If you could go back in time to the early stages of the solar system, about 4.5 billion years ago, you wouldn't have simply found the life-friendly world you expected Earth. Instead, there would have been three worlds with similar life-friendly conditions: Venus, Earth, and Mars. In terms of the physical conditions they have, all three of them look very similar from a planetary point of view, as they all have:
Surface gravity, a lot of volcanic activity, and an atmosphere of similar thickness and pressure to Earth.
They all possess volcanoes, oceans of water, and a complex interplay between the surface, the ocean, and clouds and haze, allowing these worlds to retain the vast amount of heat they absorb from the sun.
Moreover, in these very early stages, even their atmospheric composition is similar because they are all rich in molecules such as hydrogen, ammonia, methane, nitrogen, and water vapor. At one point, life may have appeared on all three planets, and in fact, at some point in the distant past, life may have appeared on all three planets. However, it didn't last except for one of the worlds. Venus experienced an uncontrolled greenhouse effect, boiling the ocean and making it like hell just a few hundred million years later. Mars lasted much longer before it became inhospitable: perhaps up to 1.5 billion years. These are stories about how our planetary neighbors cope with their respective deaths.
It's not just the two Martian moons we see today, Phobos and Phobos, the ringing disk after a collision may have produced three Martian moons, and only two have survived today. The idea is that the innermost moon of Mars was destroyed and fell back to Mars a long time ago. This hypothetical Martian transient moon, proposed in a 2016 article, is now the main idea for the formation of the Martian moon and helps explain the vast difference in topography between the northern and southern hemispheres of Mars. **labex univearths |Université de Paris-Diderot is remarkable for the fact that worlds so different from each other today may have had such a similar history in their early stages. Not only Earth, but also Mars, may have experienced catastrophic early collisions, with Earth creating our moon and Mars creating three moons, the largest of which may later return to Mars. The two remaining Martian moons, Phobos and Phobos, and Mars' severe asymmetry in both hemispheres, are two key pieces of evidence supporting this ancient Martian impact theory, with similar stories taking place on icey, moon-rich planets like Pluto. Venus may have suffered an impact that severely altered its axial rotation, but is still a planet without a moon.
What is certain, however, is that the three worlds – Venus, Earth, and Mars – are certainly influenced by external influences and internal geological processes, because:
Mountains formed over vast highlands, huge basins spanning steep lowlands, rift valleys formed, and volcanic activity, including lava flows, covered the surface.
All three places boast molten liquid interiors that have led to incredible volcanic activity, adding volatiles and carbon dioxide to the atmosphere and creating a relatively smooth ocean floor. The water that is expelled from the interior of the planet through this activity, as well as the water that is brought to the planet's surface by impact, will become an ocean of global scale, completely covering the lowest elevations.
The radius of the Earth in visible light on the right and Venus seen under infrared on the left is almost the same, and the physical size of Venus is about 90-95% of the Earth's. Despite producing a similar amount of internal heat, Earth exhibits plate tectonic activity, while Venus currently has only one plate that does not move. However, volcanic activity in both worlds is very active, and there is evidence that volcanoes have resurfaced on Venus since 1990. In the early days, Venus may surprisingly resemble Earth, but it may not last long. NASA Magellan However, when you compare Venus to Earth and Mars, even a casual observer will notice three main differences:
Their orbital distance from the Sun, their planetary rotation speed, and their physical dimensions.
The first factor, Venus's proximity to the Sun, may have doomed it from an early stage of the planet's history. While Venus is 95% the size of Earth and 72% of Earth's distance from the Sun, the latter number means that Venus receives about twice as much energy from its parent star as Earth.
Of course, back in the early days of the solar system, the sun was much cooler and much less bright than it is now, emitting only a fraction of the energy it emits now. However, over time, all stars heat up because as they consume nuclear fuel in their cores, the cores grow, heat up, and the rate of fusion inside them increases. Over time, the temperature and luminosity of the star also increase.
Nuclear fusion begins after the protostar, which will become the Sun, shrinks and cools sufficiently, but the Sun's luminosity and energy output, once it stabilizes to horizontal values about 50 million years after formation, will gradually increase over time. 4.5 billion years ago, it was only 70% of what it is today; In the future, the Sun's energy output will continue to increase on timescales of more than a billion years. **r.Helder et al., Pal Ontologische Zeitschrift, 2021 in the early stages of our solar system's history, which may have created initially life-friendly conditions on Venus. Once the planet's surface becomes cool enough to solidify and allow liquid water to exist on it, the same life-friendly conditions that ultimately led to the emergence of biological activity on Earth may also play a role on Venus. But unlike Earth, Venus is very close to the Sun, which means that as the Sun's energy output begins to increase, Venus starts to get hotter, going from its surface to the top of the atmosphere at a much faster rate than the outside world.
