Brazilian researchers develop a model of the origin of water on Earth

Findings of the study were presented at the UK-Brazil-Chile Frontiers of Science conference held in the United Kingdom by the Royal Society, FAPESP and the Brazilian and Chilean Academies of Sciences.

By Elton Alisson, in Chicheley, England

Agência FAPESP – Researchers from the Universidade Estadual Paulista (Unesp), Guaratinguetá campus, in collaboration with colleagues from the Universidade Tecnológica Federal do Paraná (UTFPR) and the Astrobiology Institute of the National Aeronautics and Space Administration (NASA), have developed a more precise model to determine the origin of water and life on Earth.

Conducted within the scope of the research project “Orbital Dynamics of Minor Bodies,” funded by FAPESP, the model was described in an article published in The Astrophysical Journal of the American Society of Astronomy and presented on February 24, 2014, at the UK-Brazil-Chile Frontiers of Science conference.

Organized by the Royal Society of the United Kingdom together with FAPESP and the Brazilian and Chilean Academies of Sciences, the event ran until February 26, 2014, at the Royal Society facility in Chicheley, a village in the county of Buckinghamshire in southern England. Its purpose was to promote scientific and interdisciplinary collaboration among young researchers from Brazil, Chile and the UK in fields on the frontiers of knowledge.

“We have developed a model to analyze the possible sources of water from space, and we are determining what the most likely contribution of each of them is to the total amount of water found on Earth today,” Othon Cabo Winter, researcher in the Group on Orbital Dynamics and Planetology at Unesp Guaratinguetá and study coordinator, told Agência FAPESP.

According to Winter, it was believed until recently that comets had delivered most of the water found on the planet when they collided with the Earth during the formation of the Solar System.

Computer simulations of the amount of water that these celestial objects made of ice could have delivered to Earth – based on measurements of the amount of deuterium (the heaviest hydrogen) in their water – have revealed, however, that the comets were not the largest sources and that they could not have contributed as significant a portion of water to the planet as that previously estimated, Winter explained.

“According to the simulations, the comets’ contribution to the delivery of water to Earth would be 30% at most,” said the researcher. “More than that is unlikely.”

The researcher went on to say that in the early 2000s, international studies were suggesting that, in addition to comets, other planetesimals (objects that gave rise to the planets), such as carbonaceous asteroids – the most abundant type in the Solar System – could also have had water and delivered it to Earth through interaction with planets and planetary embryos during the formation of the Solar System.

This hypothesis has been confirmed in recent years by observations of asteroids from Earth and by observations of meteorites (pieces of asteroids) that entered the Earth’s atmosphere.

Other possible sources of water on Earth proposed in recent years are silicate grains (dust) in the solar nebula (cloud of cosmic gas and dust from the cosmos directly related to the origin of the Solar System) that encapsulated water molecules during the initial stage of Solar System formation.

This “new” source, however, has not previously been validated and included in the models of water distribution by primordial celestial bodies, such as asteroids and comets.

“We have included these silicate grains of the solar nebula along with the comets and asteroids in the model we developed and examined the contribution each has made to the quantity of water that reached Earth,” explained Winter.

Computer simulations

According to Winter, the water on Earth from each of these possible sources has a different amount of deuterium, which can be used to indicate its origin.

Through computer simulations, the researcher and his colleagues have managed to estimate the contribution of each of these celestial objects based on this “certificate of origin” for the water found on Earth. They have also been able to determine the volume of water that each of these sources delivered and when they did so during the formation of the Earth because the contribution from each of them was made during different periods.

“The greatest amount came from the asteroids, which contributed over 50%. A small portion (20%) came from the solar nebula, and the remaining 30% came from comets,” Winter explained.

The results of the researcher’s simulations also indicated that large planets, with large amounts of water, such as the Earth, could have been formed between 0.5 to 1.5 astronomical units – between 75 million and 225 million kilometers – from the Sun.

“This distance range from the Sun, which we call the ‘habitable zone’, allows water to remain in a liquid state,” said Winter. “Outside this region, it’s very cold, and water would freeze. Closer to the Sun, it’s very hot, and water would evaporate,” he explained.

The simulations also suggested that the model developed by the researchers appears to be more efficient in determining the amount of water and the time of its delivery to Earth by these planetary bodies than the models that indicate that the water was transferred simply by mere collisions between celestial bodies just beginning to form (protoplanetary), said Winter.

“Partial information about the possible contributions of each of these sources had already existed. However, up to that point, they had not been gathered into a single model, and it had not been determined when and how much they had contributed to the formation of the mass of water on Earth,” he said.

The importance of smaller bodies

During his presentation in England, Winter underscored the importance of exploring smaller bodies, such as asteroids and comets, through missions to space. The most recent space mission to explore asteroids, conducted by the Japanese Aerospace Exploration Agency (JAXA) with the Hayabusa probe to take samples of the Itokawa asteroid, has resulted in the publication of several articles in journals such as Science and Nature.

This year, Japan plans to launch the Hayabusa-2 space probe to extract samples of the subsoil of asteroid 1999JU3 in 2018 and bring them back to Earth in 2020.

Additionally, the European Space Agency (ESA) maintains the Rosetta space probe, which is expected to be the first object to land on a comet, the comet 67P/Churyumov-Gerasimenko, and NASA is also planning a mission to capture asteroids near the Earth.

Brazil is seeking to develop and launch the space probe ASTER in 2017 to orbit the triple asteroid 2001-SN263, formed by a central object 2.8 kilometers in diameter and two other smaller objects, one measuring 1.1 kilometers and the other 400 meters in diameter.

“Never before has there been a mission to a system of asteroids of this type,” said Winter. “All of the missions were performed to observe a single asteroid.”

By exploring asteroids and comets on missions like these, we can better explain the conditions under which the Earth formed and the appearance of life on the planet, the researcher explained.

“Because they are primordial celestial bodies, comets and asteroids hold information about what the Solar System was like during its formation,” said Winter.

One of the challenges in making these precious geological materials available for scientific study, however, lies not just with their collection but in the careful preservation of the samples, ensuring the recording and filing of several pieces of information related to each of the specimens, such as the circumstances in which they were collected and the results of analyses, underscored Caroline Smith, curator of the meteorite collection of the Natural History Museum of London, in her presentation after Winter.

According to Smith, the scientific study of meteorites began in the late 18th century with scientists such as German physicist Ernest Chladni (1756-1827).

The British Museum began its collection of meteorites 50 years after it was founded in 1753, Smith said.

Since then, with samples gathered by missions conducted by the space agencies of several countries, the collections at institutions such as the Natural History Museum of London have rapidly expanded.

“In 1961, there were approximately 2,100 known meteorites, 40% of which had a record of the time and place of fall,” said Smith. “Today, in contrast, there are 48,000 known meteorites, and only 2.4% have a record of fall.”

The growing number of meteorite samples collected and scientific studies conducted on them has resulted in significant challenges for the teams that curate these museum objects, explained the researcher.

“Some of our recent dilemmas have involved maintaining access to the collection while, at the same time, preserving the meteorites for future generations,” she said.

The article “A compound model for the origin of Earth’s water” (doi:10.1088/0004-637X/767/1/54), by Winter and colleagues, published in The Astrophysical Journal can be found at: iopscience.iop.org/0004-637X/767/1/54/article.