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Monday, January 24, 2022

Cassini 1997-2017: Voyage of endless discovery, hope for life in mysterious ocean over a billion km away

Around 5.30 pm IST today, NASA’s Cassini spacecraft will be crushed by the pressure and temperature of the final embrace of the planet it has romanced since arriving in the Saturnian world 13 yrs ago. Mankind’s giant interplanetary leap has opened the doors for some of astronomy’s greatest discoveries.

Written by Amitabha Ghosh |
Updated: September 15, 2017 5:35:59 pm
cassini, nasa, saturn, Cassini Huygens,Mars Pathfinder, hurricane irma, flybys, science news Saturn, as seen by Cassini (NASA)

In 1997, NASA celebrated its return to the Red Planet with a perfect landing of the Mars Pathfinder Mission. Mars Pathfinder was a very frugal mission, costing less than Waterworld, a Kevin Costner film from two years earlier. But a much bigger mission, more than 10 times as expensive and in development for about a decade, was also being launched in 1997 — without much press or public fanfare.

Cassini Huygens, a NASA flagship mission in collaboration with the European Space Agency, was headed to Saturn — to study the planet and its moons. Several flybys by Voyager-1 and Voyager-2, and by Pioneer, had returned enough interesting data to convince a NASA-led consortium of space agencies that the time for a major mission to Saturn had arrived. Because Saturn is so far away, the mission would be powered by a plutonium-fuelled radioactive thermal generator. The electric power needed for the mission would be less than that needed for a room airconditioner — and equal to eight 100-Watt light bulbs. Cassini would carry the Huygens probe that would attempt to land on one of the Solar System’s most interesting and Earthlike moons, Titan.

Cassini took the scenic route to Saturn — it flew by Venus, Earth and Jupiter, travelling 2 billion miles to reach its destination in seven years. By the time Cassini was ready for orbital insertion around Saturn, NASA’s next generation of Mars rovers, Spirit and Opportunity, were already on the ground.


Earth and Mars share similarities — both are terrestrial planets made of silicates, both have surface water. But Saturn is completely different.

Imagine landing on Saturn — there is no solid surface to walk on. Saturn has a metallic core, overlain with metallic and liquid hydrogen. It is made up of gas, and could theoretically float on water — Saturn’s density is 30% lower than that of water. Although the planet is about 700 times larger that Earth in volume, its density is one-eighth that of Earth.

Bored of watching just one Moon on Earth? There are 62 on Saturn. The largest of those, Titan, is larger than the planet Mercury.

If you thought Hurricane Irma was bad, think about this: the storms on Saturn are about 10 times stronger at 1,200 miles/hr, and last from years to decades.

Scared of lightning on Earth? The storms on Saturn sometimes produce lightning every 1/10th of a second.
Feeling old on Earth? A year on Saturn is 29 Earth years!


So, what were Cassini’s science achievements over its 14-year mission? The spacecraft completed about 300 orbits around Saturn, including more than 150 flybys of its moons; it discovered six new moons and two underground oceans on different moons. Like on Earth, there are four seasons on Saturn; Cassini was able to sample three of those.

The Huygens probe carried a slew of instruments, was designed to enter and survive the approximately 3-hour descent through the atmosphere of Saturn, and to also survive an ocean landing. It landed successfully on Titan, the farthest body by far on which NASA has landed a spacecraft.

Like that of Earth, Titan’s atmosphere is primarily nitrogen, with traces of methane. Because Earth’s methane has biogenic sources, the question is whether the methane on Titan is related to life. The Huygens probe revealed a fascinating world on Titan — a temperature about –180C, or colder than Antarctica, with atmospheric pressure about 50% higher than on the Earth’s surface. The methane is more near the surface, and there is probably a near-surface liquid methane source.

A picture of a volcano on Earth conjures images of hot volcanic magma and gases; imagine a cold volcano. The Cassini-Huygens probe found strong evidence of cryovolcanism — a volcano where water ice and a mix of hydrocarbons is spewed into Titan’s thick atmosphere. In fact, cryovolcanism is now thought to be fairly common in the satellites of the outer planets.


Where in the Solar System would you find life? This may perhaps be Cassini’s greatest discovery. Data from the spacecraft indicate there is a large ocean, deeper than the Pacific Ocean, under the icy crust of Saturn’s moon Enceladus. At places, the crust is just about a mile thick. What’s more is that there is likely a heat source in the interior of Enceladus, which causes the water from the ocean to erupt on the surface. The ocean on Enceladus is dark, and eight times deeper than the average depth of the Pacific Ocean.

This ecological setting is eerily similar to Earth — and on Earth, such a habitat supports life. Most Earth oceans are dark — the aphotic zone where no sunlight percolates starts merely a kilometre from the surface, whereas the oceans can be as deep as 11 km at places, like at the Mariana Trench. On Earth, there are heat sources that cause volcanic or hydrothermal activity on the ocean floor. These hydrothermal vents were once thought to be inhospitable to life: what organism would, after all, live in ice cold waters, in pitch darkness, in the high pressure of the ocean floor, in sulphur-rich acidic conditions? However, recent research funded by NASA has shown that life flourishes in the underwater volcanic vents. Since there is an absence of light, some of the microscopic organisms are chemoautotrophic, which means they derive energy from chemical reactions — in contrast to organisms on the Earth’s surface, which are photoautotrophic (deriving energy from light).

