Recent work at the University of Arizona leaves planetary scientists who have been searching for life on the Jovian moon Europa skating on thin ice.1 However, the actual problem involves thick ice. This water world encased in a continuous sheet of ice is one of NASA’s chief targets in its quest to find life beyond Earth.
Compelling evidence indicates that oceans of liquid water exist beneath this crustal ice.2 Scientists think that internal heat generated by radioactive decay and by tidal interactions between Europa and Jupiter is sufficient to maintain Europa’s subsurface oceans, providing a liquid environment for life. Recent discoveries of microbes in the ice sheets above Lake Vostok, Antarctica, further fuel speculations about the possibility of Europa’s ecosystem.3
Yet, life requires more than the presence of liquid water. It also needs an array of chemical elements and a continuous energy source. Most researchers think that Europa possesses the chemical elements to satisfy life’s requirements.4 Yet, to date, they have failed to identify an energy source adequate to sustain life on Europa.
On Earth, life receives energy from two sources: sunlight and geochemical systems. Neither of these energy sources is available to power life processes on Europa.5 Sunlight cannot penetrate Europa’s thick ice to reach the subsurface water. The absence of sunlight precludes photosynthesis. Further, Europa’s geological activity is inadequate to sustain the ongoing geochemical energy needed to drive life’s chemistry.
Some researchers suggest another possible energy source for Europan life: charged particles (electrons, protons, and ionized sulfur and oxygen atoms) energized by Jupiter’s magnetosphere. These high-energy particles collide with ice on Europa’s surface to produce oxygen and hydrogen peroxide.6 They also stimulate production of formaldehyde and other organics. If these compounds were to continuously come into contact with the Europan subsurface oceans, they might provide the energy flow needed to sustain life.
SETI (Search for Extraterrestrial Intelligence) researcher Christopher Chyba proposes tidal cracking of the ice sheets or break-through melting of the subsurface oceans as the mechanisms to carry oxidants and organics into Europa’s oceans. However, both mechanisms require a relatively thin surface ice sheet, and herein lies the problem. The recent study by University of Arizona scientists indicates that the Europan ice sheet is far too thick to allow compounds formed on the surface to migrate into the oceans.
When colliding with Europa’s surface, comets cause the surface ice to melt and vaporize. If the ice is thin, melt-through will occur, leaving no evidence of the strike. But, if the surface ice is sufficiently thick, cometary impacts will leave craters. The Arizona team modeled the impact events that produced Europa’s smallest craters. Their goal was to establish a minimum for Europa’s ice sheet thickness. Based on these modeling studies, Europa’s ice surface must be at least three to four kilometers thick.7
Even at this lower limit, Europa’s surface ice is too thick to allow contact between its subsurface oceans and its surface. The isolation of Europa’s oceans renders a radiation-driven ecology impossible.8 Without energy flux into its oceans, Europan life cannot exist.
- E. P. Turtle and E. Pierazzo, “Thickness of a Europan Ice Shell from Impact Crater Simulations,” Science 294 (2001): 1326-28; Richard A. Kerr, “Putting a Lid on Life on Europa,” Science 294 (2001): 1258-59.
- Michael H. Carr et al., “Evidence for a Subsurface Ocean on Europa,” Nature 391 (1998): 363-65; K. K. Khurana et al., “Induced Magnetic Fields as Evidence for Subsurface Oceans in Europa and Callisto,” Nature 395 (1998): 777-80; Fritz Neubauer, “Oceans Inside Jupiter’s Moons,” Nature 395 (1998): 749-50.
- J. Jouzel et al., “More than 200 Meters of Lake Ice Above Subglacial Lake Vostok, Antarctica,” Science 286 (1999): 2138-41; John C. Prisco et al., “Geomicrobiology of Subglacial Ice Above Lake Vostok, Antarctica,” Science 286 (1999): 2141-43; D. M. Karl et al., “Microorganisms in the Accreted Ice of Lake Vostok, Antarctica,” Science 286 (1999): 2144-47.
- Christopher Chyba and Cynthia B. Phillips, “Possible Ecosystems and the Search for Life on Europa,” The Proceedings of the National Academy of Sciences, USA 98 (2001): 801-4.
- Eric J. Gaidos et al., “Life in Ice-Covered Oceans,” Science 284 (1999): 1631-33.
- R. W. Carlson et al., “Hydrogen Peroxide on the Surface of Europa,” Science 283 (1999): 2062-64; Christopher F. Chyba, “Energy for Microbial Life in Europa,” Nature 403 (2000): 381-82.
- The Europan ice sheet is thicker than 3-4 kilometers because larger craters exist on its surface. The measurement of 3-4 kilometers designates merely the formal lower limit of ice sheet thickness.
- Kerr, 1258-59.