From our earliest history, humans have contemplated the cosmos. Before we had an inkling of the nature of our own solar system, we wondered at the composition of our sister planets. And long before we knew there were planets orbiting other stars, we wondered if we, earth-bound beings, were alone in the universe.
This need to understand the nature of the universe and our place in it continues to fuel modern astronomy and space exploration, says Jonathan Lunine, Astronomy. Lunine is a planetary scientist, who studies how planets form and evolve, especially the giant planets such as Jupiter and Saturn and their diverse moons.
“I look at how planets evolve in two ways,” says Lunine, “through theoretical modeling and through participation in spacecraft missions. There are so many important, amazing spacecraft missions happening right now! When I first became a planetary scientist, I never thought I’d be working on so many missions at once.”
The Cassini-Huygens Mission to Saturn and its Moons
Lunine’s favorite mission at the moment is the Cassini-Huygens Mission to Saturn, which was first launched in 2004. Huygens was the European Space Agency probe that detached from Cassini and landed on Saturn’s moon Titan in 2005. It made a variety of scientific tests and observations, as well as taking astonishing pictures of the terrain. Cassini meanwhile, has been in orbit around Saturn for 11 years. During that time, the spacecraft has analyzed the composition of Saturn’s rings and engaged in multiple flybys of a number of Saturn’s moons, including Titan and Enceladus, which Cassini discovered has plumes of ice and gas venting from its interior. “These are remarkable findings,” says Lunine. “Cassini was supposed to last four years. We’re now on year 12, and we have two more to go before the mission ends.”
From the beginning, Lunine has been intimately involved in the mission as an interdisciplinary scientist (IDS) with responsibility for studying Titan’s surface and atmosphere. “It’s been really satisfying,” he says. “Because I’m an IDS, rather than being on one particular instrument for Cassini, I’ve been able to work with many different instruments and with many different instrument teams. All of us together on Cassini have become so interactive and fluid that we can cross over to other areas. I’ve been able to work on Enceladus’s gravity and composition as well as working on Titan.”
Saturn’s Moons, Titan and Enceladus
In particular, Lunine worked with Luciano Iess from La Sapienza University of Rome and Iess’s Italian colleagues to measure the distribution of mass on Enceladus. The scientists were able to infer the presence of a subterranean ocean on Enceladus by measuring density differences. The presence of the ocean has vaulted the moon to the top of the list of possible locations for extra-terrestrial life.
Lunine’s interest in Titan predates Cassini, going all the way back to his graduate student days when he co-wrote a paper with his advisor at Cal Tech, David Stevenson and Yuk L. Yung, inferring the presence of a global ocean of methane and ethane on Titan. The paper was published in 1983 and brought Lunine to the attention of European and American scientists who had been commissioned by the United States National Academy of Sciences and the European Science Foundation to look for new missions in the outer solar system. Soon Lunine was involved in the preparatory plans for the Cassini-Huygens Mission.
“We thought there was enough methane on the surface of Titan to supply it for the whole age of the solar system,” Lunine says of that long-ago paper. “What Cassini actually found are lakes and oceans that hold hundreds of times more than the known hydrocarbon reserves on Earth, but that’s still a lot less than we predicted. I suspect it’s there, though, so the rest has got to be underground.”
The Juno Mission to Jupiter
Lunine is also interested in Jupiter and is a co-investigator on the Juno Mission to Jupiter, which was launched in 2011 and should arrive in 2016. “We want to learn about Jupiter’s interior,” he says. “We want to find out if it has a solid core of rock and ice. We know it won’t be in the form of rock and ice because of the immense pressure, but we want to know if it started out that way.”
Lunine will participate in the search for life through the Europa Multiple Flyby Mission to Jupiter’s moon Europa, aiming for a launch in the early 2020s.
Juno will obtain the information Lunine and other scientists need to understand how large planets form. The mission will also allow them to gather another piece of crucial information: the abundance of oxygen on Jupiter. “Juno is carrying a microwave radiometer that will measure by remote sensing the oxygen abundance very deep in the planet,” says Lunine. “With this mission, we’ll get inside Jupiter; we’ll have a kind of tomography of the planet that will allow us to see the structure and the abundances of elements.”
The information from Juno will be combined with information from Cassini, which in the future will slip between Saturn and its rings and take a close orbit around the planet. This will allow Lunine and the other scientists to determine the internal structure of Saturn by measuring the planet’s gravity and its magnetic fields. The combined information will give a “ground truth” for giant planets around other stars, Lunine says. “We will be able to use our solar system as a kind of Rosetta Stone for these other giant planet systems.”
The Search for Extra-Terrestrial Life
Lunine’s other great interest lies in searching for extra-terrestrial life. He shares this passion with other students, postdocs, and faculty members of Cornell’s Carl Sagan Institute, founded in 2015 and directed by his colleague Lisa Kaltenegger. “The Carl Sagan Institute provides an interdisciplinary venue at Cornell for combining results from exploration of the solar system with the mountain of new data on planets around other stars” Lunine says.
Lunine will participate in the search for life through the Europa Multiple Flyby Mission to Jupiter’s moon Europa, aiming for a launch in the early 2020s. He is a co-investigator on the near-infrared spectrometer “MISE,” which will measure the composition of organics, salts, hydrates, and other materials on the surface of Europa. This may tell him whether conditions are right for life. In addition, he is currently co-chair of the habitability working group, which evaluates all the instruments that are part of the Europa mission to decide what strategy to use in determining if Europa’s ocean is habitable.
And what if the Europa Mission finds evidence of extra-terrestrial life? That life is likely to be simple microbes, Lunine says. While that may not sound glamorous, he is quick to point out that even primitive unicellular life can tell us something profound about the cosmos. “If we find extra-terrestrial life, we want to determine if it has a separate origin from life on Earth,” he says. “We have learned from studying meteorites and doing computer models that planets exchange material through impacts of large asteroids and comets. We know, for instance, that Earth and Mars have exchanged materials. You might imagine that it would be a pretty rough trip to be slammed by an asteroid moving at speeds of tens of kilometers per second, but amazingly bacteria can survive the initial impact. They can also survive the entry into the atmosphere of another planet or moon if they are encased inside a large enough chunk of rock. It’s highly unlikely, though, that if life were present in Europa or Enceladus, it would have come from Earth in this way.”
Determining that life arose independently on another planet or moon would be a game changer for humanity. “If life had a second independent genesis in our own solar system, then that would tell you that life is a common cosmic phenomenon,” Lunine says, “that wherever conditions are right, life will form.”
If we know that, then we will know we are not alone in the universe.