Richard Greenberg, a professor of planetary sciences at the University of Arizona's Lunar and Planetary Laboratory, is an expert in celestial mechanics and carries out NASA-sponsored investigations of solar system evolution and planet formation. He is also author of the current book Unmasking Europa.
After NASA and the European Space Agency chose Jupiter’s moons — including Europa — as the next destination for a major planetary exploration mission, Greenberg answered a few questions about the upcoming missions and his book about Europa, “an active oceanic world that just might be the most likely site for our first encounter with extraterrestrial life.”
Pendick: Are you surprised at all by NASA and ESA’s decision to go to Jupiter’s moons before going to Saturn’s moon Titan?
Greenberg: I think both proposed missions are exciting and both Europa and Titan are fantastic satellites to explore. As far as the competition for the next flagship in line, it is not a surprise that Europa was selected. It has been a top priority since the Galileo mission provided evidence for the global ocean. And the prospect for it being the place where we will most likely first find extraterrestrial life makes it an objective of almost existential significance!
That said, the discoveries about the Saturn system from the Cassini mission have shown us what active and exciting places Enceladus and Titan are. And the Titan Science Definition Team did a great job of developing a model mission that is really imaginative and spectacular.
In many ways the Titan mission is at least as appealing as the Europa one. So it is interesting that NASA did not simply make an either/or decision between the two missions — as was originally expected — but instead they made the announcement a prioritization, which leaves the door open to including the Titan mission as future budgetary and policy issues are addressed.
While I personally love Europa, for all the reasons I describe in my book Unmasking Europa, I am a big fan of the Titan mission as well.
Pendick: If you had to choose a few major points, what would you say the Europa orbiter will most likely tell us — and not tell us? For example, will the mission confirm the existence of Europa’s global ocean and perhaps how deep it is, but NOT tell us whether anything lives in it?
Greenberg: I personally believe that we already confirmed the existence of the ocean with our work on explaining the cycloidal crack patterns in terms of tidal stress and also with the Galileo magnetometer evidence. The new mission will have a laser altimeter to measure the height of Europa’s tides, and the radio tracking will also measure the gravitational effect of the tides. That information about the tides will further confirm the existence of the ocean. Contrary to some reports I have read, though, I do not believe the radio and altimeter information about the tides will tell us how thick the ice is.
There will also be an ice-penetrating radar that will tell us a lot — I hope — about the structure within the icy crust, which will help us understand the dynamics of the crust and the transport of materials between the ocean and the surface, which is critical for life, as I emphasize in my book.
The spacecraft will have a narrow-angle camera for super high-resolution imaging of Europa’s surface, in addition to wider-angle cameras. I hope the folks who plan the imaging will favor the narrow-angle camera, so we can really get a handle on the active processes that must be continually resurfacing Europa.
Other remote sensing will include an infrared spectrometer to identify the critical substances mixed in with the ice and a thermal instrument to map the heat distribution.
Naturally, if I were the king of NASA, I would order a lander to plop down selectively on a site where ocean material has recently been exposed. I could imagine finding marine organisms without needing to drill down very deep. But that is evidently something saved for a distant future mission.
Pendick: Too bad we all have to wait until 2026 to find out if you are right about Europa! But that leaves 17 years for us to read all about it in your book, Unmasking Europa. Tell us why you wrote the book and what we can learn about Europa by reading it.
Greenberg: I wanted to write this book because I knew several great parallel stories that would be fun to tell. The first was a fantastic science story about a satellite with an ocean containing twice the liquid water of all Earth’s oceans and an icy crust that is continually renewed by tectonic and thermal processing that links the ocean to the surface. Most of this information was in fragmentary form in the scientific literature, so I wanted to integrate it and make it accessible to any thoughtful reader.
Also, there had been a great deal of speculation about life perhaps being possible at imaginary volcanic vents on the ocean’s floor, but no one else was talking or writing about how the openings through the ice could really enhance the prospects for life in the ocean and that the biosphere of Europa, if any, would well extend right up to the surface.
There are also policy implications that had been under-appreciated. We may not need to drill down through the ice to reach life. It could lie very near the surface. That is the good news for exploration. The bad news, of course, is that we need to be very careful to avoid contaminating such a vulnerable place.
I also wanted to tell the story of how my own interdisciplinary research group had come to assemble this picture of a permeable ice layer by interpreting images of the surface in terms of our understanding of tides, and in the context of the wonderful scientific contributions of many other colleagues from other places.
And finally I wanted to tell the story of what I learned about the place of science in the framework of a large space mission. I wanted people to get an idea of how science really works. Like any creative human endeavor, it has its glorious aspects and its ugly ones. But it was always interesting and often hilarious. I just had to tell about it. These several story lines were interwoven and seemed to me inseparable, so that is how I wrote the book.