Kevin Hand zoomed out and put planet Earth in cosmic context to tackle one of the universe’s oldest and most profound questions: Is anybody out there?
Hand, astrobiologist and National Geographic Emerging Explorer, addressed that question at 10:45 a.m. Thursday in the Amphitheater, continuing Week Three, “A Planet in Balance: A Week in Partnership with National Geographic Society.”
Hand found his place in the stars as a child. While growing up under the constellations in the Vermont sky, he realized the search for life beyond Earth is guided by the understanding of life on Earth.
“Planet Earth is teeming with life,” Hand said. “North, south, east, west, high, low, hot, cold — wherever you find liquid water on planet Earth, you generally find life.”
Various space missions have given scientists good reason to predict that vast, global liquid-water oceans exist beyond Earth. Those oceans are trapped beneath icy shells of moons orbiting planets like Jupiter, Venus and Saturn, and are changing the idea of what it takes for a world to be habitable.
In the early days of astronomy and planetary science, the idea was that in order for a planet to be habitable, it had to be at the right distance from its parent star for liquid water and oceans to exist on the surface.
“If you are too close, like Venus, you were too hot and you boiled off any water that you once had,” Hand said. “If you are too far away, like Mars, you were cold and you froze out or lost any water to space. There was this kind of Goldilocks scenario: You had to be just the right sun- and Earth-distance, so as to have a liquid ocean on the surface that could sustain life.”
Hand calls that scenario the “old Goldilocks.” The “new Goldilocks” describes habitability in terms of tidal energy, or the tug and pull that moons experience as they orbit planets.
The best example of the “tug and pull” can be seen on the moons of Jupiter: Io, Europa, Ganymede and Callisto. Liquid water is not a possibility on Callisto and Io, but Hand believes Europa has the perfect combination of tidal energy dissipation to sustain life and a surface-level ocean.
“Again, one of the key aspects of these oceans is that by merit of having liquid water, we think that they could harbor life,” Hand said. “If we have learned anything from life on Earth, it’s that where you find liquid water, you generally find life.”
However, the diversity of life on Earth depends on both liquid water and biochemistry.
“All life on Earth is connected by the same tree of life,” Hand said. “What I’m curious about is whether or not there are other trees of life, separate origins of life on worlds beyond Earth, worlds like Europa, worlds like these ocean worlds or possibly even on planets beyond our solar system. Is the origin of life easy or hard? Does life arise wherever the conditions are right? Do we live in a universe that is teeming with life?”
Hand said life itself has three components: liquid water; periodic elements; and energy, the most important of the three.
“Even though (some moons) have liquid water, they don’t necessarily have sunlight that can help power the food chain,” he said. “When we look around the surface of our planet, the energy from the sun not only contains liquid water, it also powers photosynthesis, helping to serve as the base of the food chain.”
To prove that oceans do exist on other planets, Hand referenced evidence from Enceladus, one of Saturn’s moons. Using pictures from NASA’s Cassini-Huygens spacecraft, Hand discovered that the north side of the moon’s icy shell was covered with craters — a sign of impact and age on a moon’s surface.
Alternatively, there are cracks instead of craters on the south side of the shell. Scientists discovered that water was jetting out and fracturing the ice.
“The ice of Enceladus is maybe 10 or so miles in thickness, but the tidal tug and pull that Enceladus feels as it orbits Saturn causes that ocean to be contained,” he said. “But it also fractures the ice shell and those cracks allow water to seep on up and essentially boil off into space and jet into space.”
Europa has no signs of liquid water due to its average temperature of minus 280 degrees Fahrenheit, but there is evidence of salt and a fractured ice shell. Scientists believe Europa, similar to Earth, has an iron core and a rocky mantle, but the ocean underneath is estimated to be 60 miles deep.
“It’s a global ocean and if you do the math, it turns out that the volume of liquid water within Europa’s ocean is about two to three times the volume of all the liquid water found in Earth’s oceans,” he said.
Since Hand is unable to explore extreme conditions on moons throughout the universe, he looks to Earth’s extreme conditions in northern Alaska. For more than a decade, Hand and a team of scientists have studied microbe survival and the methane gas that is seeping out of Alaska’s permafrost.
“In the summer, the lakes are open to the atmosphere and photosynthesis can occur, but during the winter, the lakes freeze over and the sun goes away because it’s too far north,” he said. “The microbial ecology takes over, and the microbes that are generating methane start to do their job.”
Hand only studies in Alaska for a few days each fall and spring, but he is working with a team of engineers to create a robot that can stay in the water all winter. Hand is working on a submersible rover that would roll upside down under the ice for increased mobility.
Exploring Alaska helped Hand imagine conditions on Europa, but the water was not deep or representative enough to make any direct comparisons. To experiment with another extreme environment, Hand explored hydrothermal vents, or hot springs, at the bottom of the ocean.
“What was astonishing was back in 1977, when these were first discovered, geologists were anticipating finding active regions that were perhaps chemically interesting, but they did not expect to find the biology that was there,” he said.
Around the vents, scientists found microbes that were feeding off the chemical energy and minerals from the hot water.
“It’s super heated, but the chemically rich composition of it allows microbes to eat it and then the crabs, the shrimp and the other creatures that we see are able to use those microbes as the base of the food chain,” Hand said.
To understand the pressure of Europa’s ocean, Hand attended a National Geographic trip to the Challenger Deep, the deepest known point in the Earth’s seabed hydrosphere. In addition to a human-occupied submersible, the team released robots that could remain underwater for long periods of time, meaning the team could attach bait to the robots and see what they attracted.
“In the deepest, darkest, most extreme environment in our planet’s ocean, we see life not just seeking out a living, we see life thriving,” he said. “What you are seeing are hundreds to thousands of little shrimp-like creatures called arthropods that came out of the darkness to feed on a fish head in a trap that we set up.”
The exploration of Europa is Hand’s “dream of dreams” mission. Once a spacecraft enters the ice-ocean interface and makes contact with life, Hand said the understanding of life beyond Earth will “change forever.”
Hand closed his lecture by sharing a 400-year-old sketch by Galileo, what he calls his “favorite image of the universe.”
At the center of the sketch is Jupiter.
“Galileo turned his telescope to the night sky, pointed it at Jupiter and he saw not just Jupiter, but these four little points of light around Jupiter,” he said. “Those four little points of light, he initially thought were just stars.”
Galileo quickly realized the lights couldn’t be stars because their positions constantly changed. According to Hand, by discovering the moons of Jupiter, Galileo helped put the “final nail in the coffin of Aristotelian cosmology.”
“With the idea that the Earth is at the center of the universe and everything revolves around the Earth, (Galileo) really opened the doorways for the Copernican Revolution, which set the stage for the Earth going around the sun, our sun being a star, the stars that we see being suns in their own right and potentially being host to planets of their own,” he said.
In the decades that followed Galileo’s lifetime, Hand said humans would come to appreciate that the laws of physics, geology and chemistry work beyond Earth. The role of the fourth major science on other planets — biology — is still unknown.
“We don’t yet know whether the science of us — whether life — exists beyond Earth,” Hand said. “Does biology work beyond Earth, or is life on Earth the only singularity for biology in this universe?”
According to Hand, there is no better time than now to add biology to the list.
“We can send out the robotic spacecraft, do the experiments, search for signs of life and see whether or not we are alone,” he said. “In doing so, we can potentially bring the universe to life.”