Why do we call Venus the sister planet of the Earth?

Answer 1

Venus and Earth are very close to the same size:
Earth = mass of 45.9742 x 10^24 kg, density 5.515 g/cm^3, radius of 6378.140 km

Venus = mass of 4.8690 x 10^24 kg, density 5.24 g/cm^3, radius 6051.9 km

Both have cloudy atmospheres. Venus' orbit is closer to ours than Mars'. And until we got some very sensitive instruments out there, everybody assumed that Venus would have a similar (though warmer) climate than the Earth -- think of how many old sci-fi/fantasy novels and movies had civilizations on Venus. But instruments discovered that Venus suffers from a runaway greenhouse effect, and surface temperatures there are on the order of 730°K = 457°C = ~855°F!! And the atmosphere, which was originally thought to be water vapor, turns out 96.4% carbon dioxide, 3.4% nitrogen, and only 69 parts-per-million oxygen! Hardly a hospitable place . . .

Answer 2

Because Venus and the Earth are the
same size, scientists call Venus Earths sister planet.
They also call Venus our sister planet
because of the striking similarities to Earth.
Venus is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass
It is the closest to Earth as far as diameter and composition

Answer 3

Venus is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass).
Both have few craters indicating relatively young surfaces.
Their densities and chemical compositions are similar.

Answer 4

It is the closest to Earth as far as diameter and composition. It also has a thick atmosphere and is in the outer regions of the 'Goldilocks' zone from the Sun (not too hot, not too cold). If it was not for the runaway greenhouse effect in its atmosphere, it would probably very similiar to Earth in other ways.

Answer 5

Similar size & scientists used to think the atmosphere on Venus could possibly support life. Now we know that not to be true. (certainly not terrestrial life.)

Answer 6

Earth and Venus are similar in their masses, diameters, and densities --

MASS (..in kilograms..)
Earth : 5.977^24
Venus : 4.87^24

DIAMETER (..in kilometers..)
Earth : 12,756
Venus : 12,104

DENSITY (..in grams per cubic centimeter..)
Earth : 5.517
Venus : 5.23

Answer 7

Hi. It is about the same size and has an atmosphere.

Answer 8

it's almost as big as the earth.

Answer 9

Venus is an inferior planet, and, like Mercury, it is always seen in the vicinity of the sun in the sky. This also means that Venus has phases. We have already seen how the phases of Venus helped Galileo determine that the Earth orbited the sun.

The planet Venus is the second planet out from the sun and it has often been referred to as the ``sister'' planet of Earth because it is similar to the Earth in size and mass. Venus is 82 percent the mass of the Earth and 95 percent of the Earth's radius. Its density is also similar to that of the Earth and 5.3 gm/cc. The orbit of Venus is the closest to circular of any planet in the solar system, and the radius of its orbit is 0.72 AU. It has an orbit (sidereal) period of 225 days. Given all these similarities to Earth it is natural to assume that Venus might resemble the Earth in many ways. This is most definitely not borne out by the facts, however.

Venus is a cloud covered world. In fact the clouds are so thick that a great deal of the sunlight reaching Venus is reflected back into space. Venus' albedo is 0.65. Venus' rotation rate was difficult to determine since the cloud cover means that no surface features are visible. Early efforts centered on trying to observe changing patterns in the clouds and from that deducing a rotation rate. Accurate rotation rates were finally determined by the technique of bouncing radar signals off of the surface of Venus (radar waves being unaffected by clouds). The doppler shift produced by the rotating planet spreads out the frequencies of the return signal. The portion of the planet that is approaching you has a blueshifted return signal, while the opposite limb (which will be receding due to rotation) is redshifted. What the radar data showed was that Venus has a very slow rotation rate (243 days) and that it sense of rotation is retrograde, that is it spins in the opposite sense from its orbit around the sun.

Recall that a local solar day (a Venusian day) would be defined from one noon to the next. The length of a solar day depends both on the rotation rate of the planet and on the orbital speed. On Venus these combine to give a Venusian solar day of 117 Earth days. The strange thing about this number is that it is equal to one fifth of the synodic period of Venus as seen from the Earth. What this means is that at inferior conjunction the same face of Venus is pointing toward the Earth. It is possible that this means that tidal interactions with the Earth have helped to set Venus' rotation rate. But there is no compelling explanation of how that would come about since the tidal force of Earth on Venus is rather small.


