Evolution, Composition, and Structure of Comets

From the lecture series: A Field Guide to the Planets

By Sabine Stanley, Ph.D., Johns Hopkins University

There have been many studies done to better understand the composition, evolution, and structure of comets. These insights help to know more about those parts of our solar system that are difficult to visit. Read on to learn about some famous comets.

The image shows a bright comet shooting through the dark sky over the Earth's surface.
Comets have very elliptical orbits around the Sun. They become visible only when their orbits bring them to the inner solar system. (Image: Triff/Shutterstock)

Composition of Comets

Comets are very important for understanding the formation of the solar system, and so there has been a concerted effort to learn about their composition, evolution, and structure.

When a comet is seen in the sky, what is typically seen is the extended sublimating atmosphere, also known as the coma, surrounding the comet. A comet’s coma can be extremely large, extending for about 100,000 kilometers across—5 to 10 times the diameter of Earth. But the real heart of the comet is the small, solid portion on the inside of the coma, known as the nucleus. This is the only part of the comet that exists when it’s in the outer solar system and not outgassing.

Halley’s Comet

The Soviet Vega 1 spacecraft mission to Halley’s Comet provided the first-ever images of the nucleus of a comet. Halley’s nucleus is potato-shaped, about 15 kilometers long and 8 kilometers wide and deep. It’s also very bumpy, with lots of mountains, ridges, and depressions. It even has a crater! Although the nucleus is solid, it’s not very compact. Halley’s nucleus is probably best described as a rubble pile of small pieces loosely held together by their gravity. And this type of structure is probably relevant for many other comets, too. The jagged shapes and broken pieces of the comet may be explained by the outgassing of the comet. There are basically eruptions occurring over the comet’s surface creating new jets of material for the coma and tail. This rips apart the solid part of the comet.

The image shows Halley's comet with its bright tail against the backdrop of stars.
Halley’s Comet was last visible in 1986. It will be seen next in 2061. (Image: Brian Donovan/Shutterstock)

Europe’s Giotto spacecraft mission to Halley’s Comet also determined its ratio of deuterium/hydrogen, or D-to-H ratio. Scientists wanted to see whether this ratio matched Earth’s oceans, because that would be a sign that the source of water on Earth is from comets like Halley. However, Halley’s D-to-H ratio was twice as high as Earth’s, suggesting comets like Halley couldn’t be the major source for Earth’s water. But measurements from other comets have shown that the D-to-H ratio varies by at least a factor of 2 to 3 from comet to comet. It may be that some combination of comets, as well as icy asteroids, needs to be mixed together to get the D-to-H ratio seen in Earth’s oceans.

Even though Halley is a short-period comet, its orbit is unusual because the orbit is retrograde and extremely elliptical. This retrograde orbit suggests that Halley may have been a long-period comet that experienced some sort of gravitational interaction that changed the direction of its orbit and made it a short-period comet. This means Halley may have originated in the Oort Cloud, rather than the Kuiper Belt.

Over 85 comets with orbital characteristics similar to Halley have been discovered, typically with high eccentricity and inclination. They are now called Halley-type comets to distinguish them from the other short-period comets that have more typical orbital trajectories. And just like Halley, they may all be examples of long-period comets from the Oort cloud that were gravitationally perturbed when they got near a giant planet. Giant planets can change long-period comets into short-period comets.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on The Great Courses Plus.

Jupiter-Family Comets

Another category of comets are known as “Jupiter-family comets”. These comets have orbits in the plane of the solar system from out near Jupiter, traveling to points inward. Jupiter-family comets are likely to have been Kuiper Belt objects that at some point got too close to Jupiter, which gravitationally perturbed their orbits inwards. Over 660 Jupiter-family comets have been discovered.

Comet Wild 2 is a good example of a Jupiter-family comet. The ellipticity of its orbit around the Sun today takes it from just past Jupiter’s orbit at greatest distance inward, to about Mars’s orbit at closest approach. The orbital period is 6.4 years. But this same comet’s orbit took 43 years as recently as 1974! Scientists have traced the orbit back in time and determined that the comet passed really close to Jupiter in 1974, at a distance of about 1 million kilometers. That is when the comet experienced a strong gravitational perturbation, changing its orbit to what is seen today. Before that, Wild 2 orbited much farther out, where the closest point was near Jupiter and the farthest point was near Uranus.

The Stardust mission was sent to Comet Wild 2 and samples were collected from the coma in 2004. It also imaged the nucleus, which is more spherical than Halley’s Comet, and around 5 kilometers in extent. In 2006, the samples from Wild 2 were then returned to Earth, where scientists studied them and found that they contained many organic compounds. They even found glycine, the simplest amino acid that is a building block for life here on Earth. This demonstrated that the chemistry in comets may have provided other early building blocks for life, well beyond water.

