What's Under Europa's Icy Crust?
Looking for Life on a Jovian Moon
In 1610, with his then-new telescope, Galileo Galilei discovered four moons orbiting Jupiter. Now the second of these Galilean satellites, Europa, is under scrutiny by a spacecraft named in his honor, NASA's Galileo. Since its two-year orbital tour of Jupiter ended late last year, Galileo's researchers are focusing on Europa in an extended-mission phase called the Galileo Europa Mission (GEM).
Among Europa's investigators are several in Southwest Research Institute's (SwRI) Space Studies Department in Boulder, Colorado. Since 1977, I have been a member of the Imaging Team, which oversees operation of Galileo's camera. Other members are Dr. William Merline and University of Colorado graduate students Beau Bierhaus and Shawn Brooks. We are in the front seat of one of NASA's most ambitious and rewarding missions of planetary exploration. Bolstered by Galileo's Europa results, one view is that this small world -- the size of our own Moon -- just may be the most likely place in the solar system, besides Earth, to find flourishing life.
Life in the solar system
Searchers for life elsewhere in the universe have naturally sought environments like Earth. Liquid water seems essential for both the origin and the sustenance of life, so candidate planets would be in a star's "zone of habitability," where it is neither so cold that any water would freeze nor so hot that it would boil. Scientists looked first to our twin planet, Venus, next toward the Sun and then to Mars, next farther away, for evidence of clement climates and life.
About 40 years ago, radio telescopic data -- confirmed later by the Mariner 2 spacecraft -- revealed Venus to be a veritable hellhole; thanks to a runaway atmospheric greenhouse effect, Venus is radically too hot to harbor life. Prospects for life on Mars have roller-coastered since Mariner 4's first pictures of a bleak, crater-scarred landscape, which dashed hopes for Martian life. Later missions revealed signs of once-flowing rivers, then new hopes that life might have survived from a watery past were negated by Viking's mid-70s on-the-surface biology experiments. There are new reasons why Mars might once have harbored life, though headlined conclusions two years ago about fossilized microbes in a Martian meteorite have since been challenged.
Giving up on our own solar system's "zone of habitability," astrobiologists next turned to possibilities for life in planetary systems around other stars. But before leaving our solar system altogether, let's not forget Europa. The outer solar system, beyond Mars and the asteroid belt, is indeed cold, only dimly warmed by the distant Sun. Basking as we do in the Sun's radiance and sustained by an ecology of photosynthesis, however, we are biased about the primacy of the Sun. Modern oceanography teaches that we may be exceptional: teeming life exists on and beneath the ocean floor, and may flourish in rocks beneath the Earth's surface. Energized by geothermal systems, life might even have originated below the Earth's surface, only later evolving upwards to take advantage of the ubiquitous sunlight.
Voyager's reconnaissance of Jupiter
A breakthrough in understanding the variety of planetary energy sources came two decades ago. As the first Voyager spacecraft sped toward Jupiter, California researchers Stan Peale, Pat Cassen, and Ray Reynolds1 were developing the theory of Jupiter's tidal forces on the Galilean satellites. The three inner moons -- Io, Europa, and Ganymede -- were known to be trapped by each other's gravitational forces in an orbital resonance, which keeps them in fixed orbital and rotational configurations, despite the tidal forces of mighty Jupiter, which would otherwise dominate their motions just as tidal (differential gravitational) interactions between the Earth and the Moon have pushed the Moon outward over the eons.
Peale et al. showed that tremendous amounts of tidal heat must be dissipated within Io and -- to a lesser extent -- within Europa. These moons are literally being powerfully wrenched. Days before Voyager reached Jupiter, Science published the predictions of Peale et al. of prolific volcanism on Io. Indeed, the first close-ups of Io showed its colorful, crater-free landscape dominated by sulfurous deposits from unending eruptions of numerous volcanoes.
Almost lost amid public excitement about Io's volcanic plumes were Voyager's mysterious views of Europa. Europa was least favorably situated for close-up imaging, so even the best pictures barely exceeded our own Moon as viewed through powerful binoculars. The smooth, ice-covered world was criss-crossed by enigmatic lines -- most of them straight, but some forming strange curlicue shapes. Europa's secrets would have to await Galileo's much higher-resolution investigations, some 14 years later.
