Venus Compared to Earth

Venus is often referred to as “Earth’s Twin” (or “sister planet”), and for good reason. Despite some rather glaring differences, not the least of which is their vastly different atmospheres, there are enough similarities between Earth and Venus that many scientists consider the two to be closely related. In short, they are believed to have been very similar early in their existence, but then evolved in different directions.

Earth and Venus are both terrestrial planets that are located within the Sun’s Habitable Zone (aka. “Goldilocks Zone”) and have similar sizes and compositions. Beyond that, however, they have little in common. Let’s go over all their characteristics, one by one, so we can in what ways they are  different and what ways they are similar.

Size, Mass and Orbit:

In terms of their respective sizes, masses and compositions, Venus and Earth are quite similar. Whereas Earth has a mean radius of 6,371 km and a mass 5,972,370,000 quadrillion kg, Venus has a mean radius of about 6,052 km and a mass of 4,867,500,000 quadrillion kg. This means that Venus is roughly 0.9499 the size of Earth and 0.815 as massive. In terms of volume, the two planets are almost neck and neck, with Venus possessing 0.866 as much volume as Earth (928.45 billion cubic km compared to Earth’s 1083.21 billion).

But when it comes to orbit, the two planets are a bit different. Earth orbits the Sun at an average distance (semi-major axis) of 149,598,023 km (92,955,902 mi), ranging from 147,095,000 km (91,401,000 mi) at perihelion to 152,100,000 km (94,500,000 mi) at aphelion. Venus, meanwhile, orbits the Sun at an average distance of 108,208,000 km, ranging from 107,477,000 km at perihelion to 108,939,000 km.

Basically, Venus orbits closer to our Sun and with an eccentricity that is less than one-third that of Earth’s (0.006772 compared to 0.0167086). In addition, Earth’s axis is tilted far more than Venus’ towards the Solar ecliptic – 23.5° compared to Venus’ 2.64°. This greater proximity to the Sun is largely responsible for Venus’ runaway greenhouse effect, and the low eccentricity (combined with the minor tilt in its axis) results in very little variation in temperature (see below).

Structure and Composition:

Being terrestrial planets, Venus and Earth have similar structures and compositions. Earth’s interior is divided into layers based on their chemical or physical properties, consisting of a core, mantle, and outer crust. Whereas the core region consists of nickel and iron, the mantle and outer crust are composed of silicate rock and minerals.

While little direct information exists about Venus’ seismology, its similarity in size and density to Earth suggests that it has a similar internal structure – consisting of a core, mantle and crust. Like that of Earth, the Venusian core is at least partially liquid because the two planets have been cooling at about the same rate.

The principal difference between the two planets is the lack of evidence for plate tectonics on Venus, possibly because its crust is too strong to subduct without water to make it less viscous. This results in reduced heat loss from the planet, preventing it from cooling.

Another major difference is that Earth’s core is divided between an inner and outer core. Whereas the outer core is believed to consist of a low viscosity liquid, the inner core is believed to be solid. The liquid outer core also rotates in the opposite direction as the planet, producing a dynamo effect that is believed to be the source of Earth’s magnetosphere (see below).

Surface Features:

Unlike other planet’s in our Solar System, the majority of Earth’s surface is covered in liquid water. In fact, about 70.8% of the surface is covered by oceans, lakes, rivers and other sources, with much of the continental shelf below sea level. In addition, Earth’s terrain varies greatly from place to place, regardless of whether or not it is above or below sea level.

The submerged surface has mountainous features, as well as undersea volcanoes, oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining portions of the surface are covered by mountains, deserts, plains, plateaus, and other landforms. Over long periods, the surface undergoes reshaping due to a combination of tectonic activity and erosion.

Venus’ surface, in contrast, has little variation in terms of elevation, with the majority covered by smooth, volcanic plains. In fact, it is estimated that if a terraforming event began to allow for water to accumulate on the surface, roughly 80% of the planet would be below sea level. The majority of the above ground landmass would be in the form of two that formed from the planet’s two main highland regions –  Ishtar Terra, located in the northern hemisphere, and Aphrodite Terra, just south of the equator.

Venus surface appears to have been shaped by volcanic activity rather than tectonic activity. Though Venus is not more volcanic ally active than Earth, its older crust means that it has several times as many volcanoes as Earth, with 167 measuring over 100 km across. Whereas Earth’s oceanic crust is continually recycled by subduction at the boundaries of tectonic plates, and has an average age of about 100 million years, Venus’ surface is estimated to be 300–600 million years old.

