering the lighter colored Maxwell mountain range.
The ejection of volcanic lava onto the surface appears to have happened in one massive ejection event. This ejection event happened between 500 and 800 million years ago when the surface of Venus was flooded with volcanic lava; older impact craters were covered over and the planet was largely resurfaced.
Scientists believe that there was one massive build up of heat, pressure and volcanic magma in the mantle.
When the internal pressure became too great there were simultaneous, massive eruptions which ejected volcanic lava onto the surface.
In contrast to Venus, volcanic eruptions happen all the time; they occur at weak points in our planet’s crust.
These weak points are located at those places where two continental plates meet.
Two Types of Volcano
As on Earth…
Venus has many shield volcanoes.
Shield volcanoes are formed from thin and runny lava (‘low viscosity’) following volcanic eruptions.
Because the lava travels great distances before solidifying, shield volcanoes have wide bases and gentle slopes.
The higher temperatures on Venus means that lava tends to travel further before solidifying.
As a result the shield volcanoes on Venus have a broader base and are less high than shield volcanoes on Earth.
Venus also has a large number of uniques features called pancake domes. Pancake domes are similar to the lava domes found on Earth…
but on Venus they are up to 100 times larger.
Pancake domes are very broad, have a flat top and are less than 1000 meters in height. Their formation is believed to have been the result of the extrusion of runny (‘viscous’) lava.
Unlike Earth, Venus only has two large ‘continents’.The continent of Ishtar Terra, the size of Australia, is located near the north pole.
The other continent, Aphrodite Terra is located just south of the equator.
It is the continents that contain most of the deep valleys and mountains.
Rising high on Ishtar Terra, shown in red below, is a huge mountain range called Maxwell Montes.
Maxwell Montes rises 11 kilometers (6.8 miles) at its highest point. The mountain range is 853 kilometers (530 miles) long by 700 kilometers (435 miles) wide. Its elevation means that it is the coolest (about 380 °C ) and least pressurised (about 45 bars) location on Venus.
The heights of the mountains and the depths of the ‘seas’ are calculated in relation to the mean planetary radius of 6,052 kilometers.
Maxwell Montes’ maximum elevation of 11 kilometers is a distance 11 kms wider than the mean planetary radius.
In the same way the ‘seas’ of Venus are calculated as distances shorter than the mean planetary radius.
This fantasy ‘terraformed’ map of Venus shows all the elevations both above and below ‘sea level’ .
The thick atmosphere of Venus guarantees that small meteorites burn up before hitting the planetary surface. Only the largest meteorites, with diameters greater than 2 kms, hit the ground without burning up.
Large impact craters appear evenly distributed across the whole planet.
This image below shows three impact craters.
Saskia (37.3 km diameter) is in the foreground, Danilova ( 47.6 km diameter) to the rear left, and Aglaonice (62.7 km diameter) to the rear right.
Chemical measurements of the planetary surface suggest that most of the rocks are made of basalt. Basalt is produced following eruptions of molten lava.
The basalt on Venus is remarkably similar to the volcanic rocks of mid ocean ridges on Earth.
Thingvellir lake in Iceland is one of the few places where a mid-ocean ridge made of basalt can be seen in shallow water.
Venus’s magnetic field is 0.000015% the size of Earth’s magnetic field. As a result of being so small Venus’s magnetic field deflects far less solar radiation than Earth’s.
Venus’s tiny magnetic field results from the interaction between electrically charged particles in the solar wind and the upper atmosphere of the planet. This interaction creates a weak induced magnetic field which deflects the solar winds round the planet.
This weak magnetic field, shown by yellow magnetic field lines, can be visualised below.
At those times when the solar winds weaken, the magnetic field expands.
The interaction of the solar winds with particles in the upper atmosphere even creates auroras. Auroras on Venus occur anywhere on the ‘night side’ of the planet, even on the equator.
In contrast auroras on Earth only occur at or near the polar regions. (see Earth as a Magnet for an explanation)
Earth’s magnetic field is generated internally…
….but Venus lacks a similar internally generated magnetic field.
To answer this question we need to look at Earth’s core.
Earth has a 5,700°C solid iron inner core as hot as the surface of the Sun. However the huge pressure caused by the gravitational force prevents the inner core turning to liquid.
Temperatures in the outer core are as ‘low’ as 4,030 °C. The lower pressures in the outer core mean that iron and other metals can now exist in a liquid form.
Differences in temperature and pressure within the outer core cause convection currents in the molten metal; cool, dense liquid iron sinks whilst the warm, less dense liquid iron rises.
The fast spin of earth’s rotation also causes swirling vortices of liquid iron in the outer core.
This complicated flow of liquid iron generates electric currents, which in turn produce our planet’s magnetic fields.
Regarding Venus, it is by no means certain that Venus has any liquid metallic outer core.
Even if Venus did have a liquid outer core the incredible slow rotation of the planet would mean that swirling vortices of liquid iron could never be generated.
Astronauts exploring Venus above the cloud tops. Fantasy or reality?
Further information for science projects can be found at European Space Agency