Venus has had more spacecraft sent to it than any other planet so we are gradually beginning to acquire considerable data about it. The very first mission to Venus found that Venus has no detectable magnetic field. Since Venus has slightly smaller mass and density as compared with the Earth it may be reasonable to assume that it has a somewhat similar interior. Therefore we would expect that Venus should have a liquid iron/nickel core. Note however that we have no seismic data to support that supposition since none of the landers survived long enough to detect Venusquakes. If we assume an liquid iron core then it must be the case that the very slow rotation rate of Venus precludes the generation of a magnetic field. This would be fine except we have already learned that Mercury does have a small magnetic field and it is also a slow rotator. Planetary magnetism is not fully understood, by any means.

The lack of magnetic field means that Venus has no magnetosphere or radiation belts. However, there is still a solar wind, and this can still interact with Venus. Without a magnetic field the solar wind simply runs into the upper part of Venus' atmosphere forming a shock wave known as a ``bow shock.'' In planets with magnetic fields the bow shock occurs out at the edge of the magnetosphere. On Venus the bow shock lies close to the top of the atmosphere.

Now let's take up the subject of the Venusian atmosphere. The earliest spectroscopic observations (that is hunting for absorption features in the reflected light spectrum) revealed abundant carbon dioxide and no evidence for water or oxygen. Space missions have subsequently established that the Venusian atmosphere is 96 percent carbon dioxide. Other gasses include Nitrogen (3 percent) and traces of hydrogen chloride (hydrochloric acid), hydrogen sulfide (sulfuric acid), water vapor, argon, and sulfur dioxide. Although the cloud cover reduces the amount of sunlight that gets to the surface of Venus, infrared observations gradually indicated that the planet was very warm in its lower atmosphere. Now that spacecraft (the Soviet Venera series) have actually landed on Venus we know that the temperature at the surface is 750 K. Temperatures like this are greater than on Mercury! The atmospheric pressure is 90 times that of the Earth.

The reason that Venus is so hot is because of the ``Greenhouse Effect.'' Solar radiation is emitted mainly in the visible portion of the spectrum and that radiation can penetrate through the cloud layer (at least partially). The planet reradiates this energy as infrared radiation. Carbon dioxide has strong absorption features in the infrared band which means that the carbon dioxide molecules absorb the outgoing radiation and retain it in the planet. The planet remains at a very high temperature due to this carbon dioxide ``blanket.'' So with high temperatures and a choking poisonous, sulfur-laden atmosphere Venus comes about as close as any place we know in realizing the classic image of Hell.

Space Probes and in particular the atmosphere experiments on the Pioneer Venus mission have given us a reasonable picture of the cross-section of the Venusian atmosphere. The tops of the clouds are about 75 kilometers up from the surface. There are high winds there, driven mainly by the differential heating from the sunlight driving large convection cells as in the Earth's atmosphere. However the coriolis forces are weaker because of Venus' slow rotation and the convection cells are larger than on the Earth. Speeds can reach up to 360 km per hour at the top of the clouds. Patterns in the clouds have occasionally been detected from the Earth as they move across the face of Venus. These patterns can be seen much more clearly in ultraviolet photos. This top layer of clouds is composed of sulfuric acid droplets at a temperature of about 290 K (nearly room temperature). From around 50 to 55 kilometers above the surface there are clouds of sulfuric acid and particles of solid sulfur at a temperature of 475 K. Lightning bolts have been suspected to occur in this region and the presence of lightning was confirmed during the passage of the Galileo spacecraft around the planet. It picked up radio signals of the type generated by lightning. Below that there is a thick haze extending down to about 23 kilometers. Below this layer of clouds the atmosphere is relatively cloud free down to the surface. At the surface the winds are mostly calm, but the atmospheric pressure is about 100 atmospheres (100 times normal on the Earth). This is roughly equivalent to being 3000 feet deep in the ocean. Photos sent back from landing craft provide pictures consistent with a dark, reddish, gloomy and stagnant atmosphere.