Atmosphere and Temperature:

Earth’s atmosphere is made up of five main layers – the Troposphere, the Stratosphere, the Mesosphere, the Thermosphere, and the Exosphere. As a rule, air pressure and density decrease the higher one goes into the atmosphere and the farther one is from the surface. However, the relationship between temperature and altitude is more complicated, and may even rise with altitude in some cases.

Earth’s temperature is also subject to variation depending on the time of day, time of year, and where on the planet the temperature is being measured from. Temperatures variations are the result of changes in Earth’s orbit, rotation, and its tilted axis. The average temperature is 14° C, with the hottest recorded temperature being 70.7°C (159°F) in the Lut Desert of Iran and the coldest being -89.2°C (-129°F) at Vostok Station in Antarctica.

Meanwhile, Venus’ surface temperature experiences little to no variation, owing to its dense atmosphere, very slow rotation, and very minor axial tilt. Its mean surface temperature of 735 K (462 °C/863.6 °F) is virtually constant, with little or no change between day and night, at the equator or the poles. The one exception is the highest point on Venus, Maxwell Montes, where atmospheric pressure drops to about 4.5 MPa (45 bar) and the temperature drops to about 655 K (380 °C).

Magnetic Fields:

It is a well known fact that Earth’s strong magnetic field is intrinsic to it being able to support life. The main part of this field is generated in the core, the site of a dynamo process that converts the kinetic energy of convective fluid motion into electrical and magnetic field energy. The convection movements in the core are chaotic, causing the magnetic poles to drift and periodically change alignment. This causes field reversals at irregular intervals averaging a few times every million years, the most recent of which occurred approximately 700,000 years ago.

The field extends outwards from the core, through the mantle, and up to Earth’s surface, where it form a dipole (the poles of which are located close to Earth’s geographic poles). At the equator of the magnetic field, the magnetic-field strength at the surface is 3.05 × 10^?5 Teslas, with global magnetic dipole moment of 7.91 × 10¹⁵ T m³. Ions and electrons of the solar wind, and cosmic rays that would otherwise strip away Earth’s atmosphere, are deflected by this magnetosphere.

During a magnetic storm, charged particles can be deflected from the outer magnetosphere, directed along field lines into Earth’s ionosphere, where atmospheric atoms can be excited and ionized, causing the phenomena known as Aurora Borealis and Aurora Australis.

Venus also has a magnetic field, though it is significantly weaker than Earth’s. What’s more, Venus’ magnetic field is induced by an interaction between the ionosphere and the solar wind rather than by an internal dynamo in the core like the one inside Earth. Venus’s small induced magnetosphere provides negligible protection to the atmosphere against cosmic radiation.

Conclusions:

So let’s review. Earth and Venus have their share of similarities, but also some rather stark differences. Let’s compare them, category by category, placing Earth’s values on the left and Venus’ on the right.

Mean Radius:                6,371.0 km                   6,051.8 ± 1.0 km   

Mass:                                5.972 37 x 10²⁴ kg       4.8675 x 10²⁴ kg

Volume:                           10.8321×10¹¹ km³       9.2843×10¹¹ km³

Semi-Major Axis:         149,598,023 km          108,208,000 km

Air Pressure:                 101.325 kPa                   9200 kPa

Gravity:                            9.8 m/s²                        8.87 m/s²

Avg. Temperature:      14°C (57.2 °F)               462 °C (863.6 °F)

Temp. Variations:      ±160 °C (278°F)            640 C ()

Axial Tilt:                         23.5°                               2.64°

Length of Day:               24 hours                        117 days

Length of Year:             365 days                       224.7 days

Rotation:                         Prograde                       Retrograde

Water:                              Yes                                  No

Polar Ice Caps:               Yes                                 No

As you can see, things are run the gambit from being very close, to very different. If people are to call Venus home someday, we’ll have to do some serious renovating to bring the planet up to code!

We have written many interesting articles about Venus here at Universe Today. Here’s The Planet Venus, Interesting Facts About Venus, What Is The Average Surface Temperature On Venus?  Colonizing Venus With Floating Cities and How Do We Terraforming Venus?

For more information, check out the Hubblesite’s News Releases about Venus, and here’s NASA’s Solar System Exploration Guide to Venus.

Astronomy Cast also has an interesting episode about the planet Venus. Listen to it here, Episode 50: Venus.