Venus, Earth, and Mars are sister planets. They are solid, surrounded by an atmosphere, similar in size, and orbit close to the Sun. All three formed about 4.55 billion years ago in the same area of the early solar nebula, with the same ingredients and probably shared similar childhoods. For hundreds of millions of years they had great volcanic activity, releasing gases that formed important atmospheres of similar composition, and there was probably liquid water on their surfaces.

In contrast, today Venus has an atmosphere almost one hundred times greater than Earth’s and a hell of 460 ºC on the surface. And Mars is a frozen desert planet with a very thin atmosphere. Only the Earth seems to have maintained liquid water, in addition to very stable temperatures, for 4 billion years!

Venus and Mars Probably Evolved Differently From Earth Because

How did these dramatic climatic evolutions occur in our neighbors? How did the Earth avoid such changes? Scientists are looking for a global explanation for the three planets.

Is Venus Earth and Mars at the same distance?

Venus, Earth, and Mars are approximately at the same distance from the Sun. This means they formed out of the same material and had approximately the same initial temperatures 4.6 billion years ago. Long ago these three planets probably had moderate enough temperatures suitable for life.

What are the characteristics that differentiate the land from Mars?

Mars is quite similar to Earth, although it is not exactly the same. The mass of Mars is one-tenth that of Earth, and its size is half. For this reason, surface gravity is less than on Earth: only 3.7 m / s2 instead of 9.8 m / s2. On Mars you would weigh half as much as on Earth.

Why does Mars have a thick atmosphere?

This bakes carbon dioxide out of carbonate rocks, which increases the greenhouse effect and the temperature spirals up. Mars is only a tenth the mass of Earth and further away. So it struggles to hang onto a thick atmosphere and water is present only as ice.

Is Venus closer to the Sun than Mars?

Venus is closer to the Sun and Mars is further away, but distance alone isn’t enough to account for the different conditions on these planets. The surface temperature on Venus, for example, is higher than on Mercury, even though it receives only 25 per cent as much energy from the Sun.

What are the similarities between Venus and Earth?

‘ cloudtops in UV(left) and Venus’ surface imaged with radar(right). Venus is about 95% the size of the Earth and has 82% of the Earth’s mass. Like the Earth, Venus has a rocky crust and iron-nickel core. But the similarities stop there.

What is the difference between the time of Earth and Mars?

Mars has a similar axial tilt and rotation period to Earth. Therefore, it experiences spring, summer, fall, and winter seasons in the same way as Earth, and its day is roughly the same length.

Some keys

A key factor in these differences is found in the surface temperature of each planet. It is a complex parameter because it affects the atmosphere and in turn depends on it through the so-called greenhouse effect. This effect depends on the existing atmospheric gases and their ability to trap the solar energy that reaches the planet and that is re-radiated by it outwards. On Earth the main greenhouse gas is water vapor and, secondly, carbon dioxide; together they increase the global temperature of the planet by 15º C. On the contrary, on Mars the greenhouse effect is small, although perhaps it was not like that in the past, when temperatures were higher. On Venus, on the other hand, it is very large, given its great abundance of carbon dioxide, and we think it has been that way for most of its history.

But the greenhouse effect also depends on more complex factors. One is the reflectivity of the planet, which changes a lot between different areas and also fluctuates with events as variable as clouds on Earth or dust storms on Mars. Another factor is solar radiation, which we also know varies over time. Chaotic variations can even occur, as appears to be the case on Mars, due to abrupt variations in the inclination of its axis. This is produced by the complex gravitational influences of Jupiter and the other planets. The Earth is closer to the Sun and also has a companion, the Moon, which gives great stability to its orbit, so it is free from such changes.

Keeping the balance

Another important factor in understanding the differences between these not-so-twin planets lies in the interaction between atmosphere and surface and the resulting balance. An example is the so-called carbon cycle, which seems to control that the temperature on Earth does not change greatly over time. Carbon dioxide (CO2) in the atmosphere is in continuous exchange with the ocean, where it is transported by erosion and eventually settles to the bottom. Geological processes incorporate it into the rocks of the earth’s crust and, later, through fissures and volcanism, it escapes into the atmosphere, completing the cycle. The interesting thing is that this cycle is self-regulating. For example, if the temperature were to increase, the erosion and transport of CO2 towards the ocean and the earth’s crust would increase, with which the CO2 resident in the atmosphere would decrease, also reducing its greenhouse effect, and counteracting the change in temperature: a perfect balance!

Now, this recycling takes time, about 300,000 years, little in terms of normal planetary evolution, but … what if there are sudden changes? We could think of cataclysmic events, such as exceptional volcanic eruptions or impacts with minor bodies in the Solar System. There is another burning example: the problem of climate change due to the increase in man-made CO2, which has occurred in just 200 years, a very short time on a planetary scale. These events could escape the self-regulatory capacity of the planet, especially if they are prolonged in time.

What is wrong with Mars and Venus?

In our neighbors, Mars and Venus, their carbon cycle is long lost. On Venus, closer to the Sun, the high temperature must have completely evaporated water from the oceans through a process called the runaway greenhouse effect. Without oceans, the exchange of CO2 towards the crust was interrupted, and all the carbon available on the planet passed into the atmosphere. Mars, on the other hand, has no plate tectonics; smaller than Venus and Earth, its interior cools faster. Although in the past there were oceans of liquid water on its surface, carbon could not be returned to its atmosphere. In the absence of this self-regulating recycling and active volcanism, thermal variations led to uncontrolled cooling and much of the atmosphere collapsed.

Challenges and pending issues

Although the evolution of the three planets has been very complex, we are beginning to understand some key mechanisms. Of course, there are many open questions with few answers. The super-rotation of the clouds on Venus or the development of dust storms on Mars are two current examples. The complex interactions between numerous processes require careful mathematical modeling, and much more, and more accurate data.

The atmosphere is a chaotic and complex system, and the scientific task of understanding it in its entirety and in the three sister planets promises to be long, although yes, exciting. And necessary.

Since the development of the space age, Venus and Mars have been in the crosshairs of space agencies. The palm is undoubtedly taken by our red neighbor, visited more than forty times, the latest being NASA’s Phoenix mission, whose data has yet to reveal important clues about the presence of liquid water under the Martian surface. As for Venus, it is currently being flown over by the Venus Express mission of the European Space Agency (ESA), which is discovering surprising phenomena in its atmosphere, such as the presence of lightning!