‘Dark comets’ could reveal how Earth got its oceans

The water that makes up the oceans acted as a key ingredient for the development of life on Earth. However, scientists still do not know where the water here on Earth came from in the first place.

One leading idea is that space rocks such as comets and asteroids delivered water to the Earth through impacts. As a planetary scientist, I’m curious about the kinds of space objects that could have led to the formation of the oceans. For the past few years, I’ve been studying a type of object that I called a dark comet – which could be just the culprit. In a new study my colleagues and I published in December 2024, we discovered two classes of these elusive dark comets.

What is a comet?

The solar system is teaming with small bodies such as comets and asteroids. These space rocks were fundamental building blocks of planets in the early solar system, while the remaining leftovers are the comets and asteroids seen today.

These objects are also avenues by which material can be transported throughout the solar system. These small worlds can contain things such as rubble, ice and organic material as they fly through space. That’s why researchers see them as good potential candidates for delivering ices such as water and carbon dioxide to the Earth while it was forming.

Traditionally, the difference between comets and asteroids is that comets have beautiful cometary tails. These tails form because comets have ice in them, while asteroids supposedly do not.

When a comet gets close to the Sun, these ices heat up and sublimate, which means they turn from ice into gas. The gas heats up because of the sunlight and is then blown off the comet’s surface in a process called outgassing. This outgassing brings with it rubble and small dust grains, which reflect sunlight.

Asteroids, on the other hand, do not have cometary tails. Presumably, they are more like classic rocks – without ice on their surfaces.

What is nongravitational acceleration?

The outgassing material from the surface of a comet produces a cometary tail and a rocketlike recoil. The fast moving gas pushes on the surface of the comet, and this causes it to accelerate. This process drives comets’ motion through space on top of the motion set by the gravitational pull of the Sun.

So, when comets outgas, they have what planetary scientists call nongravitational acceleration – motion that isn’t caused by the gravity of objects in the solar system. Planetary scientists typically measure the nongravitational accelerations of comets after detecting their cometary tails.

What are dark comets?

Our team identified a class of small bodies in the solar system that take some of the properties of both comets and asteroids. We called them dark comets.

These dark comets have nongravitational accelerations like comets, so they experience a rocketlike recoil from comet outgassing. However, they don’t have the dusty tails that most comets have.

In other words, they look like typical asteroids, but gravity alone can’t explain their motion.

The first interstellar object, ’Oumuamua, was the first comet or asteroid-size body that was detected in the solar system that came from outside of the solar system.

’Oumuamua displayed this same mysterious combination of no dust tail but a cometlike nongravitational acceleration, which led to many theories trying to explain what the object could have been. One option is that it was outgassing like a comet but not producing a dusty tail.

Since ’Oumuamua was first spotted in 2017, my colleagues and I have identified other dark comets within the solar system. In our study, we found seven new dark comets, bringing the total to 14.

Now that we’ve found more dark comets, we’ve noticed that they come in two flavors. Outer dark comets are larger – about a mile wide in size – and on more elliptical orbits farther out in the solar system. Inner dark comets are smaller – typically 1,000 feet in size – and on circular orbits close to the Earth.

Contributions to Earth’s oceans

It’s still not clear exactly what these dark comets are. They may not even be traditional comets if they don’t have icy surfaces.

However, the most likely answer for their nongravitational accelerations is that they outgas water, like a comet, but don’t produce a dusty tail – at least not one we can see when we look at them with our telescopes.

If this is the case, there are sure to be many more of these objects, parading around like asteroids, still yet to be identified.

Since scientists don’t know for sure where the Earth’s water came from, if there really are lots of dark comets that have water near Earth, it is possible that these dark comets contributed water to the early Earth.

These dark comets could tell researchers more about the origins of Earth’s oceans and the development of life here on Earth.

Reasons to be excited for the future

This research is really just the tip of the iceberg, because we only just started finding these dark comets in 2023.

The Vera C. Rubin Observatory’s Legacy Survey of Space and Time, which comes online in 2025, will start scanning the entire southern sky almost every night to spot anything that moves. This telescope, located on a mountain in Chile’s Atacama desert, is home to the largest camera ever built.

It will give astronomers almost five orders of magnitude greater sensitivity for detecting moving objects in the night sky. It will likely help my colleagues and me discover lots of new dark comets in the near future.

Telescopes that are already operating, such as the Hubble Space Telescope and the James Webb Space Telescope, could also help my team watch for outgassing or ice on the surface of the 14 dark comets we’ve already identified.

Finally, the JAXA Hayabusa2 extended mission is slated to rendezvous with one of the inner dark comets, 1998 KY26, in 2031. Therefore, we will be able to see the surface of a dark comet in exquisite detail.

Darryl Z. Seligman is a Postdoctoral Fellow in Physics and Astronomy at Michigan State University. He is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-2303553. This research award is partially funded by a generous gift of Charles Simonyi to the NSF Division of Astronomical Sciences. The award is made in recognition of significant contributions to Rubin Observatory’s Legacy Survey of Space and Time.

Michigan State University provides funding as a founding partner of The Conversation US.

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