How was it when Venus and Mars became uninhabitable planets?

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If you could go back into the early stages of the solar system, some 4.5 billion years ago, you would find only one world conducive to life, but three. Venus, Earth and Mars all seemed very similar from a planetary point of view, as they all had a substantial surface gravity and an Earth-like atmosphere. There were volcanoes, watery oceans and complex interactions that allowed these worlds to retain the heat absorbed by the Sun.

In addition, the compositions of their atmosphere were similar, all rich in hydrogen, ammonia, methane, nitrogen and water vapor. For a time, conditions were favorable for life on all three worlds, but it did not last. Venus experienced an uncontrolled greenhouse effect, boiling its oceans after perhaps 200 million years. But Mars lasted much longer before becoming inhospitable: over a billion years old. These are their stories.

It is remarkable that worlds so different from each other may have had similar stories in their infancy. Earth and Mars have probably experienced catastrophic early collisions, with Earth creates our moon and Mars creates three moons, the most important of them probably having fallen back on Mars at a later date.

The three worlds – Venus, Earth and Mars – were shaped by external impacts and internal geological processes, they formed mountain ranges over vast highlands and large basins extending through the spectacular lowlands. which caused large amounts of volcanic eruptions, adding both volatile substances and carbon dioxide to the atmosphere and creating relatively smooth seabed. The liquid water that has survived has become an ocean on the planet scale, completely covering the lowlands.

When you compare Venus to Earth to Mars, there are three major differences:

1. their orbital distances to the sun,
2. the rate of their planetary rotations,
3. and their physical sizes.

The proximity of Venus to the Sun probably sentenced her early. Although Venus & nbsp; 95%, the size of the Earth and the Venus-Sun distance represent 72% of the Earth-Sun distance, this last figure translates into the fact that Venus receives twice the energy that the Earth receives. The water vapor in the atmosphere of Venus allowed it to retain more heat from the Sun, which resulted in an additional increase in the amount of water vapor in the atmosphere. After only 200 million years, this resulted in a greenhouse effect that faded away, resulting in the evaporation of surface water from Venus. He never recovered.

But at a greater distance from the Sun, Mars seemed to have the opposite problem. Mars is much smaller than the Earth, barely more than half its size, but it turns more than 50% of its distance from the Sun, which means that it only gets 43% of the Earth's size. energy that we get here on Earth. With such a small amount of incident energy coming in, you might think that liquid water would be impossible, and that Mars would be destined to be frozen forever.

Fortunately, we know without a shadow of a doubt that this was not the case! There is enormous evidence of the presence not only of liquid water passed on Mars – in the form of sedimentary rocks, spheres of hematite, dry river beds with elbows, and so on. & nbsp; – – but also current liquid water. On the slopes of the crater walls, although the claimed detection is controversial, there are water flows that leave salt deposits still today.

These proofs tell us something about the first conditions on Mars: there must be a substantial atmosphere with a strong greenhouse effect, sufficient to keep oceans, rivers and lakes liquid on the surface. It had to generate much larger surface pressures than the current weak atmosphere of Mars, and a phenomenal job of trapping heat from the Sun to prevent the world from freezing.

Such an atmosphere is impossible today. The sun emits a constant stream of charged particles known as the solar wind, which constantly reverberate in the Martian atmosphere. Because its surface gravity is so much lower than that of the Earth, it is easy to flip particles into the atmosphere of Mars and into the abyss of interstellar space. Thanks to NASA's Mars Maven mission, we can even measure how much Mars is losing its atmosphere today.

It's a fast process! According to our calculations, it would only take tens of millions of years, perhaps even a hundred million years, to transform Mars from a Earth – like atmosphere into an atmosphere unable to support the Earth 's. liquid oceans, temperate climates and life.

So, how did Mars manage to stay so long in its water-rich state: about 1.5 billion years old?

The answer lies deep beneath the surface: in the Martian nucleus. Mars and Earth have some very important things in common:

• they both turn on an inclined axis,
• about once every 24 hours,
• and contain metal-rich cores at extremely high temperatures and pressures.

At the very beginning of the solar system, before so much of the heat of the Martian nucleus was emitted into space, it probably produced an active magnetic field around Mars, similar to what our nucleus creates. around the Earth. Such a magnetosphere would protect the planet from the solar wind by diverting the overwhelming majority of the wind around Mars, leaving the atmosphere virtually untouched.

For about 1.5 billion years, that was the current state of affairs. Mars had seasons, liquid water, a weather cycle, tides and the same ingredients as Earth for life. We know that life took root on Earth in a few hundred million years at most and that Mars was at least six times that time in a world rich in oceans. The possibility of a life spent on Mars is alluring.

But the changes undergone by Mars were fast and fast. Planets are born with a fixed amount of internal heat, which radiates during their lifetime. A planet like the planet Mars, whose diameter is equal to half that of the Earth, is born with about 10 to 15% of the internal heat of our world and will see a larger percentage of it emerge much more quickly than the Earth.

