Europa: A brine martini – shaken and stirred.

Notes from the First Astrobiology Science Conference

Mars is no longer the only place in our solar system where life may have existed. In the past decade increasing interest has focused on Jupiter’s moon Europa. While current astrobiology research on Mars focuses on the mystery of where Mars’ ancient supply of water went, attention on Europa’s obvious and abundant water supply focuses on how much, if any, is liquid.

Europa has an abundant supply of water ice – this much has been known for many years. Just how this water is deposited on Europa and how it behaves has been the source of speculation in recent years. Voyager images showed a world with tantalizing hints of a sub-ice global ocean. Galileo has served to provide further evidence – so much so that the structure of the ocean and its interaction with the outer ice surface can be modeled.

Richard Greenberg began the discussion of life and Europa with a presentation titled “Habitability of Europa’s Crust and Ocean“. The outer 150 km of Europa is ice with an external surface that is rather young. According to Greenberg, Europa has been wholly resurfaced in the time since dinosaurs have disappeared from Earth. This resurfacing takes two main forms: tectonic and chaotic.

In concert with – and indeed because of this activity, Greenberg feels that some sort of ocean interaction with the surface leads to the creation of a variety of potentially habitable environmental niches. While some believe that Europa’s ice layer is thick, Greenberg is among those who think that is rather thin – and has the evidence to back up his contention.

The thick ice model favored by some would isolate Europa’s ocean from the surface, a concept based upon conservative thermal models. Newer ideas now suggest a much thinner ice cover – one where the ocean is potentially very habitable. Despite the radically different models, both models are based on Galileo data.

The orbital mechanics governing the motions of the Galilean moons (Io, Europa, Ganymede, and Callisto) and giant Jupiter lead to an interrelated series of orbital resonances. The closer satellites, Io and Europa, receive the brunt of the effect. These resonances lead to pushing and pulling of the internal structure of the moons. In Io’s case massive, nonstop volcanism results.

These resonances cause eccentricities in the motion of Europa which lead to the formation of tides. Europa fares somewhat better than Io with much less hellish results, but still receives a substantial amount of energy pumped into it. Enough energy is pumped into Europa to keep much of its ice layer in liquid form. A tidal bulge is formed that varies in size depending upon where the moon is in relation to Jupiter. These changes cause a squeezing and shaping of the moon and the induction of tides.

These tides have important effects. They may exert a torque that causes non-synchronous rotation in Europa. Evidence shows that positions on Europa’s surface are stable for up to 10.000 years so this non-synchronous rotation is rather slow process.

Tidal stresses cause heat to be pumped into Europa, which leads to a variety of stresses on its surface. These stresses lead to crack formation. Cracks are linear ridges often shown to have a bright stripe in the middle. According to Greenberg, a global ocean is required to create cracks like this. The ridges associated with these cracks form quickly (in less than 20,000 years) by means of a tidal process that pumps water and slush up to the surface. Cracks open and water rushes up. As water reaches the surface it boils and freezes at same time. This process is repeated again and again leading to the formation of the ridge features.

Surface features take a variety of forms. Dilational features are distinguished by bands which are places where broad cracks open up due to external forces and the motion of layers below the surface. Slip faults can seen in places where plates have slid past each other. This process is driven by tides which open, shear, close – again and again – causing large pieces of ice to “walk” across Europa.

Cycloidal (or arcurate) cracks form when pieces of ice fracture as the stress direction changes. This leads to the formation of arcs. This process proceeds, stops for a while, and starts up again with new stress orientation. Again, according to Greenberg, this would only happen with a thin ice layer – not with thick ice. Studies of these cracks – and the models that have been developed – serve
as the best evidence to date for a thin ice cover since these things will not happen with thick ice.

Chaotic terrain is common on Europa and involves the reworking of previous surface features. Rafts of crust float around on top of the ocean. Chaotic terrain is also in the process of being broken up by cracks that have formed after the initial break up event. An interplay of tectonics and chaotic processes leads to a seemingly haphazard process of surface re-working. The net result is a crazy surface patchwork of geometric lines morphing into jumbled chunks of ice – and vice versa.

