Venus- the Hot Planet

Planet Venus is named after the Roman goddess of love and beauty.

Head of Venus de Milo Louvre Ἀφροδίτη τῆς Μήλου

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.

Venus conjunction with the moon in the night sky© T.V’Soske/Cosmotions.com

Both Venus and Earth are rocky planets….

cross section of Venus and Earth showing crust, mantle and core

with a similar size, mass, density…

Earth and Venus compared

Image of Venus shown without cloud cover

and force of gravitational attraction.

height of a slam dunk compared on Venus and Earth

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.

early Earth showing clouds and tectonic activity© Don Dixon at Cosmographica.com 

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.

early earth covered by an ocean of liquid water© Don Dixon at Cosmographica.com 

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.

Astronaut wearing an exosuit on surface of venusPyro Firearms: Deviant art

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.

Most submarines, such as the Russian Akula attack submarine Щука-Б donot dive to depths greater than 450 metres or at a pressure exceeding 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.

graph showing temperature and atmospheric pressure change with height on Venus

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.

layers of the earths atmospherecsep10.phys.utk.edu

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!

composition of atmospheres of venus and 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.

surface of Venus from venera 13

The 20 kilometer deep cloud blanket surrounding the planet….

Venus image with marble white cloud cover

prevents spacecraft taking optical photos of the surface.

 Venus Express aerobraking about dense clouds of Venus

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.

venus express orbiting Venus

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.

venus cloud cover esa

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.

Lightning storm on surface of Venus

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.

average winds speeds around Earth

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!

Venus retrograde motion compared with earth direct motion

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….

Planetessimal collides with early Venus

 knocked Venus upside down.

retrograde rptation of Venus resulted from a collision with a planetessimal billions of years ago

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!

Orbit of Venus and Earth around the SunModified from Astrobob.areavoices.com

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.

Venus in transit across the face of the Sun

Volcanic Activity

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.

estla regio

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.

Plains of Lakhsmi and Maxwell Montes on Venus

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.

  • Why was Venus resurfaced  by one massive ejection of magma?

Scientists believe that there was one massive build up of heat, pressure and volcanic magma in the mantle.

how magma build up in crust of venus before being ejected onto planetary surface

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.

earth showing tectonic plates earthquakes and active volcanoes

These weak points are located  at those places where two continental plates meet.

Formation of Volcanoes on Earth showing movement of plates

Two Types of Volcano

As on Earth…

 from Mauna Loa Observatory

Mauna Kea shield volcano, Hawaii

Venus has many shield volcanoes.

Sapas Mons

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.

diagram of a shield volcano

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.

Earth shield volcanoes and venusian shield volcanoes compared

Venus also has a large number of uniques features called pancake domes. Pancake domes are similar to the lava domes found on Earth…

Novarupta lava dome Alaska

Novarupta Lava Dome, Alaska

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.

pancake domes venus

Continents

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.

topography of Venus showing Ishtar Terra and Aphrodite Terra

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.

Ishtar_terra_topo

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.

Maxwell Montes range of mountains on Ishtar Terra, VenusDavid P. Anderson Southern Methodist University

  • So how are elevations on Venus calculated? 

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.

Venus showing mean planetary radius of 6052 kms

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’ .

terraformed Venus showing mountains and seas

Impact craters

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.

Venus impact crater distribution

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.

Crater farm Venus

Rocks

Chemical measurements of the planetary surface suggest that most of the rocks are made of basalt. Basalt is produced following eruptions of molten lava.

Venera 13 landing showing surface of Venus

The basalt on Venus is remarkably similar to the volcanic rocks of mid ocean ridges on Earth.

Mid ocean ridge of North Atlantic showing location of Iceland

Thingvellir lake  in Iceland is one of the few places where a mid-ocean ridge made of basalt can be seen in shallow water.

mid Atlantic ridge, Thingvellir lake, Iceland

Magnetic field

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.

magnetic fields of venus and earth deflecting solar radiation from sun

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.

magnetic field of Venus with strong solar winds

At those times when the solar winds weaken, the magnetic field expands.

magnetic field of Venus expands as solar winds weaken

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.

Auroras on Venus shown on nightside of the planet

In contrast auroras on Earth only occur at or near the polar regions. (see Earth as a Magnet for an explanation)

Aurora australis superimposed over the southern polar region

Earth’s magnetic field is generated internally…

earths magnetic field in period of normal polarity between reversals

….but Venus lacks a similar internally generated magnetic field.

  • Why is this?

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.

cross section of earth showing solid inner core and liquid outer core

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.

Diagram of convection currents in outer core of Earth

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.

Coriolis force and convection currents of Earth's outer core

Regarding Venus, it is by no means certain that Venus has any liquid metallic outer core.

cross section of Venus showing crust, mantle and 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.

Venus Fun Stuff

Astronauts exploring Venus above the cloud tops. Fantasy or reality?

Further Reading

Further information for science projects can be found at European Space Agency

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