Today is the one-year anniversary of the end of the Cassini mission at Saturn. On September 15th, 2017, the Cassini spacecraft burned up in the atmosphere of the planet it had studied for thirteen years.
Cassini was more than just a robotic explorer. For many of us, myself included, it is a mission we grew up with. While I’m sad Cassini’s mission ended, I can’t help but be amazed by the incredible work the Cassini team performed over the past two-plus decades.
I was watching coverage on NASA TV right at the moment when the final signals from Cassini reached Earth. That bittersweet moment marked the end of an incredible era at Saturn. Below, you’ll find my top ten favorite discoveries made during the mission. Let’s start off with number 10.
10- Cassini’s launch on August 20th, 1997
Let’s start today’s countdown with where the mission began. The Titan IV rocket was used just this one time to launch a scientific payload, and what a payload it was. Cassini lifted off from SLC-40, which is now the site of many SpaceX launches. Cassini got quite the sendoff with a spectacular night launch, pictured first in today’s post. With that, this small bus-sized spacecraft began its seven-year trip to Saturn.
9- The hexagon and a storm at Saturn’s north pole
There is a massive hurricane at the center of a uniquely shaped hexagonal polar vortex that has been active since Saturn was imaged by the twin Voyager spacecraft during flybys in 1980 and 1981. Scientists don’t know exactly how long this storm has been raging, but it doesn’t show signs of slowing down.
Cassini captured breathtaking images of this storm during the final years of its mission, thanks to the sun illuminating the north pole of the planet.
The winds of this storm swirl around at speeds of more than 220 miles per hour; with speeds up to 330 mph at the eye of the storm. Even more staggering, the eye is about 50 times larger than the eyes of the most powerful hurricanes here on Earth. The first picture in today’s post shows a close-up view of the eye of the storm in false-color. You wouldn’t want to get caught in that storm.
8- Giant storms in Saturn’s atmosphere are driven by water convection
Continued observation of this planet by the Cassini spacecraft yielded incredible results. Instead of flybys giving us a glancing view of worlds, as the Voyager spacecraft accomplished, Cassini was able to continuously study Saturn as well as the moons in this system for thirteen years.
This continuous observation from Cassini showed that Saturn’s incredibly powerful storms are driven by a simple process that drives weather here on earth, water. Having an understanding of the fundamental science that drives physical processes here on Earth directly relates to how we are able to study our solar system. This series of images are almost-true color, and they show a massive storm that raged on Saturn from 2010 to 2011.
7- Measuring Titan’s seas
Radar measurements showed that Ligeia Mare, Titan’s second largest sea, is in the neighborhood of 560 feet deep. Radar waves were able to determine the depths of this sea because the liquid that flows there isn’t water. It’s actually a sea that’s filled with primarily Methane which isn’t reflective like water.
Right now, scientists believe that these methane lakes on Titan may be on top of a large hydrocarbon aquifer underneath the surface of the moon. The seas on Titan are named after creatures in nautical mythology around the world.
6- Imaging the “vertical structures” in Saturn’s rings
In 2009 it was discovered that Saturn’s rings are thicker and more varied than previously thought. Pictured here are some of the vertical structures, which are over 1.5 miles high. This disproves the notion that the rings have a thin disc-like structure.
Carolyn Porco, a Cassini Imaging team leader, said that “we thought the plane of the rings was no taller than two stories of a modern-day building and instead we’ve come across walls more than 2 miles high.” The second picture in today’s post shows Saturn at a distance as seen by Cassini in 2016. The scale of the ring structures, from the tiny dust and ice particles to larger rocks, is easy to lose when you look at the rings from a distance. Closer inspection reveals complexities in the rings that are utterly breathtaking.
5- The Flight of Cassini over Iapetus on September 10th, 2007
Saturn’s moon Iapetus has striking surface coloration and a long equatorial ridge that runs like a seam across the moon. Iapetus was discovered by none other than Giovanni Cassini, the person that the Cassini spacecraft is named after.
The ridge on Iapetus is about 12 miles or 20 kilometers wide and stretches over 800 miles or 1,300 km across the equator. The peaks on this ridge reach higher than Everest, topping out at 6 miles high.
The Cassini spacecraft passed within 1,000 miles of Iapetus, which was a perfect opportunity to take images like the one in today’s post that show the ridge and contrasting surface features.
