NASA-supported scientists have provided new information about how the early Earth may have acquired some elements necessary for the planet to become habitable. They also suggest a new role for Jupiter in the distribution of these elements throughout the young solar system. The study, published today in Science Advances, examines this history by looking at the ratio of phosphorus to nitrogen in iron meteorites and in younger objects known as chondrites.
Debjeet Pathak
Rice University
Our solar system formed from gas and dust that swirled around the proto-Sun more than 4.5 billion years ago. This gas contained the raw materials needed to form planets, moons, and ultimately life as we know it. Two elements of particular importance for life are nitrogen and phosphorus.
In the earliest stages of the solar system, gas and dust coalesced into bodies known as planetesimals. As these objects orbited the young Sun in this chaotic environment, planetesimals collided, leaving shattered remnants throughout the system. Eventually, many of these pieces were incorporated into planets and moons. Other pieces survive today as asteroids, still orbiting the Sun, and – if they have impacted the Earth and been recovered – as meteorites. These meteorites provide a window into the early solar system at a time before the Earth existed. Chondrites and iron meteorites are two different classes of these meteorites.
As their name suggests, iron meteorites are dense, metallic objects and are primarily made of iron-nickel alloy. Chondrites, on the other hand, are stony objects and they are responsible for most of the meteorites that have been found on Earth.
Each type of meteorite originates from planetesimals that formed at different times in our system. The oldest generation of planetesimals are the source of iron meteorites. Chondrites came from a second generation of planetesimals that formed 2-3 million years later.
Understanding how the Earth was made and the timing of its formation is important for astrobiologists who study how and when our planet became habitable for life as we know it. The young Earth needed to have a supply of life’s ingredients, including nitrogen and phosphorus, for the first living cells to form.
There is debate between scientists over where Earth’s stock of life-essential elements came from. Some evidence points to chondrites in the outer solar system traveling inward to arrive at Earth late in our planet’s formation process. However, the new study tells a different story.
Using laboratory experiments and geochemical models, the team reconstructed a map of phosphorus-nitrogen (P/N) ratios across the early solar system and found differences between the first (iron meteorites) and second (chondrites) generations of planetesimals.
The experiments and subsequent geochemical modeling showed that the first generation had a higher ratio of P/N in the outer solar system, with that ratio decreasing toward the inner solar system. This trend was reversed in the second generation of planetesimals, with higher P/N ratios in the inner solar system.
The thought is that during the formation of the first generation of planetesimals, there was an outward flow of material that raised the P/N ratio in the outer solar system. Then came Jupiter.
Rajdeep Dasgupta
Rice University
As Jupiter formed and grew to a tremendous size (and gravitational influence), the planet restricted the movement of phosphorus and nitrogen from the inner to outer solar system. This meant that when the second generation of planetesimals appeared, those in the inner solar system were left with a higher P/N ratio than their cousins further out.
“For our own solar system, Jupiter’s presence and growth history, indeed, seem to have played a critical role in determining the distribution of the basic chemical ingredients necessary for habitable worlds,” said Rajdeep Dasgupta of Rice University in Houston and senior author on the study. “It remains an open question whether a life-essential element budget similar to Earth’s can be established without a Jupiter-like planet in the population.”
Geochemical accretion modeling further shows that Earth’s present-day P/N signature is best reproduced by the inner solar system planetesimals, either those related to iron meteorites or those related to chondrites.
“The study suggests that Earth acquired its inventory of the life-essential elements phosphorous and nitrogen primarily from the inner solar system, without requiring a significant contribution from outer solar system chondrites,” said study lead author Debjeet Pathak, graduate student at Rice University.
For more information on astrobiology at NASA, visit:
https://science.nasa.gov/astrobiology
Karen Fox / Molly Wasser
Headquarters, Washington
202-358-1600
karen.c.fox@nasa.gov / molly.l.wasser@nasa.gov
