Europa Diary IV: Walking on Thin Ice

Europa Diary IV:

Walking on Thin Ice


Europa, false color.
Jupiter’s moon Europa.Credit: NASA

The Europa Focus Group is a collaboration of scientists who study Jupiter’s moon, Europa. This ice-covered world may be one of the few places in our solar system other than Earth that has a water ocean, and liquid water is believed to be one of the key factors in the development of life. Astrobiologists and other scientists eager to learn more about Europa recently headed to Alaska’s North Slope.

The scientists studied the region’s unique terrain, providing insight for future missions to the icy landscape of Europa. Flying in small aircraft to study geographical features, driving snowmobiles over glacial terrain, digging bore holes to get a glimpse of ice history — all the activities pursued by these hardy adventurers may someday be duplicated on the surface of Europa by robotic spacecraft.

Matt Pruis , a support scientist with NorthWest Research Associates in Seattle, Washington, attended the North Slope conference and kept a journal of the events.

Saturday, April 26, 2003

Today is the final day of the Europa Focus Group’s Arctic Ice Field Conference. We met this morning to discuss a gauntlet of exciting and controversial topics, such as “Snowball Earth:” the theory that several times in the history of our planet, the Earth was enshrouded in sheets of ice from the equator to the poles.

But Europa’s ice shell was, of course, the main focus of the morning. In trying to unravel the history of Europa’s icy features, various interpretations have been suggested. One interpretation is that there may be convection of “warm” ice within a thick Europan ice shell. According to this theory, slightly warmer ice moves up like an elevator through colder ice.

Model of Europa's Subsurface Structure
Two models consistent with images of Europa’s surface include a subsurface layer of liquid water or perhaps warmer, convecting ice.

Such convection might allow any microbes from the deep ocean or warmer ice below to travel up to the surface of the moon, where there is sunlight and possible organic nutrients. The ice surface gets intensely irradiated because of Europa’s proximity to Jupiter’s magnetic field, and this relentless radiation bombardment may produce organic and other oxidant molecules at a sufficient rate to provide an energy source for a Europan biosphere.

Although we don’t currently understand the process, this solar-derived organic “soup” on Europa’s harsh surface may get drawn back down into the ice below, potentially ending up in the deep ocean. This proposed cycling of Europa’s icy crust may mean that the signatures of life can be found everywhere on Europa. Or, conversely, they may be concentrated into very small regions, making them difficult to detect (unless you know where to look).

After the morning’s thought-provoking conjectures, we spent the afternoon at the Iñupiat Heritage Center. There we were greeted by Kenneth Toovak, an Iñupiat Elder and a board member of the Barrow Arctic Science Consortium (BASC). Kenneth has been helping researchers study the ice for decades; he has a deep understanding of the polar environment and a wealth of personal experience.

As we look toward studying the icy surfaces of other worlds, Kenneth urged us not to forget to monitor the conditions of our own planet. He’s concerned that the climate on the North Slope is changing, and he wonders what those changes will mean for his people. Changing habitats could affect the seals, walruses and whales on which they depend, and changes in the snow pack and erosion of the coastline would affect their homes and hunting camps.

He related several stories of how, in the last couple of years, there has been less anchor ice to keep the ice floes from drifting out to sea. The whale hunters use the ice edge for a launching platform on their expeditions. A couple of years ago, 140 people were trapped on a floe that detached from shore, and they had to be rescued by helicopter.

Kenneth told us an amazing story of a young man who was caught on the wrong side of one of these detachments in the 1930s or 40s. After a cold night on the ice floe, floating continually further from shore, a thin (less than 10 centimeter thick) skim of ice formed, extending towards the shore. The young man had a stick for testing ice, and he found that the ice was too thin for safe passage. However, he heard a voice urging him to go forward. For the next 24 hours the voice kept urging him, and he managed to keep moving over this thin ice. Finally, the next morning he reached the near-shore ice and safety.

I could imagine the young man making this trip, since in this morning’s discussions Hajo Eicken had related a story of seeing polar bears moving over thin ice. Thin ice is somewhat elastic, so if you keep moving and redistributing your weight, it may be able to support you during your journey.