The image is a picture of Tempel 1 comet taken 67 seconds after the  Deep Impact impacted it with a probe.
This image, taken 67 seconds after Deep Impact mission impacted the comet Tempel 1 with a probe, shows ridges, scalloped edges and possible impact craters from long ago. (Image: NASA/JPL-Caltech/UMD/Public domain)

Comet Tempel 1 is another example of a Jupiter-family comet traveling roughly between the orbits of Mars and Jupiter. And it completes an orbit even more quickly, at every 5.5 years. It’s about half as long as Halley’s Comet, and not very bright. In fact, it can’t ever be seen from Earth with the naked eye.

The NASA Deep Impact mission visited Tempel 1 in 2005. To study the composition of the comet, the mission actually impacted the surface with a probe and studied the material that was churned up. Spacecraft instruments detected a wide range of minerals in the dusty part of the comet including silicates, carbonates, and metal sulfides like pyrite, also known as fool’s gold. It also detected complex organics like polycyclic aromatic hydrocarbons, or PAHs that is seen on Earth in coal and tar.

Comet Churyumov- Gerasimenko, called 67P for short, is another Jupiter-family comet, but is distinct from Wild 2 and Tempel 1 because of its shape. It’s what is called a contact binary, which means it’s actually 2 large pieces that are connected by a neck region. This structure happens when 2 comets have a gentle collision, resulting in them orbiting each other very closely, eventually touching and becoming one object. The larger lobe is about 4 kilometers across in its longest dimension, and the smaller lobe is about 2.5 kilometers across.

The European Space Agency sent the Rosetta mission to study Comet 67P. In 2014, Rosetta became the first spacecraft to orbit a comet’s nucleus. It orbited 67P at a closest distance of about 10 kilometers. But a real highlight of the Rosetta mission is that the spacecraft also carried a lander, named Philae, the first spacecraft ever to land on a comet.

Learn more about Comets, the Kuiper Belt, and the Oort Cloud.

Landing on Comet 67P

Now, landing on a comet and staying there is harder than one might think. Because comets have so little mass, gravity is really low and escape velocity is really slow. If one wanted to launch off of Comet 67P, they would only have to reach a velocity of 2.2 miles per hour. So Philae was designed to have a downward thruster to push the craft down and counter any rebound from the landing. And there was a harpoon system that was supposed to latch onto the comet and hold the craft in place. But both the thruster and the harpoon system failed! So, when Philae touched down on the surface, it rebounded. The rebound wasn’t fast enough for escape velocity, but the bounce lasted almost 2 hours, taking Philae 1 kilometer above the surface. Then Philae landed on the surface again. On this second touchdown, it bounced again, but this time for only 6 minutes.

Ultimately, Philae did land, and stay landed, so that was great. But it was about 1 kilometer from the target landing site. And unfortunately, the bounces meant that Philae ended up coming to a stop in the shadow of a cliff. This resulted in very little sunlight being able to reach its solar panels, and Philae’s battery died after 57 hours. But even in that short time, Philae helped to answer the question: What is the surface of a comet like? Philae returned amazing close-up images and analyzed the composition and structure of the comet. It was learned that at Philae’s landing site, 67P has an outer rigid crust about 10 to 50 centimeters thick. The crust is made of ice and regolith and is not very strong, kind of like crunchy snow. The surface is also very uneven, showing lots of jagged edges, cliffs, and crevasses. Rosetta also found 67P to contain many different organic compounds, including the amino acid glycine.

It’s hard to discover comets until they travel into the inner solar system and develop their bright tails. However, with advancements in science and technology, these comets can now be studied in detail to help increase the understanding of our solar system.

Learn more about near-Earth asteroids and the asteroid belt.

Common Questions about Comets

Q: What are the three components of a comet?

The three components of a comet are: nucleus, coma, and tail.

Q: Why is Halley’s Comet so famous?

Halley’s Comet is famous because it was the first time that astronomers discovered that comets could be periodic. Since it was known when to expect it, several countries took advantage of the opportunity, and many spacecraft missions from the Soviet Union, Europe, and Japan were sent to study the comet.

Q: Which spacecraft launched a probe that smashed into comet Tempel 1?

The NASA Deep Impact mission visited Tempel 1 in 2005. To study the composition of the comet, the mission impacted the surface with a probe and studied the material that was churned up.

Q: Did NASA land on a comet?

The European Space Agency sent the Rosetta mission to study Comet 67P. The spacecraft also carried a lander, named Philae, the first spacecraft ever to land on a comet.

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