“Unveiling Secrets: How Mysterious ‘Dark Comets’ May Hold the Key to Earth’s Oceans”

The water that constitutes the oceans served as a crucial element for the emergence of life on Earth. Yet, scientists remain uncertain about the origin of the water on Earth.

A prominent hypothesis is that celestial objects such as comets and asteroids transported water to Earth via impacts. As a planetary scientist, I am intrigued by the types of celestial bodies that may have contributed to the creation of the oceans. For several years, I have been examining a specific category of object that I refer to as dark comets – which might be the missing link. In a recent study published by my colleagues and me in December 2024, we uncovered two categories of these enigmatic dark comets.

What constitutes a comet?

The solar system is filled with minor bodies like comets and asteroids. These celestial rocks were essential building materials of planets during the early solar system, while the leftover fragments are the comets and asteroids observed today.

These objects also serve as channels for transporting material across the solar system. As they traverse space, these small worlds may contain various items such as debris, ice, and organic substances. This is why researchers regard them as promising candidates for delivering ices like water and carbon dioxide to Earth during its formative years.

Typically, the distinction between comets and asteroids lies in the fact that comets possess stunning cometary tails. Such tails develop because comets contain ice, whereas asteroids are believed not to have ice.

When a comet approaches the Sun, the ices within it warm up and sublimate, transitioning from solid to gas. The gas heats due to sunlight and is subsequently expelled from the comet’s surface in a phenomenon called outgassing. This outgassing carries with it debris and diminutive dust particles, which reflect sunlight.

Asteroids, in contrast, lack cometary tails. They are presumably more akin to standard rocks – devoid of ice on their surfaces.

What is nongravitional acceleration?

The outgassed material from a comet’s surface generates a cometary tail and a rocket-like thrust. The rapidly moving gas exerts force on the comet’s surface, leading to acceleration. This mechanism influences the motion of comets through space, in addition to the movement governed by the gravitational attraction of the Sun.

Thus, when comets outgas, they exhibit what planetary scientists refer to as nongravitational acceleration – motion not attributable to the gravitational forces from other objects in the solar system. Planetary scientists often measure the nongravitational accelerations of comets after observing their cometary tails.

What are dark comets?

Our team identified a category of small bodies within the solar system that possess characteristics of both comets and asteroids, which we termed dark comets.

These dark comets experience nongravitational accelerations similar to those of comets, thus undergoing a rocket-like push from comet outgassing. However, they lack the dusty tails typical of most comets.

In essence, they resemble ordinary asteroids, but gravity alone cannot account for their movement.

The first known interstellar object, ’Oumuamua, was the first comet or asteroid-sized entity detected in the solar system originating from outside it.

’Oumuamua exhibited this same perplexing combination of no dust tail yet displayed a comet-like nongravitational acceleration, prompting numerous theories to explain its nature. One possibility is that it was undergoing outgassing like a comet but did not generate a dusty tail.

Since ’Oumuamua’s initial observation in 2017, my colleagues and I have detected additional dark comets within the solar system. In our study, we encountered seven new dark comets, raising the total to 14.

Now that we have discovered more dark comets, we have recognized that they come in two varieties. Outer dark comets are larger – roughly a mile in width – and follow more elliptical orbits located farther in the solar system. Inner dark comets are smaller – averaging 1,000 feet in size – and maintain circular orbits nearer to the Earth.

Contributions to Earth’s oceans

The true nature of these dark comets remains uncertain. They might not even qualify as conventional comets if they lack icy exteriors.

Nevertheless, the most plausible explanation for their nongravitational accelerations is that they expel water vapor, similar to a comet, yet do not create a dusty tail – at least not one visible through our telescopes.

If this holds true, there are undoubtedly many more such objects, resembling asteroids, that are still to be discovered.

Given that scientists are uncertain about the origins of Earth’s water, if a significant number of dark comets contain water near our planet, it is plausible that these dark comets played a role in supplying water to the primordial Earth.

These dark comets could provide researchers with insights into the beginnings of Earth’s oceans and the evolution of life on our planet.

Reasons to be thrilled about the future

This investigation is merely the beginning, as we have only recently begun identifying these dark comets in 2023.

The Vera C. Rubin Observatory’s Legacy Survey of Space and Time, set to become operational in 2025, will commence scanning the entire southern sky nearly every night to detect any moving objects. Situated on a mountain in Chile’s Atacama desert, this telescope is equipped with the largest camera ever constructed.

It will offer astronomers nearly five orders of magnitude enhanced sensitivity for observing dynamic entities in the night sky. This advancement will likely aid my colleagues and me in uncovering numerous new dark comets in the foreseeable future.

Operational telescopes, such as the Hubble Space Telescope and the James Webb Space Telescope, could additionally support my team in monitoring for outgassing or ice on the surfaces of the 14 dark comets we have already located.

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Ultimately, the JAXA Hayabusa2 extended mission is expected to meet with one of the inner dark comets, 1998 KY26, in 2031. Consequently, we will have the opportunity to view the surface of a dark comet in remarkable detail.

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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 grant is partially financed by a generous donation from Charles Simonyi to the NSF Division of Astronomical Sciences. The award acknowledges significant contributions to Rubin Observatory’s Legacy Survey of Space and Time.

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

This article is published again from The Conversation under a Creative Commons license. Read the original piece.