Seeding Life in the Oceans of Moons

Plumes of salty water ice emerge from Enceladus's cracked ice shell
Credit: NASA/JPL/Space Science Institute

Title: Impacts on Ocean Worlds Are Sufficiently Frequent and Energetic to Be of Astrobiological Importance

Authors: Shannon M. MacKenzie et al.
First Author’s Institution: Johns Hopkins University Applied Physics Laboratory
Status: Published in PSJ

A steroids 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 article explores this possibility using nearby analogies for potentially habitable exoplanets: 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.

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.Credit: JPL

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.

Ocean Worlds in Our Neighborhood

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 liquid water, several moons of Jupiter and Saturn have it as well. These moons are beyond the balmy habitable zone, so their surfaces are covered in icy crusts, but beneath those crusts are subsurface oceans of liquid water, making these moons “ocean worlds” (see Figure 1). 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.

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). Credit: MacKenzie et al. 2024

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 the authors modeled both types of impactors. 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 be, and likely have been, deposited on the ocean world moons.

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 a="" acids="" amino="" as="" been="" br="" conditions="" crater="" craters="" diameter="" earth="" few="" for="" have="" hundreds="" in="" is="" kilometers="" labs="" largest="" melt="" mimicking="" months="" of="" on="" pools.="" possible="" short="" so="" synthesis="" synthesized="" the="" thousands="" to="" years="">
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.

Original astrobite edited by Sonja Panjkov

Source: Astronomical Society/AAS-Nova

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About the author, Annelia Anderson:

I’m an Astrophysics PhD 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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Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

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NASA’s NExSS Coalition to Lead Search for Life on Distant Worlds

The search for life beyond our solar system requires unprecedented cooperation across scientific disciplines. NASA's NExSS collaboration includes those who study Earth as a life-bearing planet (lower right), those researching the diversity of solar system planets (left), and those on the new frontier, discovering worlds orbiting other stars in the galaxy (upper right). Credits: NASA

NASA is bringing together experts spanning a variety of scientific fields for an unprecedented initiative dedicated to the search for life on planets outside our solar system.  

The Nexus for Exoplanet System Science, or “NExSS”, hopes to better understand the various components of an exoplanet, as well as how the planet stars and neighbor planets interact to support life.

“This interdisciplinary endeavor connects top research teams and provides a synthesized approach in the search for planets with the greatest potential for signs of life,” says Jim Green, NASA’s Director of Planetary Science. “The hunt for exoplanets is not only a priority for astronomers, it’s of keen interest to planetary and climate scientists as well.”

The study of exoplanets – planets around other stars – is a relatively new field. The discovery of the first exoplanet around a star like our sun was made in 1995. Since the launch of NASA’s Kepler space telescope six years ago, more than 1,000 exoplanets have been found, with thousands of additional candidates waiting to be confirmed. Scientists are developing ways to confirm the habitability of these worlds and search for biosignatures, or signs of life.

The key to this effort is understanding how biology interacts with the atmosphere, geology, oceans, and interior of a planet, and how these interactions are affected by the host star. This “system science” approach will help scientists better understand how to look for life on exoplanets.

NExSS will tap into the collective expertise from each of the science communities supported by NASA’s Science Mission Directorate:

• Earth scientists develop a systems science approach by studying our home planet.
• Planetary scientists apply systems science to a wide variety of worlds within our solar system.
• Heliophysicists add another layer to this systems science approach, looking in detail at how the Sun interacts with orbiting planets.
• Astrophysicists provide data on the exoplanets and host stars for the application of this systems science framework.

NExSS will bring together these prominent research communities in an unprecedented collaboration, to share their perspectives, research results, and approaches in the pursuit of one of humanity’s deepest questions: Are we alone?

The team will help classify the diversity of worlds being discovered, understand the potential habitability of these worlds, and develop tools and technologies needed in the search for life beyond Earth.

Dr. Paul Hertz, Director of the Astrophysics Division at NASA notes, “NExSS scientists will not only apply a systems science approach to existing exoplanet data, their work will provide a foundation for interpreting observations of exoplanets from future exoplanet missions such as TESS, JWST, and WFIRST.” The Transiting Exoplanet Survey Satellite (TESS) is working toward a 2017 launch, with the James Webb Space Telescope (JWST) scheduled for launch in 2018. The Wide-field Infrared Survey Telescope is currently being studied by NASA for a launch in the 2020’s.

NExSS will be led by Natalie Batalha of NASA’s Ames Research Center, Dawn Gelino with NExScI, the NASA Exoplanet Science Institute, and Anthony del Genio of NASA’s Goddard Institute for Space Studies. The NExSS project will also include team members from 10 different universities and two research institutes. These teams were selected from proposals submitted across NASA’s Science Mission Directorate.

The Berkeley/Stanford University team is led by James Graham. This "Exoplanets Unveiled" group will focus on this question: “What are the properties of exoplanetary systems, particularly as they relate to their formation, evolution, and potential to harbor life?”