As an incredibly volcanically active planet, the water vapor spewing into Venus' atmosphere undoubtedly helps retain the heat emitted by Venus, which can also be absorbed from the Sun. Not only do these dual effects make the Earth hotter, but hotter planets cause a faster rate of evaporation on the planet's surface, which further increases the amount of water vapor present in the atmosphere. From a planetary perspective, it didn't take long for this to lead to what we call the "runaway greenhouse effect," where rising temperatures create the conditions that cause the Earth's temperature to rise even further. After just 200 million years, but possibly up to hundreds of millions of years, the surface of Venus has become hot enough that any liquid water on its surface inevitably boils. From a planetary point of view, it never recovered.
From asteroids to the Moon to Venus, Mars, Titan, and Earth, the surfaces of six different worlds in the solar system display a wide variety of properties and histories. While only Earth has liquid water rainfall and liquid water bodies that accumulate on the surface, other planets have other forms of precipitation and surface fluids, both in the present and in the distant past. Perhaps, a long time ago, the Earth had liquid water along with other worlds such as Mars and Venus, and perhaps life on the planet's surface. **mike malaska;ISAS JAXA, NASA, IKI, NASA JPL, ESA NASA JPL Since it is much farther from the Sun than Venus or Mars, Mars suffers from problems that can only be described as the opposite of Venus. Mars is much smaller than Earth, only half the size of Earth, but its orbit is more than 50% from the Sun, meaning it receives only 43% of the energy input (per square meter) we get from Earth, which is a little more than four times the amount of energy per unit area that Venus receives from the Sun. With so little incident energy reached, you might think that liquid water is impossible on such a planet, especially when the Sun is much colder, and therefore Mars is destined to freeze forever.
Fortunately for us, we can believe beyond a doubt that this is not the case! There is plenty of evidence that there is not only a large amount of liquid water on Mars – in the form of sedimentary rocks, hematite spheres, dry riverbeds with oxbow bends, canals that seem to flow into large depressions such as Valles Marineris – but also the current liquid water. On the slope of the crater wall, although some dispute the purported detection results, we observe from orbit what appears to be evidence of water currents, which leave saltwater deposits behind even today.
Recurring slope lines, such as the one on the south-facing slope of the crater at the bottom of the Merras Canyon, have not only been shown to grow over time and then fade away as the dust of the Martian landscape fills them, but are also known to be caused by the flow of salty liquid water, as the dry water flow leaves a new salt trail. In these flows, life processes not only once took place, but probably still occur today, as dormant organisms are awakened by flowing liquid water. The full body of evidence we have at the University of Arizona teaches us some profound information about the early conditions of Mars. Even in the early days, there must have been:
The massive Martian atmosphere, which creates a strong greenhouse effect in that world, is able to maintain enough temperature and pressure to sustain the surface's liquid oceans, rivers, and lakes.
In the early days, the surface pressure on Mars must have been much greater than what the current rarefied atmosphere could produce, and that atmosphere would also have to do an amazing job of trapping the sun's heat. Without these conditions, early Mars would have been a frozen world: the evidence clearly contradicts this, because the evidence for liquid water in the past is simply too large and too convincing.
Unfortunately, such atmospheric conditions on Mars are not possible today. The sun emits a steady stream of charged particles, called the solar wind, which constantly hit the Martian atmosphere. Because its surface gravity is much lower than that of Earth, it is easy to knock the particles found out of the Martian atmosphere into the abyss of interstellar space. Thanks to NASA's expert mission to Mars, we even measured the rate at which Mars is losing its atmosphere today, and can conclude how quickly the atmospheric stripping process will transform the once Earth-like atmosphere into the modern atmosphere of Mars.
The Earth (right) has a strong magnetic field that protects it from the solar wind. Worlds like Mars (left) or the Moon do not, and are often hit by high-energy particles emitted by the sun, which continue to strip particles from the air from these worlds. The solar wind radiates outward from the sun, putting every world in our solar system at risk of being stripped of its atmosphere. This has happened on Mars in the past, and it is likely that it will also happen on our planet if the Earth loses its core magnetogenerator. NASA GSFC It turns out that the process of atmospheric loss on Mars is very fast. In fact, if you were to just naively calculate, assuming that the flux of the solar wind today, and the rate at which the Martian atmosphere is stripping today, exists throughout the history of Mars, you would (erroneously) conclude that Mars is losing its atmosphere even faster than Venus becoming a planet with runaway greenhouse effects. If that's the only impact in your calculations, you'll find that it would only take tens of millions of years, perhaps hundreds of millions of years at most, for Mars to transform it from an Earth-like atmosphere to one that can't support a liquid ocean, an temperate climate, and life.
But this does not seem to reflect how natural history unfolds on Mars. Instead, there is evidence that it retains a thicker atmosphere – thick enough for liquid water to stay on its surface – for more than 1 billion years and up to 1.5 billion years. So, how did Mars maintain its water-rich state for so long? The answer to this puzzle lies beneath the surface: the Martian core. Mars and Earth have some very important things in common:
They all rotate on an inclined shaft, approximately every 24 hours, and contain metal-rich cores at ultra-high temperatures and pressures.