Given the data on Venus' atmosphere and what we know of the atmosphere here on the Earth we can reconstruct a tentative history for the atmosphere of Venus. Here on the Earth we have almost no atmospheric carbon dioxide whereas on Venus it makes up 96 percent. Both planets obtained their atmospheres from early volcanic outgassing so their subsequent histories must have taken far different paths. On the Earth the water in the atmosphere condensed out to form the oceans. The carbon dioxide was washed out of the atmosphere by dissolving into the water. Such carbonated water will eventually form carbonate rock such as limestone. Thus most of the original carbon dioxide in the Earth has been trapped back into the rocks. If you took all the rocks and got their carbon dioxide out then you would have about as much as on Venus. Presumably Venus was just a bit too hot for liquid water to form. Without the rain to scrub out the carbon dioxide it just remained in the atmosphere and warmed the planet further. Once the process is started it doesn't get turned around, so Venus just got hotter and hotter. This is known as a ``runaway greenhouse effect.'' What happened to the water on Venus? The process of ``photodissociation,'' in which incident solar energy breaks molecules apart, has caused the water vapor to mostly separate into hydrogen and oxygen. The oxygen has interacted chemically and is out of the atmosphere while the hydrogen has escaped. The implications of this history are that the presence of liquid water in the early atmosphere of a planet is very important in determining the subsequent nature of its atmosphere.

Space probes have provided data about the surface of Venus from two sources: landers and radar mappers. All of the landers have been Soviet Venera probes. The harsh conditions on the surface of Venus make it very difficult for spacecraft to survive there but some of them have lasted long enough to return data and a few pictures. The pictures reveal a barren landscape of boulders and rocks. The rocks show some signs of weathering but do tend to have angular edges suggesting that weathering hasn't been that strong. It is, however, present. This weathering is not due to rain (since there isn't any) but presumably due to the effects of the high pressure atmosphere and the light surface winds. The rocks appear to be basaltic (lava type) rocks. Venera 9 revealed slabs of rock lying on regolith. Both the rock and the regolith have low albedo, again consistent with igneous rock. Rock at the Venera 13 and 14 sites appeared to be volcanic tuff, a material formed out of volcanic ash. Needless to say there was no sign of life.

Some indications of the nature of the surface of Venus has come from bouncing radar beams off of the planet from the Earth. This is possible only when Venus is closest to the Earth (inferior conjunction), and, you will recall, this means that at best information about only one side of the planet could be obtained by this technique. The surface of the whole planet is mapped by satellites orbiting and sending down radar beams. The first such mapper was the Pioneer Venus orbiter. It revealed lowlands and rolling plains plus three major highland regions (continents). The rolling plains cover about 65 percent of the surface, the lowlands 27 percent, and the continental highlands 8 percent. The plains have eroded meteorite craters and many lava-filled craters that may be of volcanic origin. The lowlands appear to be very smooth and may represent basaltic flows similar to the Earth's ocean basins or the moon's maria. The highlands are named Ishtar Terra, Aphrodite Terra and Beta Regio. There are a few large volcanoes there. Apparently Venus is a geologically active world although less so than Earth.

The Magellan spacecraft mapped the surface of Venus with high accuracy using radar, and provided us with the best information so far. The analysis of the data found many interesting features. There is no evidence of active plate tectonics like we see on Earth. Nothing looks like continental plate spreading as in the Earth's mid-ocean ridges. However, there is evidence of past crustal fracturing and folding. It may be that the high temperatures on Venus keep the surface somewhat flexible and less likely to form rigid plates. Many large shield volcanoes have been seen and some seem to have new material around them suggesting ongoing volcanic activity. Many surfaces do not have many impact craters. Using the method of crater counting to date the surface implies that these are relatively young surfaces, say 200 to 400 million years old.


We can construct a tentative geological history of Venus: as with the other planets it formed by the coalescence of small planetesimals during the accretion phase of planet formation. The planet was massive enough to be liquid and hence underwent differentiation. There followed the epoch of significant bombardment. The continents formed sometime after this, perhaps due to the extrusion of magma from the mantle. Magma flowing out from beneath the crust formed the surfaces of the lowland areas and part of the rolling plain area. This extensive geological activity gradually died out but survives to this day in the form of active shield volcanoes in the Beta Regio area.

In comparing terrestrial worlds we develop some general ideas. Apparently larger planets have more geological activity. The have a greater internal temperature because they cool slower. They have a thinner crust and a partially liquid mantle and core. These last two features would be needed for active plate tectonics or active vulcanism, and the liquid core is probably needed for the generation of a significant magnetic field (if the planet is rotating fast enough). Thus we expect more massive planets to have younger surfaces. The Earth, obviously, is quite geologically active. Venus appears also to be active but at a reduced rate. The Moon and Mercury are geologically dead. We shall see in the next section that the intermediately-massed planet Mars was once active and is now dead.