Somewhere beyond the layers of cloud-tops, veiled below the horizon, lies a series of massive craters on the southern surface of the planet Jupiter. Their explosive history and recent origin can be traced to the fragments that once formed the Shoemaker-Levy 9 comet, before its close encounter with Jupiter in 1994. Scientists celebrated it as the first ever observation of extra-terrestrial impact.
Like the Earth, Jupiter bears the scars of past surface impacts. The first observation of Shoemaker-Levy 9 began thirty years before its fateful collision. While many comets orbit from the Kuiper Belt, Oort Cloud, or beyond, some fall within the planetary system itself. Shoemaker-Levy 9, though discovered in March 1993, was caught by Jupiter’s gravity during the late 1960s or early 1970s, reorienting its orbit away from the sun.
Astronomers Eugene Shoemaker, Carolyn Shoemaker, and David Levy accidently discovered the comet while searching for near-Earth objects at the Palomar Observatory in California. What they uncovered was a tight, elliptical path falling towards the planet: its orbit was the consequence of its trajectory while being captured by Jupiter. Prior to its detection, Shoemaker-Levy 9 had already skimmed its cloud tops and broken into at least 23 fragments thanks to gravitational tear. The fragments varied in size, from tens of meters, to a few kilometres. Broken, but not destroyed, the pieces maintained their orbit in unison.
Using the best tools of their time, experts found that not only had Shoemaker-Levy 9 previously grazed Jupiter, but that it was soon to return at a distance of 45,000 kilometres from its center. Given the planet’s enormous size, and the comet’s path of motion, this implied a 99.99 per cent chance of collision. Astronomers became excited at the prospect of seeing an event that occurs but once every six thousand years.
With numerous telescopes, two nearby probes, and even the distant Voyager 2 trained on the anticipated meeting, media coverage of the event grew. Fate would have it that only minutes after the collision began, Jupiter was at just the right angle for direct Earth observation of the impacts.
Six hundred times more powerful than the world’s entire nuclear arsenal, the pieces slammed into the planet at sixty kilometres per second. The resulting plumes reached 3,000 kilometres into the sky at a temperature of over 24,000 degrees Celsius, creating dark crescents in the cloud tops that could span the diameter of the Earth. From July 18 to July 22, 1994, scientists exploited the opening of the clouds to find what lay beneath them.
Shoemaker-Levy 9 revealed the presence of diatomic sulphur (S2), carbon disulphide (CS2), ammonia (NH3), and hydrogen sulphide (H2S) in Jupiter’s atmosphere. It generated seismic waves that permeated the planet at hundreds of kilometres per hour, amplifying radiation emission from the surface. Based on data collected from the impact, Jupiter contained less water than previously thought — or at least, farther within its depths. The scars from the collision remained for months, and were even more prominent than the Great Red Spot.
These events were critical in demonstrating the conception of Jupiter as a “cosmic vacuum cleaner,” a body that attracts objects away from the inner solar system. Because of its sheer mass and location, Jupiter would be well-suited to pulling asteroids, meteors, and comets such as Shoemaker-Levy 9 away from any chance of meeting the Earth. Some scientists speculate that the number of extinction events on Earth due to impacts has been significantly reduced by Jupiter’s influence. However, a debate remains as to whether Jupiter pulls more of these objects than it blocks towards the Earth with its gravity. Regardless of theoretical implications, the comet’s punch into the planet not only made a spectacle for all to see, but also marked an important day in astronomy.
Drifting away from the explosions of Shoemaker-Levy 9, next week we will awaken to the serenity of the red desert from another planet, and the tale of a land lost on it. Until then, the glow of comet dust trailing through a sea of darkness awaits you.