Polar bears move across this thin ice on their broad, snowshoe-like paws. When the ice gets thinner still, the bear will resort to crawling on its knees and elbows to make it across. But if the bear stops its motion, the ice fractures and the polar bear gets to swim. This is not such a worry for the polar bear, with its thick fur and layer of insulating fat, but it would have been the end of the young man who performed the long walk.

In some ways, using Earth’s ice fields as an analog of Europa is like “walking on thin ice.” We take the risk of assuming that the features we see on Europa have a similar formation history as features on Earth. But sometimes different features, while seemingly quite similar in structure, have vastly diverse histories. As we move forward in our quest to understand Europa’s icy realm, new information will generate new theories. We will shift our stance if our position becomes untenable. That’s the nature of scientific progress.

In thinking over the events of the last few days, I now appreciate how different the Arctic Ocean actually is from the surface of Europa. The Arctic ice pack is 1 to 5 meters thick. It has extremely high gradients of temperature, from the frigid Arctic air above to the comparatively warm upper ocean below. The ice weakens, cracks and is deformed by interaction with the ocean and air currents.

It is generally believed that the ice shell on Europa is significantly thicker than this – that it is perhaps 20 to 100 kilometers thick. With thick ice, processes within the Europan ice shell become important. For instance, this additional thickness would allow the ice to support “loads” such as ice ridges or ice mountains many times larger than those seen on Earth.

Ice Probe
Missions beyond Europa orbiters, like a probe to drill into the Europan oceans, may not have to go far into the ice to find evidence of life.

Yet we don’t see such large loads on Europa. Europa only has about one-fourth the gravity of Earth, so features on Europa could be very large. The planet Mars, for instance, has one-third the gravity of Earth, and volcanoes on Mars reach immense sizes. The largest volcano on Mars – Olympus Mons – is 24 kilometers high (higher than three Mount Everests) and 550 kilometers in diameter (about the size of Arizona). Such large volcanoes can exist on Mars both because of the low gravity and the lack of surface tectonic motion.

Since Europa has even less gravity than Mars, it is quite a quandary why the ice features on Europa are only a couple hundred meters tall. Perhaps the low relief of the features is due to the ice relaxing over long geologic time scales.

This is just one instance of the limitations we face in trying to interpret what we see on Europa. Even if we incorporate everything we know about Earth’s ice features in our Europa models, for true understanding we will need to include data that goes beyond our own planet.

By constantly moving forward in our search for knowledge, by continuing in our quest for exploration, we hope to move past the “thin ice” of Earth-Europa conjecture. We’ll have to wait for a spacecraft to visit the icy moon to get some final answers, and that won’t happen until 2011 at the earliest. Our journey will be long and perilous, but just imagine how thrilled we’ll be when we finally reach the far-off shore of Europa!


Support for the conference was provided by the NASA Astrobiology Institute. Dr. Ronald Greeley (Arizona State University) organized the meeting; Dr. Hajo Eicken (University of Alaska) organized and led the field excursions. The Ukpeagvik Inupiat Corporation allowed the use of their meeting facilities and provided access to key field areas.

Matthew Pruis is a third-year graduate student in Marine Geology and Geophysics at the University of Washington’s School of Oceanography. He received his Bachelor of Science in Applied Geophysics from the Michigan Technological University. His graduate advisor is H. Paul Johnson . In addition to his graduate work, Matt also works at NorthWest Research Associates, Inc. as a research scientist. In this multi-part Europa Diary series, Matt Pruis chronicled his impressions for the Astrobiology Magazine, as part of his participation in the Europa Focus Group’s recent journey to Alaska’s North Slope.

Related Web Pages

Europa Diary I: Landing on Alien Terrain
Europa Diary II: Life on Ice
Europa Diary III: On Polar Bear Time
Europa Diary IV: Walking on Thin Ice
Alaska Europan Photo Gallery (Credit: Matt Pruis/ Jere Lipps )
Ice on Europa
Iñupiat native people
Barrow Arctic Science Consortium
Ukpeagvik Inupiat Corporation
Interpreting Europa’s Features
Infrared Spectroscopy: An Overview
Galileo Project Home

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