Daniel Apai leads the “Earths in Other Solar Systems” team from the University of Arizona. The EOS team will combine astronomical observations of exoplanets and forming planetary systems with powerful computer simulations and cutting-edge microscopic studies of meteorites from the early solar system to understand how Earth-like planets form and how biocritical ingredients  — C, H, N, O-containing molecules — are delivered to these worlds.

http://otherearths.org

The Arizona State University team will take a similar approach. Led by Steven Desch, this research group will place planetary habitability in a chemical context, with the goal of producing a “periodic table of planets”. Additionally, the outputs from this team will be critical inputs to other teams modeling the atmospheres of other worlds.

Researchers from Hampton University will be exploring the sources and sinks for volatiles on habitable worlds. The “Living, Breathing Planet Team," led by William B. Moore, will study how the loss of hydrogen and other atmospheric compounds to space has profoundly changed the chemistry and surface conditions of planets in the solar system and beyond. This research will help determine the past and present habitability of Mars and even Venus, and will form the basis for identifying habitable and eventually living planets around other stars.

http://sol.hamptonu.edu/project/the-living-breathing-planet/

The team centered at NASA’s Goddard Institute for Space Studies will investigate habitability on a more local scale. Led by Tony Del Genio, it will examine the habitability of solar system rocky planets through time, and will use that foundation to inform the detection and characterization of habitable exoplanets in the future.

http://www.giss.nasa.gov/projects/astrobio/

The NASA Astrobiology Institute's Virtual Planetary Laboratory, based at the University of Washington, was founded in 2001 and is a heritage team of the NExSS network. This research group, led by Dr. Victoria Meadows, will combine expertise from Earth observations, Earth system science, planetary science, and astronomy to explore factors likely to affect the habitability of exoplanets, as well as the remote detectability of global signs of habitability and life.

Five additional teams were chosen from the Planetary Science Division portion of the Exoplanets Research Program (ExRP).  Each brings a unique combination of expertise to understand the fundamental origins of exoplanetary systems, through laboratory, observational, and modeling studies.

A group led by Neal Turner at NASA’s Jet Propulsion Laboratory, California Institute of Technology, will work to understand why so many exoplanets orbit close to their stars. Were they born where we find them, or did they form farther out and spiral inward? The team will investigate how the gas and dust close to young stars interact with planets, using computer modeling to go beyond what can be imaged with today's telescopes on the ground and in space. 

A team at the University of Wyoming, headed by Hannah Jang-Condell, will explore the evolution of planet formation, modeling disks around young stars that are in the process of forming their planets. Of particular interest are “transitional” disks, which are protostellar disks that appear to have inner holes or regions partially cleared of gas and dust. These inner holes may be caused in part by planets inside or near the holes.

A Penn State University team, led by Eric Ford, will strive to further understand planetary formation by investigating the bulk properties of small transiting planets and implications for their formation.  

A second Penn State group, with Jason Wright as principal investigator, will study the atmospheres of giant planets that are transiting hot Jupiters with a novel, high-precision technique called diffuser-assisted photometry. This research aims to enable more detailed characterization of the temperatures, pressures, composition, and variability of exoplanet atmospheres.

http://science.psu.edu/news-and-events/2015-news/FordWright4-2015

The University of Maryland and NASA’s Goddard Space Flight Center team, with Wade Henning at the helm, will study tidal dynamics and orbital evolution of terrestrial class exoplanets. This effort will explore how intense tidal heating, such as the temporary creation of magma oceans, can actually save Earth-sized planets from being ejected during the orbital chaos of early solar systems.

Another University of Maryland project, led by Drake Deming, will leverage a statistical analysis of Kepler data to extract the maximum amount of information concerning the atmospheres of Kepler's planets.

The group led by Hiroshi Imanaka from the SETI Institute will be conducting laboratory investigation of plausible photochemical haze particles in hot, exoplanetary atmospheres.  

The Yale University team, headed by Debra Fischer, will design new spectrometers with the stability to reach Earth-detecting precision for nearby stars. The team will also make improvements to Planet Hunters, www.planethunters.org, a web interface that allows citizen scientists to search for transiting planets in the NASA Kepler public archive data. Citizen scientists have found more than 100 planets not previously detected; many of these planets are in the habitable zones of host stars.

A group led by Adam Jensen at the University of Nebraska-Kearney will explore the existence and evolution of exospheres around exoplanets, the outer, ‘unbound’ portion of a planet's atmosphere. This team previously made the first visible light detection of hydrogen absorption from an exoplanet's exosphere, indicating a source of hot, excited hydrogen around the planet. The existence of such hydrogen can potentially tell us about the long-term evolution of a planet's atmosphere, including the effects and interactions of stellar winds and planetary magnetic fields. 

From the University of California, Santa Cruz, Jonathan Fortney’s team will investigate how novel statistical methods can be used to extract information from light which is emitted and reflected by planetary atmospheres, in order to understand their atmospheric temperatures and the abundance of molecules.

Editor: Sarah Loff