1. Introduction

It seems appropriate for a journal devoted to cosmology to encompass the field of astrobiology and to move beyond the "anthropic principle" of quantum physics, the standard astroparticle physics and astrophysics that dominate the field of classical cosmology.

In the stndard "Big Bang" cosmology the universe began 13.8 billion years ago. Therefore, according to this model, life arose on Earth in the last 30% of the history of the cosmos. How life arose, is as yet unknown, though we can say with certainty that life emerged out of baryonic matter. Life as we know it is based on carbon. How carbon was incorporated into life is unknown, nor do we fully understand the full chemical pathways which led to life. However, beginning with the work of Stanley Miller and his landmark contribution in 1953, and the subsequent discoveries in organic chemistry, it is likely that the full recipe for life will someday be discovered.

With a few exceptions (Burchell 2010; Joseph 2009a; Rampelotto 2009; Sharov 2010), attempts to understand the origin of life have assumed the Earth is the cradle of life. However, if life was not delivered to this planet via mechanisms of panspermia, and ruling out, for the moment, the possibility of the interplanetary exchange of life (Mahaney and Dohm 2010), then certainly it is reasonable to assume that life could have begun independently on other planets and moons, including Titan (Naganuma and Sekine 2010), Io (Schulze-Makuch 2010), Mars (Levin 2010), and Europa. The same steps of chemical evolution that eventually led to the evolution of the first cells on Earth could also have taken place on other worlds, limited only by resources, the availability and abundance of water, distance from the sun, and the capacity to develop a life-promoting atmosphere (Lal 2010), although as in the case of Europa habitbility does not necessarily need an atmosphere as we argue in this paper. Thus it is reasonable to consider the phenomenon of life and its evolution in the Solar System and, more appropriately, in the context of the space sciences, including cosmology. It is indeed timely to begin inserting in a universal context not only chemical evolution, but to go beyond and to begin considering the phenomenon of life on Earth, in the Solar System and, more appropriately, in the context of the space sciences, including cosmology—the proper perspective for the new science of astrobiology.

2. A Return to the Moon: Preliminary Steps in the Exploration of our Solar System

In spite of the fact that Neil Armstrong’s first step on the Moon was such a tremendous leap forward in the exploration of the Solar System, unfortunately low-Earth orbits have constrained all subsequent efforts during following four decades.

Fortunately, unlike forty years ago when the exploration of the Solar System was the exclusive realm of two nations (America’s NASA and the Soviet Union’s Roskosmos), today there are major international efforts in progress, including the European Space Agency (ESA), the Indian Space Research Organisation (ISRO), China National Space Administration (CNSA) and the Japan’s Independent Administration on the Exploration and Aviation of Space Study and Development Organization (JAXA). Humans may again set foot on the moon. Naturally, the next step would be other planets, perhaps using the moon as a base and stepping stone to the stars.

We are now aware of the presence of water on the lunar surface. Since water can accumulate on the Moon, and given evidence that early Mars may have also been flush with water, then it is certainly reasonable to assume that this precious resource so vital to life, may have also accumulated on other moons and planets.

Thanks to the Galileo and Cassini Huygens missions we are now in possession of vital information of the moons of the outer Solar System. We now have several intriguing candidates which may harbor at least primitive life, including Titan (Naganuma and Sekine 2010) and especially Europa where an ocean of water may freely circulate beneath its overlying icy crust.

What makes these discoveries even more intriguing are super-Earths orbiting in the habitable zones of other solar systems. We are rapidly learning that our outer Solar System may be a typical of other solar systems. If true, then life may be everywhere, and on some planets, these living cells may have evolved in ways similar to life on Earth (Joseph 2009b).

3. Penetrators as Possible Landers on the Moons of the Outer Solar System

Penetrators are instruments that consist of small projectiles that can be delivered at high velocity to reach just beneath the surface of planets or their satellites for probing samples of surface and subsurface chemical elements and biomarkers. So far delivering them to Mars has encountered technical difficulties. The MARS-96 robotic spacecraft scientific mission was launched in November 1996. The spacecraft had a malfunction in the third stage of the rocket and re-entered the Earth's atmosphere, falling into the Pacific Ocean (Surkov and Kremnev, 1998). Unfortunately, in December 1999, a second attept by NASA failed to land successfully in the north-polar region (Gavit and Powell, 1996).

The penetrator technology continues to undergo technological advances that will make them vital to forthcoming missions for the exploration of the Solar System. The development of penetrators by the UK Penetrator Consortium is expected to make a variety of in-situ measurements at widely separated locations on the Moon and beneath the lunar surface (Smith et al., 2008, Gowen et al, 2009).