This cross-sectional view of the four terrestrial planets (plus Earth's Moon) shows the relative size of the core, mantle, and crust of the five worlds. There are compelling similarities between Earth and Mars in that they both have a crust, mantle, and a metal-rich core. However, Mars is much smaller in size, which means it initially contains less heat overall, and it loses heat at a greater rate (by percentage) than Earth. NASA JPL was supposed to have what was called a magnetogenerator in its core back in the early days of the solar system, before most of the heat in the Martian core was radiated into space: something that Earth still actively possesses. This generator generates a coherent, active magnetic field around the planet; That's why the compass hands are always deflected to face the magnetic "poles" on Earth: the active magnetogenerator at our core. Such a magnetosphere, if it had ever orbited Mars, would protect the Earth from the solar wind, diverting the vast majority of charged particles away from the Martian surroundings and leaving the atmosphere largely unaffected.
For about 1.5 billion years, this was the state of our neighboring planets. In the early days, Mars had:
Season. Liquid water, weather cycles, tides.
and the components of life that are innate to the earth.
We know that life has taken root on Earth very early, maybe within 200 million years of the formation of the Earth, and of course within 700 million years, while Mars has more than 1 billion or even 1.5 billion years of time, and it is a world rich in oceans. This presents an important and enticing possibility for Mars: at one point, it is also a world full of life in a water-rich environment.
The hematite sphere (or "Martian blueberry") was photographed by the Mars rover Opportunity. This photo was taken in the lowlands of Mars at low altitudes, where liquid water is thought to have once covered the now exposed surface. The past of water is the most favorable situation that led to the formation of these spheres, and very strong evidence comes from the fact that many spheres were found connected together and should only happen if they had a water source. In other words, early Mars was a promising candidate for life, and perhaps even a world where life has appeared and flourished. However, the changes that Mars underwent after the start of a series of life-friendly conditions were swift and thorough. Planets are born with a fixed amount of internal heat that radiates over the course of their lifetime. A planet like Mars, even though it is only half the diameter of Earth, has only about 10-15% of the heat inside a terrestrial planet. Its surface area-to-volume ratio is much larger than Earth's, so Mars sees a greater proportion of its internal radiation on shorter time scales than a planet like Earth can release its internal heat.
This leads to a "nightmare scenario" of a living world: its core becomes cold enough that its magnetogenerator goes extinct. It was this event that happened on Mars about 3 billion years ago, which caused the magnetic field that once existed to be imprinted in the Martian rocks at the time, but never since. Then, once the protective magnetic field around Mars is gone, the solar wind begins to hit the Martian atmosphere, causing those fragile particles to be stripped away. In a short period of time, that is, in just 100 million years or so, the atmosphere is almost completely knocked into interplanetary space. Once the surface pressure drops below a critical value – a value set by the triple point of water – the Martian ocean cannot remain liquid and will either freeze under the surface or sublimate off. The days of the presence of large amounts of liquid water on Mars are over.
The animation shows the transformation of Mars from a wet, ocean-rich world to the dry, barren Red Planet we see today. The whole process took about 100 million years and occurred shortly after the internal core generators of Mars stopped, causing the solar wind to strip away the Martian atmosphere. **eso/m.It is entirely reasonable for Kornmesser that in the first few hundred million years of our solar system's history, we had three life-friendly worlds: Venus, Earth, and Mars. Venus is likely to have experienced a relatively rapid death because its proximity to the Sun created an atmosphere rich in water vapor that captured enough heat to create a runaway greenhouse effect that ruined its chances of early life. But the situation is much better on Mars, where in 1.5 billion years our solar system could have two inhabited planets where single-celled life developed and took root. In all likelihood, whatever life first progresses to**—whether on Earth or on Mars—random asteroid impacts will kick matter from that inhabited world into interplanetary space, where primitive life forms may be transported to another uninhabited world.
From this point of view, it can be assumed that perhaps all earthlings originated from Martians, or that any life on Mars could have its ultimate origin traced back to Earth. Mars's magnetic field has always protected it from the sun, allowing rivers and sediment accumulations and hydrogeological processes to take place. It's just that because of its small size, it cools down quickly, loses its magnetic protection, then loses its atmosphere, and eventually becomes uninhabitable.
In another 1 billion or 2 billion years, the Sun will heat up enough to make Earth suffer a fate similar to that of Venus: the Sun's energy output is so great that our planet's oceans are boiling. However, if the Earth were a little smaller and less massive, we would suffer a fate similar to that of Mars, where the ends of our core generators would cause our atmosphere to be stripped away. As it stands, the Earth seems "just right" fit for life to emerge and flourish over a long period of time: we are not too close to the Sun, we are not too small, and the liquid water on our surface right now has lasted for about 4+ billion years and will probably last for a billion or two years. In terms of planetary habitability, maybe we really have a case of "Goldilocks".
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