Asteroids: Fossils of the Early Solar System
1. Introduction
Asteroids are a group of small rocky objects that orbit around our sun. Our entire solar system formed from a giant molecular cloud. These molecular clouds contain gas clouds and dust materials, which weigh about 106 to 108 solar masses. Due to some sort of shockwave, these molecular clouds begin to collapse, and the gravity makes the clouds fall inwards, and due to angular momentum, they start to spin and form a flattened protoplanetary disk. At the centre, the materials start to accumulate more and form our sun. Accretion of these molecular clouds in the protoplanetary disk forms planetesimals such as planets, asteroids, comets, dwarf planets and other solar bodies. In the Tauri phase, the star creates strong solar storms, which clean up the molecular clouds which didn't form as planetesimals. From these, we can infer that most of the asteroids we see now were created along with our sun and other planetesimals.
During the formation of the solar system, most of the asteroids were trapped between Mars and Jupiter, these asteroids are called main belt asteroids. Trojan asteroids are found at the Lagrange points of the planets, and some asteroids orbit within our inner solar system. Generally, the asteroids range between a few meters and several kilometres in diameter. Asteroids are sometimes also called Minor planets. The first discovered asteroid was Ceres, and it is also the largest known asteroid so far, which is also classified as a dwarf planet. Ceres was discovered by Giuseppe Piazzi on January 1st, 1801, at the Palermo Observatory. In the early days, some planetesimals like Pluto and Ceres were classified as planets in the same category. However, as astronomers learn more about space, they theorise some criteria only if an object follows those criteria, it will be considered a planet, but Pluto and Ceres failed the criteria and became dwarf planets and asteroids. This is also applied to all other objects in our solar system, and they are classified accordingly. An object to be classified as an asteroid should follow some criteria, like it should orbit around the Sun, should not be a planet or dwarf planet, have no atmosphere, be primarily rocky or metallic, not display cometary activity and so on. This review aims to study, how the research of large asteroids like Ceres and 4 Vesta to recently discovered asteroid 2024 YR4 have contributed to shape our understanding various aspects of our solar system. These asteroids believed to be the fossils of our early solar system preserving various chemical and physical information. This paper also discusses about the contribution of asteroid discovery from ground-based telescopes to space telescope. We conclude this review by discussion of future aspects of asteroid research.
2. Asteroids
2.1 Distribution of Asteroids in our solar system
| Group | Distance | Region |
|---|---|---|
| Vatira | < 0.7 | Orbits inside Venus’s orbit |
| Atira | < 1.0 | Orbits inside Earth’s orbit |
| Aten | < 1.0 | Earth crossing asteroids with < 1 AU |
| Apollo | > 1.0 | Earth crossing asteroids with > 1 AU |
| Amor | 1.017 – 1.3 | Approach Earth but do not cross its orbit |
| Main Belt | 2.2 – 3.2 | Largest population between Mars and Jupiter |
| Trojan | ~5.2 | Share Jupiter’s orbit at L4 and L5 points |
The Table 2.1 showcases the major asteroid groups in our solar system with its distance from the sun and its region of orbit. From this we can infer that asteroids do not randomly orbit our sun. They are distributed across our solar system as groups at different distances from the sun. This distribution of asteroids is due to the combination of dynamical processes, resonances and gravitational influences, especially from Jupiter and Saturn. As discussed in the Introduction, these asteroids are created from the molecular clouds. The inner region produced rocky and metal rich objects, while in the outer regions they formed icy and gaseous objects. As discussed, most of the asteroids are found between Mars and Jupiter. As bodies near Jupiter begin to form into rocky planetesimals like planets, the strong gravitational influence of Jupiter inhibited the accretion into planet by stirring up velocities of the small bodies and made them collide into each other, which in turn becomes asteroids belt now. Jupiter’s strong gravity created orbital resonances, where its gravitational pull disrupted the stability of some asteroid’s orbit, leaving behind some gaps in the asteroid belt. These gaps are called Kirkwood gaps. During the migration of giant planets, they sent some materials inward, becoming NEO’s and Vatria’s. Some bodies become resonance with giants becoming Hilda, Trojan, Cybele, Alinda and some objects sent outward and become Centaurs, KBO’s. The thermal difference in the early solar system left most of the asteroids in inner belts are S type, while C type asteroids are found mostly in outer belt. These are the reasons why there are differences in the composition of materials in different regions and reasons for the structure of asteroids groups.
