Meteorite

A meteorite is a rock that originated in outer space. It has fallen onto the surface of a planet or a moon. This journey begins when a meteoroid enters a planetary atmosphere. As the object moves, friction and pressure cause it to heat up. This process causes the object to radiate energy. This creates a bright fireball, which many people call a shooting star. Astronomers call the brightest examples of these fireballs bolides.

When the object finally settles on a surface, it is called a meteorite. There are two ways scientists categorize these arrivals. If the object is observed during its flight and then recovered, it is a meteorite fall. All other recovered meteorites are known as meteorite finds. Most meteorites are quite small. They usually arrive at the surface at their terminal velocity. This means they only create a small pit upon impact. However, larger objects can create massive hypervelocity impact craters.

As a meteoroid enters the atmosphere, it undergoes a process called ablation. This is when the heat melts the surface of the rock. Ablation can sculpt the meteorite into unique shapes. Some meteorites develop shallow, thumbprint-like indentations called regmaglypts. If the object maintains a fixed orientation, it may develop a conical nose cone shape. As the object slows down, the molten surface layer solidifies. This forms a thin, dark layer known as a fusion crust.

Scientists traditionally divide meteorites into three broad categories based on their makeup. Stony meteorites are composed mainly of silicate minerals. About 86% of meteorites are chondrites, a type of stony meteorite. These contain small, round particles called chondrules. These particles were likely melted while floating in space. Chondrites are roughly 4.55 billion years old. They are often called the building blocks of the planets. Another stony group is the achondrites, which make up about 8% of meteorites.

Iron meteorites make up about 5% of observed falls. These are largely composed of ferronickel alloys like kamacite and taenite. Most iron meteorites likely come from the cores of ancient, molten planetesimals. When these planetesimals collided, their metal cores broke apart. Iron meteorites are very durable and often survive atmospheric transit intact. This is why they are frequently responsible for large impact craters. Examples include the Barringer Meteor Crater and the Odessa Meteor Crater.

Stony-iron meteorites represent the remaining 1% of meteorites. These contain large amounts of both metallic and rocky material. One type is called a pallasite, which may come from the boundary of a planetesimal's core. Another type is the mesosiderite. While rare, these objects provide a look at the complex layers of early space bodies.

Space rocks hit Earth with surprising frequency. Roughly 35 to 40 objects with a diameter of one meter enter the atmosphere every year. This averages out to about one event every ten days. Most of these objects disintegrate before they reach the ground. However, very large stony objects can reach the surface. These objects can weigh tens of millions of tons. Such impacts are very rare but can be extremely energetic.

Meteorites are vital for understanding the chemistry of our solar system. Some meteorites come from the Moon or Mars. These are the only materials from other planets that humans have recovered. Scientists have found complex organic compounds in meteorites. These include substances found in DNA, such as uracil and cytosine. In 2018, researchers found liquid water and prebiotic substances in 4.5 billion-year-old meteorites. In 2019, scientists even detected sugar molecules. These findings suggest meteorites may carry the ingredients for life.