American Scientist

To the Editors:

In "Tides and the Biosphere of Europa" (January–February), Richard Greenberg notes that cracks occur rather regularly in the surface ice and extend down to the salt water below. Their timing may even be predictable. If this is correct, then why do so many of our probe designs rely upon expensive gnawing and heating systems to burrow down to the oceans below when we might take advantage of tides and volcanoes to get free access to the liquid? If the most promising ecological niches are in these cracks, that would be all the more reason to aim for these zones.

Erik Baard
Long Island City, New York

To the Editors:

Richard Greenberg's article describes both the geological evolution and the current state of Jupiter's moon Europa, but it left me a bit perplexed. He says: (1) Europa's surface is predominantly water with a depth of about 150 kilometers. (2) There is a relatively thin crust of ice, but most of the water is liquid. (3) Stresses, caused by the tides induced by Jupiter, create cracks in the crust and expose the liquid to energy from the sun and nutrients—oxidizers and fuels—from external sources such as Jovian radiation and comets. (4) This exposure to energy and material could create habitable niches in the liquid phase.

Yet R. W. Carlson, R. E. Johnson and M. S. Anderson (Science, 286:97–99, October 1, 1999) say that spectra measured in the laboratory indicate that hydrated sulfuric acid is a major component of Europa's crust. They conjecture that the sulfuric source could be external or even endogenous. But the same material exchange that would provide nutrients to any living organisms would also supply sulfuric acid to the liquid water and so convert it to aqueous sulfuric acid. If this exchange is close to a steady state or even an equilibrium state, that would imply a relatively large concentration of sulfuric acid. Equilibrium between a likely candidate solid hydrate—octahydrate or hemihexahydrate—and aqueous sulfuric acid would require a liquid sulfuric acid mole fraction in the range of 0.1–0.13. I suspect that it would take a most unusual organism to survive in such an acidic environment. On the other hand, as Carlson et al. pointed out, acidity has a bright side; it would make it far easier for the tides to maintain a liquid phase beneath the crust since sulfuric acid solution can remain liquid at considerably lower temperatures than water.

Frank Zeleznik
Rocky River, Ohio

Dr. Greenberg replies:

Erik Baard grasps a key implication of my article. So much attention and effort has been directed to the problem of drilling down through the ice because those activities began when the thick-ice model was still in vogue. Now, there are several groups whose programs and financial support depend on continuing those efforts. In fact, there is a good chance that a lander on Europa could scoop up fresh oceanic material right at the surface, if the landing location were chosen well.

Dr. Zeleznik raises an interesting point. The near-infrared spectra, upon which Carlson et al. based their results, do show that the crystalline structure of the water ice is bound with some cations, but the data cannot distinguish conclusively whether the substances are hydrated salts, sulfuric acid or something else. It is not clear which substances, or how much of each, are on Europa. This is a hotly debated topic, but I believe the hydrated salts are the leading contender. In any case, improving our knowledge of what substances are on the surface and how they relate to what is in the ocean will be a critical part of determining whether Europa truly is habitable.