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Module 2 - Planetary Formation and Differentiation

Comet Fragment Slams into Jupiter

In July 1994, 21 chunks of comet Shoemaker-Levy 9, which had broken apart a year earlier, slammed into Jupiter. The Hubble telescope recorded this spectacular event. These images, beginning at lower right, chronicle the results of one such collision. Hubble began snapping pictures of the impact area just five minutes after the collision. Nothing can be seen. Less than two hours later, a plume of dark debris is visible [bull's-eye pattern, image second from bottom]. Two impact sites are visible in the next picture, taken a few days later. The final snapshot shows three impact sites, the newest near the bull's-eye-shaped region. *Image Credit*: R. Evans, J. Trauger, H. Hammel and the HST Comet Science Team and NASA

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Impacts!! 24 image_header 2022-02-03T18:51:03+00:00
Planetesimals and protoplanets grow by impacting other bodies, called accretion. Impacts can be very powerful, depending on the mass and make up of the impactor. Basically, the bigger the impactor, the more powerful the impact. Also, metallic meteorites are stronger and more durable during impact than stony meteorites, which are more durable than icy bodies. (Mentally compare what would happen if you were to drop a cannon ball, a chunk of concrete, and a dirty snowball off of a building.)

The power of the impact comes from the impactor itself. A chunk of rock hurtling through space carries a lot of kinetic energy, which is the energy of motion. A car going down the highway also carries kinetic energy, but cars are carefully designed to absorb that kinetic energy (when it strikes a tree or plummets head first off of a cliff) by bending in crumple zones. In contrast, the impacting body (meteoroid, asteroid, etc.) does not contain crumple zones, and its kinetic energy must be transferred to the surroundings upon impact, creating light, a loud boom, and possibly a crater ± impact melt (if large enough).

Impacts well demonstrate one of the fundamental laws of physics, called the law of conservation of energy. Energy can neither be created nor destroyed. It can only be transformed from one kind of energy to another.

As a body (meteoroid, asteroid, etc.) moves through space, it comprises an independent system. That system has a total energy (E_t) associated with it that includes other kinds of energy:
- kinetic energy (E_k): energy of motion. Kinetic energy can also refer to motion of individual particles (atoms, electrons), which increases with increasing temperature.
- gravitational potential energy (E_gp): energy of position relative to another body and depends on its mass and distance from the center of mass from another object.
- Internal energy (E_i): energy of the chemical bonds holding the body together.
If this hypothetical body were to approach a planet with an atmosphere like the Earth, friction and pressure heat up the object, giving off light and appearing to someone on the ground as a shooting star. Break up of the approaching object into smaller pieces begins as it moves through the atmosphere. The total energy of the approaching body is conserved, but the energy of the system is transformed and transferred to the surroundings. The new forms of energy include:
- Light energy (E_l)
- Internal energy of the surroundings
When the body impacts, the total energy of the impactor is almost entirely transformed and transferred to the surroundings. The impactor usually breaks up upon impact into smaller pieces, although with iron meteorites there can be more:
- Mechanical work (E_d) of the impact may create a crater, throwing up pieces of the substrate. Those pieces of the substrate would each have their own kinetic energy and gravitational potential energy.
- Sound energy (E_s): Impacts are loud and send out shockwaves, including sound waves.
- Heat, which increases the kinetic energy (E_k) of the substrate and may lead to melting, possibly forming impact melt (molten rock formed during impact).
The light and mechanical energy of an impact was visible in 1994 when a fragment of the Shoemaker-Levy 9 comet slammed into the dark side of Jupiter.

Remember.. The scale of an impact (size of the crater, amount of melting) correlates with the size of the impactor: the larger the impacting body, the more energy that can be transferred to surroundings.

Early in the history of the Solar System, the collision of planetesimals and proto-planets, therefore led to not only the accretion of mass to form the planets, but also to the energy to locally, or to wholesale melt a body, potentially forming a magma ocean.

The rate of impacts has dropped off dramatically since the early days of our Solar system. That doesn’t mean NASA is not preparing for a potential impact by mapping the distribution and orbits of Near Earth Objects. You can learn more about planetary defense efforts by listening to this Science Friday podcast.

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