Meanwhile, researchers continued to speculate about Europa. Though the tidal pulling and tugging on Europa's interior is much less than for Io, the rate of heating is hardly trivial. Given other evidence that Europa contains abundant water, tidal heat just might keep the water liquid. Beneath its icy crust, Europa could have an ocean. So, unexpectedly, the solar system has a second potentially habitable zone -- out near Jupiter.
Europa reveals her mysteries
As Galileo approached Jupiter in 1995, some researchers were expecting a Europan "ocean;" some even had the temerity to discuss possible life ("fish") in the Europan seas. Others calculated that the tidal heat would be lost too rapidly to sustain high enough temperatures for liquid water. We simply knew too little about Europa, and the processes within it, to decide. Galileo's engineers and scientists had to make tough choices about what to study during the spacecraft's orbital tour, because the main antenna's failure to deploy left a shrunken radio "pipeline" for sending data back to Earth. Europa was given priority.
What we have seen of Europa during the first 15 Galileo orbits (11 in the prime orbital tour) has been stunning, if only to exemplify the artistry of nature. Like Jack Frost's windowpane creations writ large on Europa's surface, an amazing variety of shapes, forms, and patterns bedazzles the eye (see accompanying pictures). How the topographic shapes were actually formed, however, tests the interpretive powers of geomorphologists. And implications for the geophysics of Europa's interior are yet more difficult to fathom. Just as the tastes and textures of an exotic fruit are unknowable from the form of its skin, so scientists cannot know what lies beneath the intricate, low-relief topography of Europa. But our curiosity is whetted, so we try.
Galileo's low-sun pictures exaggerate the apparent relief. Actual ridge heights rarely exceed a couple hundred meters. Most of Europa's topography is more like Kansas than the Appalachians. Polar researchers look at Europa and perceive familiar arctic landscapes of shifting ice plates. Some analogies break down in detail, but many Galileo geologists believe that we are observing a thin ice crust floating on top of liquid water2. Indeed, water may have reached the surface locally, forming a once-again-frozen "pond." Analysis of small cracks adjacent to large topographic loads implies a layer of brittle ice, less than 1 kilometer thick, above Europa's supposed ocean.
The ice is much thinner where it has obviously foundered. Nowhere is the crustal disruption more dramatic than in the chaotic zones of Conamara Chaos, first imaged during Galileo's sixth Jupiter orbit. Here, ice blocks several kilometers across, bearing the ubiquitous ridges characteristic of Europa, have broken off and floated; they seem to have twisted, tipped, and even foundered in a rough matrix, perhaps the once-again-frozen ocean that was temporarily exposed at Europa's surface.
Yet the conclusion that Europa does indeed have an ocean close to the surface is not entirely secure. It is notoriously difficult to judge rates from images of static geological formations. (Consider glaciers flowing viscously down high mountain valleys on Earth.) Does the brittle ice rest atop liquid water, or does it instead cover slush or simply warmer ice? Some Galileo geologists believe that the surface topography reflects subsurface solid-state convection. Yet the heat that warms and mobilizes the ice -- simulating water -- implies that liquid water itself might exist still farther below, down perhaps 20 kilometers. That would devastate any hopes for near-term sampling of Europa's ocean. Conceivably the technology might be developed to break through tens or hundreds of meters of ice in a future mission to Europa, but 20 kilometers seems formidable. Soon, a new mission to Europa may fly sounding experiments to determine how close the water -- if indeed it exists -- really is to the surface. Perhaps thin spots exist, even if the ice is generally very thick. So far, Galileo has taken sharp pictures of only a tiny fraction of Europa's surface.
How old is Europa's surface?
Possibly Europa was once the thin-ice, watery world -- even teeming with life -- that we imagine or wish it to be, but then it subsequently cooled and froze and has remained unchanged for eons. In this view, Europa presents a tableau of a once-fascinating world, now dormant and long dead. Can we tell if Europa is still alive?
