Comets

Comets: Why study them? What can they teach us about finding life beyond Earth?

Universe Today has explored the importance of studying impact craters, planetary surfaces, exoplanets, astrobiology, and solar physics, and what this myriad of scientific disciplines can teach scientists and the public regarding the search for life beyond Earth. Here, we will explore some of the most awe-inspiring spectacles within our solar system known as comets, including why researchers study comets, the benefits and challenges, what comets can teach us about finding life beyond Earth, and how upcoming students can pursue studying comets. So, why is it so important to study comets?

Dr. James Bauer, who is a Research Professor in the Department of Astronomy at the University of Maryland, tells Universe Today, “For star gazers, comets are some of the most attention-grabbing objects in the sky. They move, they change their shape, appearance, and their brightness, as they travel through their orbits. Yet they are scientifically important for other reasons. When they venture towards the Sun (approach their orbital perihelion), they show what they are made of, by emitting gas and dust from the comet nucleus. They are the most accessible, least altered solar system bodies, and they are accessible because they come close to the Sun and Earth. They have retained a significant portion of their volatiles over time, and they have likely played a significant role in transporting volatile material through the solar system, for example from the outer solar system to the inner planets.”

While comets have been explored via spacecraft for only the last few decades, their observational history dates back several thousand years, including Halley’s Comet, which becomes visible from Earth every 75-79 years, and was most famously illustrated on the Bayeux Tapestry in the 11th century that depicted the Norman invasion of England in 1066, known as the Battle of Hastings. Like most astronomical phenomena throughout history, the cometary observations were initially perceived as either positive or negative omens, whether it be for fortune or health.

In fact, it was the legendary Greek philosopher, Aristotle, who proclaimed during the 4th century BCE that comets were atmospheric phenomena. This belief went unopposed until a series of physicists and mathematicians made their own scientific assertions about comets, including the French mathematician, Jean Pena, who deduced that comets were of celestial origin as opposed to terrestrial origin. This was later confirmed by Tycho Brahe, who used the Great Comet of 1577 to measure its parallax and deduced that comets are of astronomical nature, as well. Since the dawn of the Space Age, several spacecraft missions have visited comets up-close, including Halley’s Comet on a few occasions, offering scientists incredible opportunities to learn more about these mysterious balls of ice. But, even with advanced ground- and space-based exploration technologies, what are some of the benefits and challenges of studying comets?

“Comets change,” Dr. Bauer tells Universe Today. “Throughout their orbits, as well as their behavior in different orbital passes near the sun (perihelion passages), they often have variations in their behavior. This makes them exciting to study. Each variance provides more information as to the cause and nature of its behavior. It also makes the behavior difficult to interpret. For example, if you measure an unusually strong presence of a species, unless you have observed the comet over a broad timescale, you cannot assume it is a regular feature of the comet and not something from a short-term outburst. This makes it difficult to interpret the same exciting variances in behavior.”

Comets are massive planetary bodies comprised of ice and dust that orbit the Sun in the Oort Cloud and are remnants of the formation of the solar system, making them approximately 4.6 billion years old. While comets put on fantastic displays when they travel close to the Sun, they originate in the far reaches of the solar system known as the Kuiper Belt, which is home to the dwarf planet, Pluto. As their orbits take them closer to the Sun, the increasing heat causes the volatiles within the comet’s core, known as its nucleus, to burn off and evaporate into the cosmos, producing the spectacular tails we see from Earth.

Dr. Bauer tells Universe Today, “Several decades ago, the paradigm for comet activity was that water sublimation drove the activity in most cases, as the comet came within distances near 3 or 4 AU to the Sun. Over the last decade with data from observatories like Spitzer, Akari, and NEOWISE this paradigm began to erode. These observatories showed CO [carbon monoxide] and CO2 [carbon dioxide] driven activity, or at least that these species were responsible for a significant portion of the outgassing for comets.”

Along with their spectacular displays, comets have been hypothesized to have brought the necessary ingredients to Earth for life to emerge in a process called panspermia, and some scientists even postulated using comets to disperse the ingredients for life throughout the Milky Way Galaxy. Regarding the ingredients, the European Space Agency’s Rosetta spacecraft discovered in 2020 that comet 67P/Churyumov–Gerasimenko contained phosphorus, which is one of the most crucial ingredients for life to emerge. But if comets brought life to Earth, could this be the same for other worlds throughout the cosmos? All things considered, what can comets teach us about finding life beyond Earth?

Image of Comet 67P/Churyumov-Gerasimenko taken by the European Space Agency’s (ESA) Rosetta spacecraft on Jan. 31, 2015. (Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0)
Composite image of the surface of Comet 67P/Churyumov–Gerasimenko taken by the ESA Rosetta spacecraft’s OSIRIS instrument in 2016. (Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA – CC BY-SA 4.0)

Dr. Bauer tells Universe Today, “Observations show us that comets carry the chemical building blocks of life, and those chemical species can also be found in exo-solar systems; they are ubiquitous.  Many classes of organic compounds have been detected in comets, compounds that are also found in spectra of exo-systems. Our currently single example of an exo-comet, 2I/Borisov, looked like a fairly typical Oort cloud comet, and so we might expect that primordial chemistry might be similar in exo-systems. However, continued studies, including of future exo-comet discoveries, may yield surprises.”

Researchers who study comets come from a myriad of scientific and technical disciplines, including astronomy, chemistry, computer science, physics and astrophysics, just to name a few. They use a combination of ground- and space-based observatories and telescopes to study the geological and chemical compositions of the cometary nuclei and tails. Examples of past and current instruments used to study comets include NASA’s Chandra X-ray Observatory, OSIRIS-REx, and the Pan-STARRS survey, with Dr. Bauer mentioning to Universe Today about upcoming instruments, including the Vera C Rubin Observatory, NEO Surveyor, Comet Interceptor, and SPHEREx. Therefore, with all these scientific backgrounds and instruments needed to study comets, what advice does Dr. Bauer offer upcoming students who wish to pursue studying comets, and what additional information can he share regarding studying comets?

“My advice is for students to learn to program well in several standard languages using machine learning libraries, so that they can program the complex analyses to find patterns within the data,” Dr. Bauer tells Universe Today. “The next generation of observatories will provide a very large quantity of data about comet behavior and characterization, but a clear picture of the furthest cometary populations, those in the Kuiper Belt and the Oort cloud populations, remains elusive. Some constraints from these cometary populations have been made, but the sizes and distributions of these furthest populations may need to be characterized by the generation of platforms that follow, a few decades from now.”

How will comets help us better understand our place in the universe in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!

Laurence Tognetti

Laurence Tognetti is a six-year USAF Veteran who earned both a BSc and MSc from the School of Earth and Space Exploration at Arizona State University. Laurence is extremely passionate about outer space and science communication, and is the author of “Outer Solar System Moons: Your Personal 3D Journey”.

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