This Friday, Sept. 15, 20 years after its launch, NASA’s Cassini satellite will descend into Saturn’s atmosphere, eventually burning up to leave nothing behind. The mission produced a number of groundbreaking discoveries, many of which revolve around Saturn’s rings and moons. News Bureau physical sciences editor Lois Yoksoulian spoke with Illinois astronomy professor Leslie Looney about the significance of the Cassini mission.
What are some of the most important things we have learned from Cassini?
The Cassini spacecraft was the first mission to orbit Saturn and provide an in-depth study of Saturn, its rings and its many moons. Titan, Saturn’s largest moon, is the only one in the solar system to have an atmosphere. Even though the average temperature of Titan is -290 degrees Fahrenheit, Cassini found liquid on the surface in lakes, rivers and within the atmosphere. This liquid is not water, but rather mostly pure methane. Could life develop using liquid methane instead of liquid water? Maybe, but it would take much longer. So, Titan gives us a snapshot of how early life on Earth might have started out, but if Earth had been in a deep freeze.
The other fascinating moon is Enceladus, whose surface is mostly water ice without an atmosphere. It frequently spews water and water vapor from the surface in plumes, likely from heating deep inside the moon due to gravitational forces from Saturn. The existence of these plumes implies that Enceladus has an ocean under the icy crust. Even more exciting is the recent detection of hydrogen gas from the vapor plumes. It takes a lot of energy to form observed hydrogen gas from water molecules, so Enceladus produces a lot more heat than we previously thought. Life needs a liquid and an energy source, and Enceladus has both, making it a very appealing target for follow up observations.
Cassini was sent to answer many questions, but what new ones have been raised as a result of the mission?
Even in the two examples given above, it is clear that we have more questions concerning Saturn’s ring system than we did when we started. The details of the liquid cycle on Titan or the process and evolution of the plumes on Enceladus are significant questions that invite further study. With each closer look at Saturn's moons, we realize that there are niches in which life could flourish, but we need to explore more deeply to understand the likelihood. Titan and Enceladus alone are reasons for a return trip, not even mentioning some of the large questions on the origin, age and expected evolution of Saturn’s rings.
Many people may not realize that Cassini’s fate was planned and not the result of a malfunction. Why is it important to crash satellites into planets?
We are trying to be conscientious solar system citizens. If Titan or Enceladus have a chance of native life, we need to keep our spacecraft far away because there is a chance that viable Earth bacteria could hitchhike their way into Saturn’s orbit aboard our satellites. If a contaminated spacecraft crashed into Saturn, we may be inadvertently destroying what we are trying to seek out. You’d think that we would be able to sterilize spacecraft, but it is challenging and costly.
Now that Cassini is out of fuel, NASA has arranged an elaborate orbit that will have it fall into Saturn and the dense atmosphere will destroy the satellite. It is sad, but this death spiral will also allow us to collect data during the descent, transmitting details that may raise even more exciting questions. At the same time, we may be saving life we don’t even know about yet.