To answer this question SwRI researchers have focused on Europa's craters, or rather its virtual lack of them. Comets and asteroids crash onto planetary surfaces, leaving impact craters. Impact craters are rare only on those few worlds (like the Earth and Io) where rapid geological forces erode, cover up, or otherwise destroy craters soon after they form. Terrestrial craters have been dated, like the 50,000-year-old Meteor Crater in Arizona and the Chicxulub Crater in the Yucatan formed by the impact that rendered dinosaurs (and most other species, too) extinct 65 million years ago. Some lunar craters were dated through analysis of Apollo moon rocks. These crater ages agree with estimates, from telescopic surveys of Earth-approaching asteroids and comets, of the current bombardment rate. Since roughly the same numbers of asteroids and comets strike other planets in the inner solar system, the greater densities of craters on them suggest that their surfaces are rather old compared with the Earth's. Geological activity on Venus generally ceased about half a billion years ago; Mars and Mercury seem to be older still.
The same principle can be applied to Jupiter's moons. Ganymede and especially Callisto -- which is literally saturated with craters -- are old compared with crater-free Io and compared with Europa. But just how old, in absolute terms? We cannot compare directly with the crater densities on terrestrial planets because Jupiter is struck by tens of times as many comets as it is struck by asteroids (asteroids dominate in the inner solar system. The gravity of Jupiter, which has scattered comets away from the inner solar system, has thus protected us from what might otherwise have been a continuing, life-threatening cometary bombardment.)
Gene and Carolyn Shoemaker, co-discoverers of the famous Shoemaker-Levy 9 comet that impacted Jupiter in July 1994 (Gene Shoemaker tragically died last year in an automobile crash on a lonely track crossing the Australian Outback while researching terrestrial craters), led the telescopic survey of comets in near-Jupiter space, defining the current impact rate. Other scientists, such as SwRI's Dr. Hal Levison, have calculated the dynamical evolution of comets, moving in toward Jupiter from their place of origin in the Kuiper Belt beyond the orbit of Neptune. Evidently, a crater 20 km diameter or larger is formed on Europa about every million years. So far there seem to be fewer than 10 such craters on Europa. If so, its surface averages just 10 million years old -- and much younger still in the nearly crater-free chaotic regions.
Only two years ago, Gene Shoemaker had concluded from the first Galileo images of Europa that Europa's surface might be a billion years old. Work at SwRI demonstrated, however, that the 5- to 10-km "pits" visible were really depressions formed locally where the ice had collapsed due to internal geological activity. They were not impact craters, after all, and so should not be counted in calculating surface age. These inferences have been verified by subsequent, closer pictures of Europa's amazing complex of internally caused pits, domes, and moats.
Real impact craters are rare on Europa. One of the largest is named Pwyll. About 26 km across, it is surrounded by dark, reddish-brown materials dredged up from below when the comet struck. A dramatic pattern of bright radial rays emanating from Pwyll stretches across many hundreds of kilometers, reminiscent of rays (visible through good binoculars) surrounding the lunar crater Tycho. One Pwyll ray is draped across part of the Conamara Chaos region. Evidently Pwyll, which is the youngest of the large Europan craters and thus may have been formed roughly a million years ago, postdates much of Conamara Chaos. Yet other parts of the Chaos may encroach on the ray. Perhaps the broken, floating blocks of ice were active only a million years ago.
Large craters like Pwyll are far too sparse to use for relative age-dating of the restricted localities of most recent activity on Europa. So we wish to count the more frequent, much smaller craters. But our studies at SwRI have shown that most of the small craters are, actually, not due to impacts of small comets on Europa but are instead "secondary craters" created by material ejected from the few larger craters. Like the pits, they can't be counted as primary comet impacts. It appears that small comets must be quite rare. And the most recent activity we've seen on Europa happened only a few hundred thousand years ago. Regions not yet imaged by Galileo may be younger still.
Indeed, Europa's geological activity may be continuously, if episodically, ongoing: somewhere Europan ice is cracking even as you read this article. Far from being a frozen tableau, Europa is still warmed by Jupiter's tides and remains active. Very likely, it has a watery ocean and perhaps life today. The remaining dilemma is whether we'll be able to reach Europa's oceans with modern technology: Is there near-surface access, or is this ocean tens of kilometers down? Solving that mystery, however, must await the next spacecraft mission to this remarkable Jovian moon.
1. Peale, S. J., P. Cassen, and R. T. Reynolds. Melting of Io by tidal dissipation. Science, 203:892-894, 1979.
2. Carr, M.H., M.J.S. Belton, C.R. Chapman and 19 others. Evidence for a subsurface ocean on Europa. Nature, 391:363, 1998.
Published in the Fall 1998 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.