***The billion-dollar question, then, is: if life is found in hydrothermal vents under the oceans on Earth, does life lurk in the dark depths of the massive underground ocean on Enceladus? If yes, what are the attributes of these life forms? Are they microscopic? Are they chemoautotrophic? Hopefully, a follow-up mission to Cassini in the coming decades will provide the answers.


Oct 15, 1997, Liftoff: Titan IVB/Centaur lifted off with the Cassini orbiter and the ESA’s Huygens probe on board.

Apr 25, 1998, First Venus flyby: Cassini-Huygens performed flyby of Venus, going within 176 miles (284 km) of the Venusian surface. Gravity assist accelerated the spacecraft by about 7 km/s, to help it reach Saturn

Jun 24, 1999, Second Venus flyby: After another trip around the Sun, Cassini-Huygens performed a second flyby of Venus for another gravity assist, this time coming within 600 km of the planet

Aug 17, 1999, Earth-Moon flyby: Nearly 2 years after launch, Cassini-Huygens flew within 1,100 km of Earth, gathering a 5.5 km/s speed boost

Dec 1999-Apr 2000, Through the asteroid belt: Cassini-Huygens became the seventh spacecraft ever to negotiate the belt

Dec 29, 2000, Exploring Jupiter: Cassini-Huygens made its closest approach to Jupiter at 10 million km on Dec 30, providing, along with the Galileo spacecraft already orbiting Jupiter, unique insight into the Jovian system

Oct 31, 2002, Camera tested OK: Cassini captured an image of Saturn during a camera test 20 months before reaching the planet. It was then 285 million km from Saturn, nearly twice the distance between the Earth and Sun

Apr 7, 2004, Watching Saturnian storms: Three months from arriving at Saturn, Cassini observed two storms merging into a larger storm — only the second time this phenomenon had been observed on Saturn

May 31, 2004, Two new moons: Cassini discovered two previously unknown moons — Methone and Pallene, 3 and 5 km in diameter, taking the number of known moons of Saturn to 60. This was to increase

Jun 10, 2004, First up, Phoebe: Phoebe was the first of Cassini’s many moon flybys. The spacecraft flew within 2,000 km of the dark moon, 1,000 times closer than Voyager-2, which got to within 2.2 million km of Phoebe in 1981

Jun 30, 2004, Saturn orbit insertion: Cassini, still carrying the Huygens probe, became the first spacecraft to orbit Saturn

Oct 24, 2004, First encounter with Titan: Spacecraft beamed back information and pictures after successfully skimming the hazy atmosphere of Saturn’s moon Titan from 1,200 km above Titan’s surface

Dec 23, 2004, Huygens probe detaches: The Huygens probe detached from the Cassini orbiter to begin a three-week journey to Titan

Jan 13, 2005, Descent to Titan: Huygens landed on Titan — the first and only landing on any world in the outer solar system. The descent lasted two hours and 27 minutes, and the battery-powered probe lived for another 72 minutes on Titan’s surface, and transmitted spectacular images that revealed meteorology and geology that was extraordinarily like the Earth’s

Mar 8, 2006, Liquid water on a moon: Scientists announced evidence of liquid water reservoirs feeding a huge cloud of vapour over the south polar regions of Enceladus, a small moon of Saturn. Earlier in February, Cassini had discovered something like an atmosphere on Enceladus.

May 31, 2008, Primary mission over: In four years at Saturn, Cassini revealed a wealth of new knowledge about the planet, its rings, and its moons

Feb 2, 2010, Mission extended to 2017: The extension enabled Cassini to observe seasonal changes in the Saturn system over almost half of the planet’s 30-year orbit around the Sun

Jun 21, 2011, Enceladus’s hidden ocean: Cassini found more evidence for a largescale saltwater reservoir beneath Enceladus’s icy crust

Mar 1, 2012, Hint of fresh air at Dione: Cassini “sniffed” molecular oxygen ions around Saturn’s moon Dione. The ions are sparse — one every 11 cubic cm

Mar 5, 2014, 100th Titan flyby: Each flyby provided a little more knowledge of Titan and its striking similarities with Earth. Titan is really early Earth in deep freeze

Jul 27, 2014, 101 geysers on Enceladus: 101 distinct geysers suggested it is possible for liquid water to reach from the moon’s underground sea all the way to the surface

Apr 12, 2017, Energy for life? Scientists announced the indication of hydrogen gas, which could potentially provide a chemical energy source for life, in the subsurface ocean of Enceladus

Apr 23, 2017, Final Titan flyby: Cassini passed 979 km above Titan’s surface on its 127th and final approach

Apr 26, 2017, Grand Finale Began: Cassini signalled the end of its historic mission with 22 daring loops through the gap between Saturn and its innermost ring, coming closer than ever before, and exploring a whole new Saturnian world in the process

Sept 15, 2017, The Grand Finale: A month shy of 20 years in space, Cassini will make its final plunge into Saturn’s atmosphere. It will be crushed and vapourised by the pressure and temperature of Saturn’s final embrace.

Dr Amitabha Ghosh is Chair, Science Operations Working Group, Mission Operations, NASA Mars Exploration Rover Mission.

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