Title:Impacts on Ocean Worlds Are Sufficiently Frequent and Energetic to Be of Astrobiological Importance
Authors: Shannon M. MacKenzie, Alexandra Pontefract, R. Terik Daly, Jacob J. Buffo, Gordon R. Osinski, Christopher J. Cline II, Mark J. Cintala, Kathleen L. Craft, Mallory J. Kinczyk, Joshua Hedgepeth, Sarah M. Hörst, Abel Méndez, Ben K. D. Pearce, Angela M. Stickle, and Steven D. Vance
First Author’s Institution: Johns Hopkins University Applied Physics Laboratory, 1001 Johns Hopkins Road, Laurel, MD 20723, USA
Status: Published in The Planetary Science Journal [open access]
Asteroids and meteorites are usually associated with doom and destruction (rest easy, dinosaurs), but they may have also been essential for the emergence of life on Earth. It is popularly theorized that some of the base building blocks of life, like volatiles and organics, were delivered here by meteorites and that the energy of these impacts synthesized even more, like HCN and amino acids. Expectedly, the same should be true for other planets. Today’s paper explores this possibility using nearby analogies for potentially habitable exoplanets, namely our solar system’s ocean worlds.
Why do meteorites carry organics?
The solar system formed from one massive cloud of gas and dust, so the composition everywhere is approximately the same. However, early Earth was an extremely hot ball of magma that destroyed its organic matter. Luckily, organics were able to survive in objects like meteorites in the cold outskirts of the solar system.
Ocean Worlds in our Neighborhood
Figure 1: Saturn’s moon, Enceladus, with a liquid water ocean beneath the icy crust. Jets on the surface are strong indicators of hydrothermal vents on the ocean floor. Image credit: NASA Astrobiology Program
In the search for extraterrestrial life, we start by looking for the basic necessities – and water is a big one. Though Earth is the only planet in our solar system with water, several moons of Jupiter and Saturn have it as well. They are beyond the balmy habitable zone so the surfaces of these moons are covered in icy crusts, but beneath those crusts are subsurface oceans of liquid water, making these moons “ocean worlds”. On their own, the presence of water makes these moons astrobiologically interesting, and they will also elucidate ocean worlds that are further away.
Today’s authors studied typical impact events on Jupiter’s moon Europa, and Saturn’s moons Enceladus and Titan to determine 1) if organics could survive the impacts, and 2) what processes could occur in the resulting melted material in the impact craters before it refreezes.
Surviving the Impact
To evaluate survivability, the authors modeled the maximum pressure of an impact on an ocean world’s ice crust for a range of impact velocities and angles. Around Jupiter and Saturn, most impactors are either icy or rocky objects that originate from the Kuiper Belt or Oort cloud, so they modeled both. Rocky impactors create higher pressures (shown in gray in Figure 2) than icy impactors (shown in black in Figure 2). From the sizes of observed craters on the ocean world moons, previous works determined the velocities and pressures of impacts, which are shown by the colored boxes in Figure 2. Finally, a number of other works have estimated the ranges of survivable pressures for biota and biologically important molecules, which are shown by the green and black bars on the right of Figure 2. Impressively, the survivable pressure ranges are within the observed and modeled pressures of impacts! So these life building blocks can, and likely have been, deposited on the ocean world moons.
Figure 2: The modeled impact velocities and maximum pressures for icy (black) and rocky (gray) impactors. Survivable pressures of various organics (green and gray colored bars on the y-axis) are within the range of observed velocities and pressures from craters on each ocean world moon (colored boxes). Image credit: Figure 1 in the paper.
Crater Melt Pools
When an impactor hits the icy crust, some of the ice will melt. The deposited organics will end up in a pool of liquid water in the crater, which is an ample environment for prebiotic chemistry until the pool freezes. From the observed crater sizes and modeled velocities, the authors estimated how much liquid water could remain in a crater and how long it would take to freeze. Freeze times ranged from a few Earth years for the smallest craters (<4 kilometers in diameter) to thousands of years for the largest craters (hundreds of kilometers). In labs on Earth mimicking the crater conditions, amino acids have been synthesized in as short as a few months to a few years, so synthesis is possible in the melt pools.
The pools eventually freeze, trapping any deposited or synthesized material on the icy surface. Other processes, like future impacts, are required to break through the icy crust and transport material to the subsurface oceans where theorized hydrothermal vents could allow more complex development.
Tangible Evidence
In summary, survivable impacts on the ocean world moons are common, and each provides an opportunity for prebiotic chemistry to arise. Unlike most objects astronomers study, the proximity of these ocean worlds means that we can thoroughly understand them through physical samples. NASA’s Cassini detected organic compounds in the plumes that burst off the surface of Enceladus, and the Dragonfly mission is set to head for Titan in 2028 to collect and analyze samples once it arrives in 2034. In the coming decades, we may witness the discovery of more precursors to life or microbial life itself in the subsurface oceans of moons in our solar system, and gain radical insight into the ocean worlds beyond.
Astrobite edited by Sonja Panjkov
Featured image credit: Bryn Cashen-Smart, University of Minnesota Department of Art, https://www.instagram.com/bryn\_allyn/
Author
I’m an Astrophysics Ph.D. candidate at the University of Alabama, using simulations to study the circumgalactic medium. Beyond research, I’m interested in historical astronomy, and hope to someday write astronomy children’s books. Beyond astronomy, I enjoy making music, cooking, and my cat.
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