Just 20 million years after the solar system formed, the Red Planet already had its outer shell

SURFACE SURVEYOR In 1997, NASA’s Sojourner rover explored the “Rock Garden,” an ancient floodplain strewn with large rocks that were swept into the valley during Mars’ wetter past. Scientists now say the Red Planet’s crust solidified much quicker than once thought. JPL/NASA

Key takeaways

  • Mars had its crust fully formed within just 20 million years of the solar system’s formation.
  • Mars likely had a 100-million-year head start on Earth in terms of potential habitability.
  • Geochemical analysis of zircon crystals in Martian meteorites indicates Mars’ crust is about 4.547 billion years old.
  • Mars cooled rapidly, which suggests its early atmosphere was thin and allowed fast heat loss.
  • While Mars solidified quickly, Earth was still molten due to events like the formation of the Moon, delaying its habitability.

Mars was a fully formed planet, crust and all, within only 20 million years of the solar system’s formation. Because of its quick development, the Red Planet likely has a 100-million-year head start on Earth in terms of habitability, according to recent studies.

Geochemical examinations of zircon crystals taken from Martian meteorites show that Mars’ oldest crust originated 4.547 billion years ago, according to a study published in Nature. That’s just 20 million years after the solar system’s planets formed from the disk of gas surrounding the sun.

The development of terrestrial planets such as Mars, Earth, Venus, and Mercury culminates in the birth of the planet’s outermost shell, or crust. The process begins with the accretion of particles from the protoplanetary gas disk, which eventually turn into molten material that forms a hot magma ocean. The magma ocean cools and crystallizes, forming a thick metallic core and an outer crust. Simulations show that the entire process takes between 30 million and 100 million years.

However, analyses of Martian zircons undertaken by planetary scientist Laura Bouvier of the University of Copenhagen indicate that Mars’ crust evolved far more swiftly. The scientists examined the isotopes in seven crystals. Isotopes are versions of an element with the same number of protons but varying numbers of neutrons, resulting in differing masses. Some isotopes are unstable and radioactively decay into other elements at predictable rates. Using the decay of uranium-235 to lead-207 and uranium-238 to lead-206, Bouvier and her team concluded that the zircons may be up to 4.476 billion years old.

The scientists next investigated another radioactive isotope decay pathway, lutetium-176 to hafnium-176, and concluded that Mars’ early crust is considerably older. Zircons formed from such rocks should have included far more hafnium than the researchers discovered in their investigations. Based on the amount of hafnium present and the lutetium-hafnium decay rates, the scientists concluded that the initial magma crust solidified about 4.547 billion years ago.

Heavy bombardment by asteroids followed, partially melting the crust, yet the impacts did not totally transform it back into a magma ocean, according to the study. The zircons retaining evidence of this previous crust formed when Mars cooled down permanently approximately 4.476 billion years ago.

“Having Mars cool so quickly puts limits on how massive its atmosphere could have been,” says Linda Elkins-Tanton, a planetary scientist at Arizona State University in Tempe who submitted a commentary accompanying the new research. Researchers can estimate how much water and carbon dioxide the magma ocean may have discharged to build an early atmosphere based on how rapidly a planet cools and how quickly the sun can remove its atmosphere. However, Elkins-Tanton believes that in the case of Mars, the atmosphere would have been rather thin, allowing for such fast heat loss.

Mars would also have solidified around 100 million years before Earth, giving it an advantage in terms of habitability. Our world was almost definitely entirely molten at the time, maybe as a result of a massive smash that produced the moon and remelted the entire planet (SN: 4/15/17, p. 18), Elkins-Tanton claims. “That restarted everything on Earth.”

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