The Hunt for Earth’s Origins
No one knows how water—the wellspring of life—arrived on Earth. No one knows if the Solar System, with a planet possessing the necessary ingredients for life within the habitable zone, is a cosmic rarity. Nor do scientists know whether the gas giants in the Solar System played a role in delivering essential materials to the habitable zone. The answers to these questions are contained in volatiles unaltered since the formation of the giant planets. Comets were long thought to be the most likely “delivery service” of Earth’s water. But new models and new data, including data from the Rosetta comet mission have cast some doubt.
Right now, astronomers are on the cusp of directly observing the process of habitable planet birth around other stars. The next generation of observatories will reveal details of the disk chemistry and planet growth in the regions where habitable worlds form. The only way to get similar data for the Solar System and learn where Earth’s water came from is to match the chemical fingerprints of inner solar system volatiles to a location in the protoplanetary disk (rotating circumstellar disk of dense gas and dust surrounding a newly formed star). Such data can distinguish between competing models of solar system formation to specify where the water came from and how it was delivered. Astrobiologist Karen Meech, at the University of Hawai‘i at Mānoa’s (UH Mānoa) Institute for Astronomy (IfA) and her team are using astronomy, planetary science, geology and astrobiology to explore these questions of where Earths’ water comes from.
Clues to Earth’s Past from Deep Inside the Earth
The idea that comets were the source of Earth’s water arose in the 1980s when the Giotto spacecraft flew through the cloud of gas surrounding Halley’s Comet. Its sensitive detectors showed that the comet’s water was enriched in deuterium, a heavy isotope of hydrogen, found in abundance in the Earth’s oceans. In the 4.5 billion years since the Earth formed, surface water has undergone a lot of processing through volcanic recycling, atmospheric escape and interaction with biology.
“It didn’t seem logical that today’s oceans should match the D/H (ratio between deuterium and hydrogen in water that yields information about its origin and geologic history) chemistry of the primordial water present at Earth’s formation,” said Meech. “During a geological field trip exploring Icelandic hot springs one of the guides said that the escaping water vapor plumes were “primordial.”
Although the statement had been wrong, this led to Meech to realize that the answer to the origin of Earth’s water would have to be pursued from multiple disciplines. Since isotope chemistry had shown the existence of primordial helium coming from Earth’s interior, Meech hypothesized that it could also be used to search for Earth’s primordial water. Lydia Hallis, an astrobiology postdoc working with Meech, has used advanced ion-microprobe instrumentation on samples from Baffin Island to show that Earth’s primordial water may be very different from the surface oceans. “We hope to mount an expedition to get samples from Greenland, because these are known to have the best primordial helium signatures,” said Meech.
Inner Solar System Material Stored in the Outer Solar System
The European Space Agency’s Rosetta mission, along with many ground-based measurements have shown that cometary D/H has a wide range of values. The relative amount of deuterium to hydrogen is temperature dependent and set during formation of the comet’s ices. The widely ranging values seen in comets suggests that they formed over a wide range of temperatures (distances from the Sun) in the Solar System.
On September 22, 2014 the Pan-STARRS1 telescope discovered a very weak active long period comet (LPC), C/2014 S3 near its closest approach to the sun. This was very unusual since LPCs usually develop long tails as their ices heat up for the first time. Dubbed a “Manx” comet for its nearly tailless appearance (like the cat), a spectrum showed that its composition was similar to inner solar system rocky material — very different from the red organic-rich surfaces of comets.
Astronomers have many models of how planets in Solar System grew, some have the planets forming in place, while others have large scale gas giant planet migration. All of the small leftovers, the rocky or icy-planetesimals were scattered by the giant planets. From where in the solar system material gets scattered into the Earth-building zone depends on which model is correct.
“This discovery of a Manx with inner solar system composition was really exciting,” said Meech. “Because this is only consistent with some of the models, Manx comets may provide a way to discriminate between formation models.”
The Proteus Mission—Searching for Origins
The large variation in D/H in comets coupled with large scale movement of these icy leftovers of the planet building process show that astronomers cannot use a single chemical fingerprint to trace the delivery of water to the inner solar system. Measurements of other protoplanetary disks and models show that multiple chemical and isotopic signatures should be frozen onto the planetary building blocks.
Tracing different chemicals can provide independent measurements of where the material originated. To access this record, astronomers need: (1) a population of icy bodies that faithfully records the history of volatile migration in the early solar system; (2) a source of volatiles that can be accessed affordably; (3) knowledge that the volatiles were not altered by heating in their parent body; and (4) measurements from multiple chemical markers with sufficient precision to distinguish between original volatile reservoirs. Main Belt Comets (MBCs), a class of icy bodies that orbit within the asteroid belt that were discovered by astronomers in Hawai‘i, meet these requirements.
MBCs are the perfect targets for this investigation because they satisfy these criteria. MBCs have emerged as significant reservoirs of primordial water and potentially other volatiles. These icy asteroids may have formed in-situ or been dynamically implanted as the giant planets grew. Unlike true comets, they have remained on stable orbits since the era of planet formation or migration and preserve a record of their accretional or growth environment, frozen in time. Occasionally an MBC gets hit by a meter-scale impactor that strips away part of its insulating regolith (loose covering). The newly exposed volatiles sublime into space over an extended period. “Unlike asteroids, the tight-lipped witnesses to events in the early solar system, MBCs are extroverts; spewing their secrets into space where a nearby spacecraft can observe them without having to land and excavate,” said Meech.
A team of scientists from the IfA and the School of Ocean and Earth Science and Technology at UH Mānoa, NASA’s Jet Propulsion Laboratory, the Southwest Research Institute, and the Japan Aerospace Exploration Agency are developing a Discovery-class mission concept called Proteus to explore multiple MBCs. Proteus will provide key missing information about the processes that created a habitable planet in Earth’s Solar System.
Interstellar Objects—Messengers from other Star Systems
On October 19, 2017, the Pan-STARRS1 telescope discovered the first interstellar object (ISO), 1I/‘Oumuamua. In spite of a very short period of visibility there was an intense observing frenzy to characterize it that has led to the publication of more than 120 papers.
“ISOs are the left-over debris from the process of building planets in other star systems,” said Meech. “When they are delivered to our neighborhood, ISOs give us a close up view of the planet building processes elsewhere that we cannot get any other way.”
‘Oumuamua has challenged many assumptions by astronomers about what small bodies from another star system would look like, because its elongated shape was atypical of a comet. Currently, UH researchers are intensely studying a new interstellar remnant, 2I/Borisov, that will remain visible for over a year. With the Vera C. Rubin Observatory, formerly known as the Large Synoptic Survey Telescope, scheduled to come on line soon in Chile, astronomers from around the world can expect to see an increase in the number of ISOs discovered. This will provide astronomers with more samples of materials from other star systems to compare with the primordial building blocks of Earth’s Solar System — helping to further address the question surrounding the formation of habitable worlds.
“The next question of whether or not they are inhabited is still off in the future,” added Meech.
An international team of scientists, led by UH Mānoa’s Karen Meech, are developing a NASA Discovery-class mission concept called Proteus to explore Main Belt Comets that should yield key missing information about the processes that created a habitable planet in Earth’s Solar System.