Japan and Russia have valuable experience with penetrator technology and its use for the search of biomarkers. For example, a penetrator probe was developed in the course of the former Japanese LUNAR-A project (Mizutani et al., 2000). It is now being developed to improve the sensitivity required to detect small deep moonquakes, as well as other types of lunar seismic events into the lunar regolith. Preliminary results indicate that newly developed penetrator technology can function properly after impact (Yamada et al., 2009).

4. Saturn's Moon Enceladus and the Possibilities for Life

Enceladus is the sixth-largest moon of Saturn and reflects almost 100% of the sunlight that strikes it. (cf., Fig. 1). High-resolution Cassini images show icy jets and towering plumes ejecting large quantities of particles at high speed (cf., Fig. 2). It also it has a variety of terrains, including canyons, some tectonically deformed terrain, and very few impact craters in the south polar region all of which indicates that this little moon is geologically dynamic and active. In addition, the Visual and Infrared Spectrometer (VIMS) instrument detected crystalline water ice along the surface which may have formed in the last 1000 years and may have been thermally altered in the recent past. Moreover, simple organic compounds were also detected. The best explanation is that a warm liquid water ocean lies beneath the surface, heated possibly secondary to tidal forces. In 2009, ammonia was found and can be produced biologically and acts as an anti-freeze. Particles of ice analysed by Cassini revealed that the surface ice was composed of salt water.

Thus there is evidence that a warm salt water ocean may exist beneath the surface of this tiny moon. Coupled with evidence of salt water, ammonia, and simple organic compounds including carbonates, there is thus sufficient motivation for discussing the possibility of life on this moon.

Fig. 1. Portions of the "tiger stripe' fractures are prominent alomg the diagonal of the image. The plumes emanate from the regions that are on or near these geologic features. Credit NASA/JPL/ Space Science Institute.

Fig. 2. The plumes of ice and water vapor from Enceladus' south pole. Credit NASA/JPL/ Space Science Institute

There is now ample evidence that many species of archaea and other microbes can survive and flourish under what until a few years ago were considered "life-neutralizing" conditions. Extremeophiles flourish in deep sea thermal vents, at the bottom of the ocean near volcanic vents, in pools of radioactive waste, in salt, and nearly 2 miles beneath the surface of the Earth in the absence of oxygen or sunlight. Therefore, it is not unreasonable to assume that microbes could also flourish on Enceladus where similar conditions prevail.

5. Europa and the Possibility of Life?

Europa is the sixth moon of the planet Jupiter and is around 3,100 kilometres making it smaller than Earth's Moon (cf., Figure 3). However, it is surprisingly like the inner terrestrial planets as it consist primarily of silicate rock and appears to have iron core. It also has an oxygen atmosphere which is held in place by Jupiter's magnetic field protecting it from the sun's solar winds. Nevertheless, there is ample evidence supporting the possibility of a subsurface warm, watery, Earth-like ocean which in turn suggests the possibility of life. The heat to keep the oceans warm is likely provided by tidal flexing secondary to Jupiter's strong gravity.

Europa is dynamically active and its surface, which is composed of water-ice, is smooth and largely devoid of craters; indicating these craters have been covered over by surface and subsurface activity. For example, the surface is striated by cracks, streaks and bands. It is believed that sulfur is present (Carlson et al., 1999; McCord et al., 1998). These may be either sulfate salts, or alternatively sulfuric acid hydrates, but there is general consensus that sulfur is present in the molecular compounds that are present the icy surface. If so, then sulfur may exist beneath the surface.

The cracks were likely formed secondary to tectonic forces. The surface is thought to be around 100 km (62 mi) thick.

Some believe that in addition to subsurface watery-eruptions that Europa's warm water oceans may periodically melt their way to the surface which may be only a few kilometers thick. Yet other believe this is impossible, believing the surface to be hard and between 10 to 20 miles thick. There is general agreement, however, as to the presence of a subsurface ocean, with estimates that it may be up to 100 km (60 mi) deep. If correct, then Europa has than more than twice the volume of Earth's oceans.

Oceans, sulfur, and sources of energy all are ingredient which could support microbial life, including sulfur-eating microbes. Europa, therefore, is an excellent candidate for life.

To the general public Europa, as a potential abode for life, was widely publicized in the 1980s both in a book form and in a film by Sir Arthur C. Clark (Clark, 1982, 1984). In Clark's book, a warning was issued to the people of Earth: "All these worlds are yours—except Europa. Attempt no landings there".

Fig. 3. A view of Europa at the beginning of the Galileo misison. The intersection lines that form a letter X highlight a dark patch underneath the intersection of both strokes of the letter. It is called the Conamara Chaos Region (cf., Fig. 4).