The fact that the progress of crack formation rotates over time as a result of non-synchronous rotation has been used to validate thin ice model. Water is pumped through cracks on a daily (1 Europan day = 3.5 Earth days) basis. Each crack can be active for as long as 10,000 years. As part of this process open water can be exposed every million or years or so. According to Greenberg there is no reason not to expect that these processes are still in effect today on Europa.

Greenberg closed by showing his own thumbnail sketch of what sorts of ecological niches putative Europan life might find in one of these cracks. His diagram showed tides moving materials (and organisms) up and down cracks, with materials delivered periodically by comets being delivered downwards.

Just under the surface, below the region where high radiation levels are to be expected, Greenberg suggests that there might be layers where light could be sufficient for photosynthesis to occur. He cautioned that the transient nature of these cracks freezing and thawing would create ecological niches of a temporary nature. As such he postulated that Europan organisms would likely need to be able to tolerate long periods of being frozen – or be able to migrate though the ocean to other cracks which were still liquid.

Robert Papppalardo picked up where Greenberg left off and focused on one of the mechanisms whereby surface features could be formed on Europa. In a presentation titled “Europa; Diapirism and the potential for Intra-Ice Biological Niches“, he focused on a series of features noticed in imagery from Europa called “lenticulae”. These features – or “spots” are roughly 10 km in size, and are equally space over larger regions of terrain.

Pappalardo postulated that these features were formed as the result of diapirs. A diapir is a mass of low-density material rising buoyantly and distorting outer surface as it pushes from underneath. On Europa these diapirs are formed from ice which is warmer and of a different composition than the surrounding ice. The material in these masses can extrude onto surface too. Pappalardo suggested that these rising masses might represent ices containing a comparatively large amount of salts

Rising material deforms and then partially melts surface creating the spot like features seen from space. Older and more organized surface features (such as those in the Thrace feature) are distorted and transformed by diapirs rising up from underneath. This leads to the creation of chaotic terrain where more organized tectonic cracks once existed.

Pappalardo likened the whole process to that of a “planetary lava lamp” with tidal heating replacing light bulb as an energy source to power the phenomenon. He closed by suggesting that the convection associated with diapirs might serve to mobilize and transport biological materials and organisms – should there be life on Europa.

Jody Deming completed the talks on ice and life with a presentation titled “Arctic Ice as an Earth Analog for Subzero Microbial Habitats Elsewhere“. The research described involves a joint effort between the University of Washington and the University of Alaska. The joint effort uses Arctic sea ice as an Earth analog for subzero microbial habitats elsewhere. Unlike the previous two presentations which dealt with large scale ice features and processes, this research looks at a scale relevant to individual organisms – in this case, microbes.

While fresh water studies in ice-covered Antarctic lakes (such as Lake Vostok) are important in understanding the environments that may exist on Europa, sea ice studies are required for a full understanding of the briny ocean that may exist on Europa.

The presence of salts depresses the freezing point of water allowing it to be liquid at lower temperatures. The brines in this research can be as cold as -35C – the lowest level where life and liquid water exist. This group is trying to push lower temperature limit for life on Earth with the hope of understanding how it might function in environments elsewhere. Sample collection is done near Barrow, Alaska and analyzed in situ in laboratory facilities located on the ice pack.

The research team has found that as sea ice gets colder, the brine channels located within the structure of the ice close off and form increasingly smaller pores. Despite this shrinkage, organisms are able to live in the brine pockets within these small pores. In addition to bacteria, the brine inclusions contain organic compounds and what some substances that may actually serve as cryoprotectants.

In addition, fluid flow is possible between these pores allowing nutrient exchange to occur between pores. As such, this ability to accommodate viable microniches at the microbial level suggests that the large kilometer sized inclusions mentioned before would be more than adequate to support life on Europa.

Related Links

° NASA Astrobiology Science Conference Website

° SpaceRef/Astrobiology Web Program and Abstract Database

° NASA ARC press release