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We often talk about the strong similarities between Earth and Mars, but it’s probably the differences that explain why one planet has life and the other doesn’t – at least, no life we haven’t found so far.
Specifically, new research suggests it could be due to the size gap. Mars’ diameter is only 53% that of Earth (just over half its size), which would prevent Mars from clinging to volatile substances that we know are vital for life, such as ‘water.
“The fate of Mars was decided from the start,” says planetologist Kun Wang of Washington University in St. Louis.
“There is probably a threshold on the size requirements of rocky planets to hold enough water to allow habitability and plate tectonics, with a mass exceeding that of Mars.”
Although there are many differences between Earth and other terrestrial planets in the solar system, it can be difficult to determine which factors promote the emergence of life and which factors hinder it.
But we can look at some of the things life on Earth needs to exist and work from there.
One thing life on Earth needs is liquid water, so the conditions that allow liquid water to be present are one of the key items on the planet’s habitability checklist. We know Mars had surface water – we’ve seen evidence of this in the Martian meteorites that made their way from Earth, mined from the Red Planet when the solar system was still young. Today, however, Mars is dusty, dry, and desolate, and all the water on its surface is frozen.
The transition from a relatively humid planet to an arid dust bowl is sometimes attributed to the loss of Mars’ magnetic field. But it is possible that other factors play a role in the retention of volatiles, such as the surface gravity of a cosmic body; Earth’s gravity, for reference, is 2.66 times that of Mars. So Wang and his team began to investigate.
Specifically, they began to examine the abundance of the moderately volatile element potassium on various objects in the solar system, using it as a tracer for other volatile elements and compounds.
This is because the isotopic ratios of potassium are an important indicator of the depletion of volatiles inside the planets, as they are insensitive to igneous processes and impact-induced vaporization.
“Martian meteorites are the only samples we have available to study the chemical makeup of the mass of Mars,” Wang said.
“These Martian meteorites have ages ranging from several hundred million to 4 billion years and have recorded the volatile evolutionary history of Mars. By measuring the isotopes of moderately volatile elements, such as potassium, we can infer the degree of volatile depletion of bulk planets and make comparisons between different bodies in the solar system. “
The team studied the isotopic compositions of potassium in 20 Martian meteorites, chosen because they appear to be representative of the silicate composition of Mars. These compositions were then compared to the known bulk silicate compositions of three other internal solar system objects of varying masses – the Earth, the Moon and the asteroid Vesta.
The results showed that Mars lost more volatiles than Earth during its formation, but retained more than the Moon and Vesta, both of which are significantly smaller and drier than Mars.
“The reason for the much lower abundances of volatile elements and their compounds in differentiated planets than in primitive undifferentiated meteorites is a long-standing question,” says planetologist Katharina Lodders of the University of Washington.
“The discovery of the correlation of isotopic compositions of potassium with planetary gravity is a new discovery with important quantitative implications for when and how differentiated planets received and lost their birds.”
This has implications for our understanding of the history of the planet, the researchers said. Previous research has shown that Mars was once very soggy. This new correlation between gravity and volatile retention could help place constraints on the amount of water Mars once had.
Additionally, the discovery has implications for our search for habitable worlds outside the solar system. One factor that influences the presence of liquid water on a planetary surface is its temperature, related to its proximity to the host star. Too close and the water evaporates; too far, and it freezes.
We can also measure the size and mass of exoplanets, based on how much starlight they block as they move between us and the star, and how much the star moves in its mutual orbit. with the exoplanet. The team’s work could therefore help us exclude exoplanets that are too small for liquid water.
“The size of an exoplanet is one of the easiest parameters to determine,” Wang said.
“Based on size and mass, we now know whether an exoplanet is a candidate for life, as a first-order determinant for volatile retention is size. “
The research was published in PNAS.
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