Did a main-belt comet collide with the source of life?

Where does the water on Earth come from? Recently, Nature magazine published an article stating that detection data from the James Webb Space Telescope (hereinafter referred to as the Webb Telescope) showed for the first time that water vapor exists around Comet Reed in the main belt of the solar system.

"Main-belt comets exist in the asteroid belt between the orbits of Mars and Jupiter." Yu Liangliang, assistant professor at the Macau University of Science and Technology, told the Science Times reporter that main-belt comets are a type of active small celestial bodies discovered by astronomers in the past 30 years. They are main-belt small celestial bodies that are believed to contain water ice inside, but strong direct evidence has been lacking before. The new findings of the Webb telescope have confirmed this view.

Water-rich comet brings new evidence

The location of the Earth in the solar system was not the place for water with a very low boiling point and very light specific gravity to stay from the beginning. This is the rule set by the sun with its high "body temperature". Unexpectedly, the Earth is a "nail house", with very exceptional water, even water accounts for 71% of the Earth's surface area.

Then the question is, the temperature of the solar protoplanetary disk where the earth was formed was very high, and water vapor should not be able to condense on the primitive earth. The mainstream view in the past was that the water on earth was brought by comets and some asteroids. Composed of ice, dust and rocky materials, comets are small celestial bodies rich in water ice. When a comet approaches the sun, its ice begins to sublimate and produce gas that erupts outward and blows away small particles of dust on the surface, giving it a hazy or cloudy halo, called a "coma", and a long tail is the "comet tail".

Yu Liangliang believes that the water molecules in the coma of Comet Reed prove that there should be a large number of small bodies with internal ice in the main belt. Their water ice is mainly buried deep underground, and usually has a low gas production. If an impact occurs, underground water ice in a local area will be exposed, which will produce a comet-like tail, thus becoming a main belt comet. "Main belt small bodies are more likely to migrate to the vicinity of the Earth's orbit than periodic comets, which to some extent supports the possibility that the icy small bodies in the main belt can 'deliver water' to the Earth."

Icy small bodies in the main belt may also be "water deliverymen"

The "delivery of water" from icy small bodies in the main belt to the Earth is also full of uncertainty, and the challenge comes from hydrogen isotopes.

Previously, scientists calculated the deuterium-hydrogen ratio (D/H) of water on Earth and three famous comets - Halley, Hyakutake and Hale-Bopp, and found that the deuterium-hydrogen ratio of cometary water is more than twice that of water on Earth. If all the water on Earth was brought by comets, the deuterium-hydrogen ratio should be the same. "This means that the water on Earth should not come entirely from comets," said Yu Liangliang.

Studies have shown that the deuterium-hydrogen ratio of chondrites from asteroids is lower than that of the Earth. "The deuterium-hydrogen ratio of water on Earth is between that of asteroids and comets. Asteroids in the protoplanetary disk are formed inside the snow line (condensation point line) of water and are water-deficient, while comets are formed outside the snow line and are rich in water." Yu Liangliang analyzed that after a certain proportion of asteroids and comets hit the Earth, the water they brought mixed together, which can explain the problem that the deuterium-hydrogen ratio of water on Earth is between that of asteroids and comets.

Yu Liangliang also proposed another possibility. "There may be some small celestial bodies with a material composition between asteroids and comets, located near the snow line. They delivered water to the Earth, so the deuterium-hydrogen ratio is also between that of asteroids and comets."

In the solar system, the snow line for water is located in the middle of the main belt. "The discovery of main belt comets and the evidence from the Webb telescope that they do have water molecules prove that there are indeed small icy bodies with a material composition between asteroids and comets near the snow line. They may have brought water to the early Earth," said Yu Liangliang.

The absence of carbon dioxide is not surprising

The water was successfully detected using the Webb telescope's near-infrared spectrometer, but it also came with a new mystery: Unlike other comets, no carbon dioxide was detected around Comet Reed. Typically, about 10% of a comet's volatile matter is carbon dioxide.

Yu Liangliang believes that this is not surprising. "According to thermodynamic models, the temperature in the main belt region can only keep water inside for longer than the age of the solar system. Carbon dioxide sublimates more easily than water and cannot be preserved for a long time." Periodic comets come from the Kuiper belt or the Oort cloud, where the temperature is low and carbon dioxide can be preserved.

Michael Kelly, an astronomer at the University of Maryland and the first author of the paper, explained that one possibility is that Comet Reed contained carbon dioxide when it was formed, but later lost it due to high temperatures. Another possibility is that Comet Reed may have formed in a warmer place in the solar system, where carbon dioxide could not condense onto the star.

In the future, main-belt comets may be able to tell us more.