Physics simulations show that living organisms could not have made it from Mars to Jupiter aboard asteroids.

Europa in particular is on NASA's radar for finding life—it has a watery, potentially hospitable surface.

One ton of small Mars rocks hit Earth every year.

New research suggests that any life we find in certain places in our solar system represents a new branch on the tree of life, not just Earth leftovers that made it to space. This is a hotly debated topic in astrobiology, because scientists have gone back and forth over whether life on Earth somehow emerged from only materials on Earth, or if something traveling from space sprinkled and seeded the Earth with life-building compounds. For Space.com, Mike Wall reported on an American Geophysical Union talk from geophysicist Jay Melosh about the moons Europa and Enceladus.

From his base at Purdue’s Earth, Atmospheric, and Planetary Sciences department, Melosh did a study by running 100,000 simulations of space objects hitting Mars. He modeled particles and pieces flying off the surface and back into space at three different speeds. “We know that about 1 ton of Mars rocks that are fist-size or larger rain down on Earth every year,” Wall writes, and so Melosh made an educated estimate for how much would rain down on Europa and Enceladus, which orbit Jupiter and Saturn, respectively.

The quantities he arrived at are tiny: under half a gram to Europa and half a percent of a gram to Enceladus. Moreover, the journey took two billion years, and the tiny bits hit the surface at anywhere from 11,000 to 60,000 miles per hour. Melosh just finds it hard to believe any form of life would survive that length of time in deep space and an impact of that nature. His calculations say the numbers are similar if something had struck the Earth instead. Mars’s much, much thinner atmosphere would allow for more powerful impacts that shot more material away from the surface faster, so running simulations with Mars values is the more forgiving model.

The reason Melosh focused on Europa and Enceladus is that each has a salty body of water just below an icy shell. If building blocks of so-called “Earth life” had arrived at the right time in the right way, these moons might have had the hospitable receiving blanket to spawn life forms. Although Titan is big in the zeitgeist of potential life-fostering moons, its gigantic surface oceans are of hydrocarbons , meaning any lifeforms that originated there would be methanotrophs. We have some of those on Earth, but relatively few, and an asteroid isn’t likely to strike to the deepest depths of the ocean and launch a cubic mile of seawater into space. (That would be awesome, though.)

Melosh considers the likelihood of life traveling from Mars or Earth to be so infinitesimally small that it’s essentially zero. Because of this, if scientists do someday find lifeforms on these moons, he believes even organic lifeforms will have their own separate origin than life on Earth. Both places might have had the same building blocks of life arrive over time by asteroid impact, but the life itself will be different, with a different origin story than life on Earth.

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