Asteroid families have clusters of asteroids which share similar orbital parameters such as semimajor axis, eccentricity, inclination and some even have similar composition. It is believed that the asteroids of the same families got separated from the same parent body, that is why they have similar characteristics. Most of the asteroid families form when a large parent body is subjected to heavy impacts and collisions and the fragments of the parents are made to orbit the sun and some families formed by cratering of asteroids. Most of the asteroid families are found in the main belt. The major asteroid families in the main belt are Flora, Eunomia, Koronis, Eos, Themis, Hygiea, Vesta, Nysa, Hungaria. Haumea is one of the most known family that was found in the Kuiper belt with a dwarf planet Haumea. If noted well, there will be mostly no major asteroid families beyond Jupiter or before Mars except few families like Hungaria, Phocaea, Ennomos and Eurybates, this is due to the gravitational perturbations of nearby planets which disrupt the formation of prominent families permanently. Beyond Jupiter the gravitational influence of the giant planets destabilizes the orbit of asteroids and prevents formation of families. The other main reason is that the main belt is mostly stable and has most of the known asteroids of our solar system and hence the collisions will be more thus creating more asteroid families.
2.2 Types of Asteroids
Based on the Spectral type, Albedo and compositions they can be classified into several types. There are three major types of asteroids: C-type (Carbonaceous), S- type (Silicaceous) and M- type (Metallic). As discussed in the previous paragraph, the thermal gradient played a vital role in the formation of different types of asteroids, S-type (17%) asteroids mostly formed inside of the solar system where the temperature is high, C-type (75%) asteroids mostly formed in the outer solar system beyond Jupiter where the temperature is low and M- type asteroids are found in the middle region. M- type asteroids are mostly the differentiated part of asteroids, where the core becomes molten and later it gets separated due to impact and collisions with other bodies. B-type, P-type, D-type and F-type asteroids are formed in the outer regions of solar nebula. G-type asteroids formed in the regions of ice, later they melted and caused chemical alteration of minerals. A-type asteroids are the fragments of the mantle of differentiated asteroids. Q- type asteroids are young formed due to the collisions. R- type asteroids are partially melted, differentiated bodies. V- type asteroids are volcanic crusts that get separated from differentiated asteroids. K-type asteroids are formed in the middle of the main belt and get altered by high temperature. L- types are formed by partial melting or peculiar conditions of solar nebula. E-type asteroids are formed in the hot region of solar system under poor oxygen conditions. T- type asteroids are formed in the colder regions of the solar system. The following table illustrates the different types of asteroids.
2.3 Physical characteristics of Asteroids
Asteroids range from a few meters to several kilometers in diameter. The largest asteroids are Ceres (~933km) and Vesta (~530 km). Most of the asteroids are in the range of 100 km and they have irregular and elongated shapes. Due to their self-gravity large asteroids tend to be nearly spherical. Like planets, asteroids also rotate on their own axis. The rotation period ranges from a few minutes to days. There is an interesting phenomenon called YORP effect, where the exposure of sunlight affects the rotation period, tilt of the spin axis and even break apart the asteroids. Most of the surfaces of asteroids are covered with regolith, a layer of loose fragments of the materials created by impacts and collisions, thermal expansion and exposure to cosmic rays. The fragments of the collisions and impacts attracted by mutual gravity and forms rubble asteroids, these asteroids have poor internal structure and cannot move faster than 2.2 hours without breaking part. These rubble piles have high porosity. Larger asteroids are partially or fully differentiated and have comparatively less porosity and have higher internal structure. Some asteroids exhibit in binary form, where two or multiple asteroids orbit each other with common barycenter and orbits sun. Some asteroids also exhibit satellite systems, where some asteroids capture another asteroid and make it its moon. Lack of atmosphere in the asteroids causes large diurnal variations where in daytime it becomes quickly hotter by sunlight and becomes extremely colder in nighttime by darkness.
3. Conclusion
As discussed in the introductions, the study of asteroids revealed many insights of early universe. They are one of the evidences of early solar system because mostly unaffected by any geological process. By studying these asteroids we can know more about the formation of our own world.