About 3 billion years ago, the core of Mars became cold enough to stop producing this protective magnetic dynamo and the solar wind began to hit the Martian atmosphere. In a short time, that is to say in a few tens of millions of years, the atmosphere has been projected into the interplanetary space. As a result, the oceans were unable to remain in liquid form and froze below the surface or sublimated.

It is quite plausible that, for 1.5 billion years, our solar system has possessed two highly inhabited planets, where single-cell life has developed and settled. It is highly likely that if one life was growing on one planet before another, a random strike of asteroids would project materials into interplanetary space and eventually transport this life from Earth to Mars or from Mars to Earth. .

If this seems unlikely to you, do not forget this: 3% of all the meteorites we found on Earth do not come from asteroids or comets, but have a Martian origin. This was only confirmed by the Mars Pathfinder mission in the late 1990s, which analyzed the soil it discovered and allowed us to determine with certainty that rocks from another planet have already been directed to the Earth. And therefore, probably, the opposite is also true.

The story of Venus was that of a quick death. It may have been born as ready to live as Earth, but its proximity to the Sun has created a very rich atmosphere of water vapor, trapping enough heat to create a glowing greenhouse effect, boiling the oceans and ruining his chances. for life.

But Mars came out much better. Its atmosphere, liquid water and rotation rate have allowed it to develop and maintain stable and life-sustaining conditions for 1.5 billion years. Its magnetic field protected it from the sun during all this time, which allowed the accumulation of rivers and sediments, as well as hydrogeological processes. It seems almost inconceivable that life was not born there, given the speed and ease with which it manifested and flourished on Earth. It's only because of its small size, which made it cool quickly, lost its magnetic protection, then its atmosphere, that it became uninhabitable.

During the few hundred million years that followed the formation of the solar system, we had three potentially habitable worlds: Venus, Earth, and Mars. If things were slightly different & nbsp; if the Sun was smaller and paler, if Venus was circling a greater distance, or perhaps if it was simply spinning faster, it might not have had the effect a fleeting greenhouse that had made it so uninhabitable.

But perhaps surprisingly, Mars was coming out a lot better. As life emerged on Earth and transformed our atmosphere, something similar was being played out on Mars. Maybe rocks, chemicals and even life have been traded between our two worlds by interplanetary impacts, and perhaps Mars has been inhabited for a billion years or more. Once he died, however, there was no return. If we look at 3 billion years in our past, Earth was the last habitable planet standing.

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If you could go back into the early stages of the solar system, some 4.5 billion years ago, you would not find a world fit for life, but three. Venus, Earth and Mars all seemed very similar from a planetary point of view, as they all had a substantial surface gravity and an Earth-like atmosphere. There were volcanoes, watery oceans and complex interactions that allowed these worlds to retain the heat absorbed by the Sun.

In addition, the compositions of their atmosphere were similar, all rich in hydrogen, ammonia, methane, nitrogen and water vapor. For a time, conditions were favorable for life on all three worlds, but it did not last. Venus experienced an uncontrolled greenhouse effect, boiling its oceans after perhaps 200 million years. But Mars lasted much longer before becoming inhospitable: over a billion years old. These are their stories.

It is remarkable that worlds as different from each other may have had a similar story in their infancy. Earth and Mars have probably experienced catastrophic early collisions, those of the Earth creating our Moon and those of Mars creating three moons, the largest of which probably returned to Mars at a later date.

The three worlds – Venus, Earth and Mars – have been shaped by external impacts and internal geological processes, mountain ranges formed at the top of vast uplands and large basins extending across the spectacular lowlands. Their liquid and molten interior has caused many volcanic eruptions, adding both volatile compounds and carbon dioxide to the atmosphere and creating relatively smooth ocean floors. The liquid water that has survived has become an ocean on the planet scale, completely covering the lowlands.

When you compare Venus to Earth to Mars, there are three major differences:

1. their orbital distances to the sun,
2. the rate of their planetary rotations,
3. and their physical sizes.

The proximity of Venus to the Sun probably sentenced her early. Although Venus represents 95% of the Earth's size and the Venus-Sun distance is equal to 72% of the Earth-Sun distance, this latter value translates into the fact that Venus receives twice the energy that the Earth receives. The water vapor in the atmosphere of Venus allowed it to retain more heat from the Sun, which resulted in an additional increase in the amount of water vapor in the atmosphere. After only 200 million years, this resulted in a greenhouse effect that faded away, resulting in the evaporation of surface water from Venus. He never recovered.

But at a greater distance from the Sun, Mars seemed to have the opposite problem. Mars is much smaller than the Earth, barely more than half its size, but it turns more than 50% of its distance from the Sun, which means that it only gets 43% of the Earth's size. energy that we get here on Earth. With such a small amount of incident energy coming in, you might think that liquid water would be impossible, and that Mars would be destined to be frozen forever.