4- Saturn’s rings, a dynamic environment that serves as an example of how moons can form
Images captured in April of 2013 “show disturbances at the very edge of Saturn’s A ring- the outermost of the planet’s large, bright rings. One of these disturbances is an arc about 20 percent brighter than its surroundings, 750 miles (1,200 km) long and 6 miles (10 km) wide. Scientists believe the arc and protuberances are caused by the gravitational effects of a nearby object.”
Also of note are the “propellers” that Cassini imaged during its mission. According to NASA, “Propellers are disturbances in the ring caused by a central moonlet.” While the Cassini mission helped answer countless questions about Saturn, it also helped raise new ones. The more we study the Saturnian system, the more intriguing and exciting the questions and answers become.
3- Evidence of “hot-water chemistry” beyond Earth
Enceladus is an icy world where there’s more than meets the eye.
After “an extensive, four-year analysis of data from the spacecraft, computer simulations and laboratory experiments led researchers to the conclusion the tiny silica (SiO2) grains most likely form when hot water containing dissolved minerals from the moon’s rocky interior travels upward, coming into contact with cooler water. Temperatures required for the interactions that produce the tiny silica grains would be at least 194 degrees Fahrenheit (90 degrees Celsius).”
This type of activity is exciting because here on Earth, hydrothermal vents are hotbeds for exotic forms of sea life. Merely seeing hydrothermal vents is no indication there is life there, but it’s a place we could look at in more detail in future missions.
Pictured here are plumes of ice and water vapor as they escape through the icy crust of Enceladus’s south pole. According to NASA, Cassini helped “reveal complex organic molecules originating from Saturn’s icy moon Enceladus, strengthening the idea that this ocean world hosts conditions suitable for life.”
2- Ocean Worlds
In 2005 Cassini discovered water vapor, which was ejected from vents near the south pole of Enceladus. This led researchers to theorize the presence of reservoirs of water under the thick icy crust.
NASA JPL scientists continued to study the gravity of the moon, looking for variations that would provide clues into the composition of the materials in the moon. “The gravity measurements suggest a large, possibly regional, ocean about 6 miles (10 kilometers) deep, beneath an ice shell about 19 to 25 miles (30 to 40 kilometers) thick.
The subsurface ocean evidence supports the inclusion of Enceladus among the most likely places in our solar system to host microbial life. Before Cassini reached Saturn in July 2004, no version of that short list included this icy moon, barely 300 miles (500 kilometers) in diameter.”
As Cassini project scientist Linda Spilker put it in this article, “Material from Enceladus’ south polar jets contains salty water and organic molecules, the basic chemical ingredients for life.”
The mysteries of Enceladus will have to wait for the next mission to the Saturnian system.
On January 14th, 2005, the Huygens lander touched down on the surface of Titan, giving us an unprecedented look at this captivating moon. This was the first, and so far, the only time a spacecraft has touched down on a world in the outer solar system.
Titan is a massive moon, more massive than Mercury, and is the second largest moon in the solar system. According to NASA, “Titan is the only moon in our solar system that has clouds and a dense atmosphere, mostly nitrogen and methane. It is also the only other place in the solar system to have an Earth-like cycle of liquids flowing across its surface.” Much of what we now know about Titan is thanks to Cassini and Huygens.
The Huygens probe performed a 2.5-hour descent and touched down in a plain littered with what could be water ice rocks. Huygens gave us the first images of Titan’s surface as it passed through the dense upper clouds. Spacecraft can’t see through this thick layer in the visible spectrum, so Huygens was essential in giving us a look at the surface of the moon. Speaking of the surface of the moon, the temperature at Titan is frigid, in the neighborhood of -290 degrees Fahrenheit or -179 Celsius.
The Huygens lander was developed by the European Space Agency and was part of the overall Cassini Huygens mission between NASA and ESA. While the scientific knowledge we gained from the lander is incredible, I think the international collaboration and cooperation needed to pull off an interplanetary landing is just as important.
“There is no strife, no prejudice, no national conflict in outer space as yet. Its hazards are hostile to us all. Its conquest deserves the best of all mankind, and its opportunity for peaceful cooperation many never come again.” John F. Kennedy, Rice University Speech.
The Cassini Huygens mission represents the best of what we can achieve. Peaceful cooperation between sovereign nations working towards a common goal of expanding scientific knowledge. As much as the science will be part of Cassini’s legacy, the partnership between nations for this mission will be just as important fifty or one hundred years from now. The policy of cooperation on massive scientific undertakings like Cassini bodes well for future missions to other worlds.
Picture credits- NASA, ESA, and personal collection.