Cassini 1997-2017: Voyage of endless discovery

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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.

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.

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.

On Saturn

Earth and Mars share similarities — both are terrestrial planets made of silicates, both have surface water. But Saturn is completely different.
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 than Earth in volume, its density is one-eighth that of Earth.
There are 62 Moons on Saturn. The largest of those, Titan, is larger than the planet Mercury.
The storms on Saturn are about 10 times stronger at 1,200 miles/hr, and last from years to decades.
The storms on Saturn sometimes produce lightning every 1/10th of a second.
A year on Saturn is 29 Earth years!

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.

Hope for life in mysterious ocean

  • 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 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.

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