Fortunately, we know without a shadow of a doubt that this was not the case! There is enormous evidence of the presence not only of liquid water passed on Mars – in the form of sedimentary rocks, spheres of hematite, dry river beds with elbows, and so on. – but also current liquid water. On the slopes of the crater walls, although the so-called detection is controversial, there are water flows that leave salt deposits still today.

This evidence tells us something about the first conditions on Mars: there must be a substantial atmosphere with a large greenhouse effect, sufficient to keep the oceans, rivers, and liquid lakes on the surface. It had to generate much larger surface pressures than the current weak atmosphere of Mars, and a phenomenal job of trapping heat from the Sun to prevent the world from freezing.

Such an atmosphere is impossible today. The sun emits a constant stream of charged particles known as the solar wind, which constantly reverberate in the Martian atmosphere. Because its surface gravity is so much lower than that of the Earth, it is easy to flip particles into the atmosphere of Mars and into the abyss of interstellar space. Thanks to NASA's Mars Maven mission, we can even measure the current loss of Mars atmosphere.

It's a fast process! According to our calculations, it would only take tens of millions of years, perhaps even a hundred million years, to transform Mars from a Earth – like atmosphere into an atmosphere unable to support the Earth 's. liquid oceans, temperate climates and life.

So, how did Mars manage to stay so long in its water-rich state: about 1.5 billion years old?

The answer lies deep beneath the surface: in the Martian nucleus. Mars and Earth have some very important things in common:

• they both turn on an inclined axis,
• about once every 24 hours,
• and contain metal-rich cores at extremely high temperatures and pressures.

At the very beginning of the solar system, before so much of the heat of the Martian nucleus was emitted into space, it probably produced an active magnetic field around Mars, similar to what our nucleus creates. around the Earth. Such a magnetosphere would protect the planet from the solar wind by diverting the overwhelming majority of the wind around Mars, leaving the atmosphere virtually untouched.

For about 1.5 billion years, this was the state of the art. Mars had seasons, liquid water, a weather cycle, tides and the same ingredients as Earth for life. We know that life took root on Earth in a few hundred million years at most and that Mars was at least six times that time in a world rich in oceans. The possibility of a life spent on Mars is alluring.

But the changes undergone by Mars were fast and fast. Planets are born with a fixed amount of internal heat, which radiates during their lifetime. A planet like the planet Mars, whose diameter is equal to half that of the Earth, is born with about 10 to 15% of the internal heat of our world and will see a larger percentage of it emerge much more quickly than the Earth.

About 3 billion years ago, the core of Mars became cold enough to stop producing this protective magnetic dynamo and the solar wind began to hit the Martian atmosphere. In a short time, that is to say in a few tens of millions of years, the atmosphere has been projected into the interplanetary space. As a result, the oceans were unable to remain in liquid form and froze below the surface or sublimated.

It is quite plausible that, for 1.5 billion years, our solar system has possessed two highly inhabited planets, where single-cell life has developed and settled. It is highly likely that if one life was growing on one planet before another, a random strike of asteroids would project materials into interplanetary space and eventually transport this life from Earth to Mars or from Mars to Earth. .

If this seems unlikely to you, do not forget this: 3% of all the meteorites we found on Earth do not come from asteroids or comets, but have a Martian origin. This was only confirmed by the Mars Pathfinder mission in the late 1990s, which analyzed the soil it discovered and allowed us to determine with certainty that rocks from another planet have already been directed to the Earth. And therefore, probably, the opposite is also true.

The story of Venus was that of a quick death. It may have been born as ready to live as Earth, but its proximity to the Sun has created a very rich atmosphere of water vapor, trapping enough heat to create a glowing greenhouse effect, boiling the oceans and ruining his chances. for life.

But Mars came out much better. Its atmosphere, liquid water and rotational speed have allowed it to develop and maintain stable and life-sustaining conditions for 1.5 billion years. Its magnetic field protected it from the sun during all this time, which allowed the accumulation of rivers and sediments as well as hydrogeological processes. It seems almost inconceivable that life was not born there, given the speed and ease with which it manifested and flourished on Earth. It's only because of its small size, which made it cool quickly, lost its magnetic protection, then its atmosphere, that it became uninhabitable.

During the few hundred million years that followed the formation of the solar system, we had three potentially habitable worlds: Venus, Earth, and Mars. Si les choses étaient légèrement différentes si le Soleil était plus petit et plus pâle, si Vénus tournait plus loin ou peut-être s’il tournait plus vite, il n’aurait peut-être pas eu l’effet de serre qui l’a rendu inhabitable.

Mais peut-être étonnamment, Mars s&#39;en sortait beaucoup mieux. Alors que la vie émergeait sur Terre et transformait notre atmosphère, quelque chose de similaire se jouait sur Mars. Peut-être que des roches, des produits chimiques et même la vie ont été échangés entre nos deux mondes par des impacts interplanétaires, et peut-être que Mars a été habitée pendant un milliard d’années ou plus. Une fois qu&#39;il est mort, cependant, il n&#39;y avait pas de retour. Si nous regardons 3 milliards d’années dans notre passé, la Terre était la dernière planète habitable debout.

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