A group of scientists from the United States conducted a series of simulations that showed what characteristics planetary systems and their stars should have to make it easier for comets to deliver the "building blocks of life" to these worlds.
Prebiotic molecules were necessary for life to arise on Earth: molecular hydrogen (H2), water (H2O), carbon dioxide (CO2), and ammonia (NH3), for example. But how did they end up on the planet? There are two hypotheses that explain their origin.
The first is the hypothesis of endogenous synthesis (from within). It states that these molecules could have arisen on the young Earth as a result of natural phenomena: lightning discharges, irradiation of the atmosphere with high-energy protons, volcanic activity. However, the efficiency of this process depends on the degree of oxidation in the atmosphere and is significantly reduced in more oxidized environments. The composition of the Earth's atmosphere has undergone great changes throughout history. It is still difficult to know exactly whether the gas envelope of our young planet was suitable for endogenous fusion.
The second hypothesis is exogenous, close to the concept known as "panspermia". It suggests that the "building blocks of life" came to Earth from outer space. They were probably brought by asteroids, comets and interplanetary dust particles. In soil samples from the asteroid Ryugu, collected and delivered by the Japanese probe Hayabusa-2 in 2020, researchers have identified a diverse set of prebiotic molecules - aliphatic amines, carboxylic acids, nitrogen-containing heterocyclic compounds. And intact amino acids found in meteorite samples (Murchison meteorite, Lake Murray meteorite and Allende meteorite) may indicate that some prebiotic molecules can survive atmospheric entry and not break down.
Scientists have long considered comets as major "delivery vehicles" for potentially important prebiotics because these bodies are known to contain large amounts of prebiotic molecules - hydrocyanic acid (HCN) as well as simple amino acids. Despite the relatively small number of collisions of comets with the young Earth (compared to asteroids), researchers have estimated that these "tail wanderers" could deliver 20 times more organic material to our planet than meteorites. The explanation is that comets have a lot of carbon-containing matter (about 10 percent) compared to C- and S-class asteroids (two percent and 0.2 percent, respectively).
However, there is often controversy surrounding comets as efficient "deliverers" of elements important for life. These objects move in space at extremely high speeds, and if they hit the planet at a speed of more than 20 kilometers per second, the probability that complex organic molecules survive the collision with the Earth's surface is close to zero. True, mathematical models show: the structures of molecules can remain unharmed if comets collide with the surface at very low speeds, thus minimizing the "thermal decomposition of raw materials".
A team of scientists from the University of Cambridge (USA), led by astronomer Richard Anslow, tried to understand this issue in detail. The researchers conducted a series of simulations and computer experiments to understand how planetary systems can slow down the movement of comets - to reduce the speed of collision so that important molecules are not destroyed under the influence of high temperatures (this is possible at too high speeds of falling comets). Scientists have found that it is easier for comets to deliver "ingredients for life" to rocky planets that are densely "packed" in their systems. In other words, they are fairly close to each other.
Anslow and colleagues showed that the minimum collision rate will always be lower for exoplanets orbiting sun-like stars (yellow dwarfs) than for exoplanets at M-class red dwarfs (the most common type of star in the Milky Way). The researchers found that there are two types of planetary systems that can slow the speed of comets by 5 to 10 kilometers per second:
- Systems with relatively massive solar-type stars, where all objects rotate slightly slower;
- Systems where planets are close together, like peas in a pod - so that a comet repeatedly passing close to planets can slow down over time.
"Our calculations have shown that comets can safely deliver the ingredients for life to planets of relatively low mass, like Earth, or even lower mass that orbit stars of solar mass or even larger. And also to planetary systems where objects are quite close together, probably even closer than in our system," explained Anslow.
The astronomer noted that in ideal conditions as a result of a slow collision of a comet with the surface of an exoplanet inside the crater would appear something like a "prebiotic soup" or "comet pond" with important chemical compounds for life.