Chaotic ice crust in the Conamara region of Europa. Conamara has the appearance ofa site at which the crust had melted allowing blocks of ice to shift positions by floating to new locations before refreeze took place (Greenberg, 2005). Credit NASA..

For example, if genetically similar, we might conclude that life may have been transferred between the planets of this solar system (Istock 2010; Menor-Salván 2009; Trifonov 2009), or that all life came from a source outside our solar system (Joseph 2009a,b, Sharov 2010). If biologically and genetically unique, this would indicate that life is not unique, but may arise independently on innumerable planets, including those well outside what are considered habitable zones and which have very hostile environments.

A return to the Jupiter System is being envisaged in the next decade with the definite option to break the "Clark Warning". This includes the Europa Jupiter System Mission (EJSM), a worldwide collaboration that will focus mainly on Europa and Ganymede--the largest satellite in the Solar System. The mission consists of two flight elements operating in the Jovian system: the NASA-led Jupiter Europa Orbiter (JEO), and the ESA-led Jupiter Ganymede Orbiter (JGO). JEO and JGO will explore Europa and Ganymede, respectively (Grassett et al., 2009).

6. Speculations About Life On Encedalus and Europa

We have argued that contact with possible biomarkers would be possible for EJSM (Chela-Flores and Kumar, 2008). In recent years, life forms have been found on Earth thriving in places with no sunlight or oxygen. Microbes have been discovered surviving on the energy from the chemical interaction between different kinds of minerals. These extraordinary microbial ecosystems are models for the types of life that might be present inside Enceladus and Europa. There are three such ecosystems found on Earth that have implications for life on Enceladus. Two are based on methanogens, Archaea, which thrive in harsh environments without oxygen. Deep volcanic rocks along the Columbia River and in Idaho Falls host two of these ecosystems, where microorganisms pull their energy from the chemicals of different rocks. The third ecosystem is powered by the energy produced in the radioactive decay in rocks found deep below the surface in a mine in South Africa.

Lake Vostok (Karl et al., 1999), is the largest of about 80 subglacial lakes in Antarctica. Its surface is of approximately 14,000 km², its volume is 1,800 km3, and it has a maximum depth of 670 m. The circulation of pure water in Lake Vostok is driven by the differences between the density of meltwater and lake water. Geothermal heating warms the bottom water to a temperature higher than that of the upper layers. The water density decreases with increasing temperature resulting in an unstable water column. This leads to vertical convective circulation in the lake, in which cold melt water sinks down the water column and water warmed by geothermal heat ascends up the water column (Siegert et al., 2001).

Similarly, Europa may also have geothermally-heated warm water under its ice-crust. Processes of the type that occur in Lake Vostok may be taking place on Europa (Singer et al., 2003).

7. Discussion

Ideally, in the search for life, sending properly equipped humans to other planets and moons would best ensure would best ensure that every possible means of discovery would be employed. Given the hazards and other difficulties that would be encountered, robotics are the most promising, including the use of robotic exploratory vehicles, such as our early suggestion of a submersible called a hydrobot (Horvath et al., 1997). In this regard, NASA has tested an autonomous underwater vehicle (AUV) called ENDURANCE for the Astrobiology Science and Technology for Exploring Planets (ASTEP) program (Doran et al., 2007). From a purely practical point of view, penetrators are the logical choice. One of the most important considerations in the search for life, then, would be selecting the most promising landing site for penetrators (Chela-Flores, 2010).

There are significant advantages from the point of view of the penetrator technology to prefer Sickle in the case of Europa. The penetrators would target Europa's flat dilational bands. Dilational bands would be steeped with sulfur, and thus, the presence of sulfur-eating bacteria would be targeted for discovery.

If the search for biosignatures is focused on the area known to be bearing sulfur patches, non-ice chemical elements of the icy patches may bear biogeochemical fingerprints of life. Non-ice elements, especially sulfur should be abundant on dilational bands chosen within this large area.

Therefore, a landing site such as Sickle is appropriate for penetrators, due to the lack of significant elevations. The tremendous costs of such a mission, when measured against the scientific return of taking advantage of the penetrator technology is more than justified given the discoveries which might be made.

Even if the sulfur patches turn out to have a small δ34S parameter (with a modulus much smaller than -70‰, hence not necessarily biological), the nature of the chemically rich icy surface would be better understood at the geochemical level.

From the many arguments that have been discussed above, we conclude that the implementation of penetrators in future exploration of the Europa and Encedalus is worthy of all the support that will be needed, both at the national, as well as at the international level. With the help of such appropriate instrumentation, we can face one of the most transcendental questions in astrobiology, namely the discovery of a second genesis of life in our Solar System (McKay, 2001; Chela-Flores, 2009).