Hydrothermal Systems

11.2.1 The Environment for Microbes Immediately After Impact - A Plausible Picture

The first obvious hydrologic consequence of impact is the formation of hydrothermal systems. The energy delivered by the impactor will heat the target rocks, generating a transient thermal anomaly [e.g., 4]. Water from deep confined aquifers penetrated by the impact, precipitation, groundwater and other sources of water can be heated and vaporized. In ice-rich regions ground-ice may be melted.

Hydrothermally-precipitated minerals have now been recognized in a number of structures including the Ries [4], Haughton [5], Manson [6] and Chicxulub [7]. Osinski et al. examined the hydrothermal alteration within localized hydrothermal pipes in the Haughton impact structure's fault systems and within the polymict impact breccia that filled the crater after impact 23 million years ago [5]. The first stage they recognize in this process is an early stage where quartz is precipitated and temperatures are above 200°C, well above the currently defined upper temperature limit for life (113°C). In the main stage that follows they recognize the precipitation of calcite, barite, marcasite and other minerals. During this stage temperatures are between 100° and 200°C. At this stage, temperatures within some of these systems are appropriate for hyperthermophiles, whose growth temperatures are above 80°C. During the late stage temperatures drop below 100°C and selenite is formed.

The authors estimate that it took some tens of thousands of years for the crater to cool below 50°C [5], making the phase of the hydrothermal anomaly a geologically short-lived event, but a long-term event when considered in context of biological colonization (new volcanic lakes, for example, can be colonized in a matter of months). Once temperatures are reduced to below 50°C the environment becomes suitable for prokaryotic mesophiles (organisms that grow best between 25 and 40°C) and eukaryotes (which have not been found above ~55°C).

In terms of understanding how these phases of water temperature affect microbial life, it is important to try to picture the real physical environment. The mineralogical signatures from Haughton suggest that hydrothermal activity occurred for many thousands of years and theoretical models give convenient curves of crater cooling over time that are uniform, but it is unlikely that these observations and calculations apply to the entire crater with the uniformity that they suggest.

For example, one can imagine a large ejecta block thrown onto the rim of a crater, isolated from the melt sheet and exposed to the wind. On the surface of such a block temperatures might have dropped to below 50°C within days. Similarly, the surface of some outlying regions of melt rocks isolated from hydrothermal pipes may have cooled down more quickly than hydrothermal systems being continuously supplied with heated water/steam from the base of the melt sheet. Osinski et al. have mapped the hydrothermal pipes in Haughton and found them to be associated with the edges of the crater, suggesting that they flowed out around the faults of the crater and were localized features.

In the Manson structure, hydrothermally-altered minerals are localized to the central uplift [6] and in this case alteration may have been dominated by geothermal heating from the uplifted material. As the subsurface heats at approximately 30°C per kilometre depth, an uplift of ~2-3 kilometres would expose material from the subsurface whose surface temperatures would be suitable for thermophiles and hyperthermophiles.

So what would such an environment look like to a field microbiologist? The microbiological view across a new crater is speculative, but we can imagine it to be heterogeneous. A plausible analogue of the environment around the edges of the crater days to months after impact may be the hot springs found in a hydrothermal region such as Yellowstone National Park [8]. Here there are hot springs with temperatures in excess of 90°C, but areas of cooled terrain in between lead to a patchwork environment fit for mesophiles, thermophiles and hyperthermophiles that varies on spatial scales of tens of metres. Changes in plumbing from subsurface geologic activity might now and then change the location of the springs. This is a microbiologically diverse environment, where water comes in the form of hydrothermally heated sources, rain and maybe snow and cool groundwater feeding into hydrologic depressions.

In the centre of a newly-formed crater, the melt sheet might sustain more homogeneous temperatures if it is uniformally heated during impact. In the 25 km-diameter Ries structure, the initial temperature of the suevite is estimated as ~550°C [4]. Degassing pipes suggest that cooling could have begun quite rapidly. Alteration products within the pipes suggest temperatures below 100-130°C [4]. Osinski et al. estimate temperatures of 650-700°C during the emplacement of the Haughton melt sheet [5]. Draining of water into the crater and the formation of pond(s) and lake(s) would have further caused localized rapid reductions of surface temperature, first resulting in steam, but when the temperature was reduced to below 100°C, causing convective cooling. Thus, very quickly, regions of the suevite would have become suitable for microbial colonization.

Hydrothermal systems are significant on Mars, not so much as a source of hot water, but as a source of any water at all [9]. The phase of hydrothermal activity presents the possibility of melting subsurface ground ice that could provide a rare source of sustained liquid water for life [10]. The impact-induced thermal anomaly may delay the freezing of lakes and furthermore, the heat gradient would drive convection with the lake [9], mixing warm water and any potential nutrients. The recent detection of substantial ground ice on Mars [11] suggests that this is a potentially important process in the Martian post-impact environment with potentially relevant exobiological implications. However, one important factor militates against too much biological optimism. Just because liquid water is formed does not mean there are any organisms to colonize it. On Earth, the dogma, 'where there is water, there is life' holds true because of the substantial microbial biosphere that covers the planetary surface. Usually, the transient presence of water leads to colonization by airborne microorganisms or those leached from the ground. On Mars, the lack of an aerial biota or substantial surface microbiota sustained by a photosynthetic biosphere means that it is more plausible to imagine transient sterile bodies of water. Nevertheless, the thermal pulse generated by impact increases biological potential and so it is a phenomenon of exobiological interest on any planet, regardless of the presence of life or not.

11.2.2 Evolutionary Significance of the Duration of Hydrothermal Systems

At any given time in the history of Earth, volcanic hydrothermal systems have existed. In Hawaii alone, new lava flows, for instance, have existed continuously for the last 70 million years [12], suggesting the continuity of post-volcanic habitats over geologic time periods. Can the same be said of impact-induced hydrothermal systems? Clearly it cannot, as there is no hydrothermal system associated with a fresh crater on Earth today. The mean impact interval required to allow for almost continuous hydrothermal post-impact crater habitats can be estimated. Osinski et al. estimate the period of hydrothermal temperatures greater than 50°C in the Haughton structure to be 'several tens of thousands of years' [5]. The current interval of impact events on this scale is once every ~107 years [13], much longer than the period of hydrothermal activity. Events with much longer hydrothermal durations are correspondingly less frequent. A bolide of 10 km size may have two orders of magnitude more energy than Haughton (such as the impactor that formed the Chicxulub structure at the so-called K/T boundary, the geologic boundary between the Crataceous and Tertiary) and so its hydrothermal phase could plausibly last for ~1 million years, but the corresponding frequency is once every 100 million years.

From the microbiological point of view we can therefore say that for present-day impact fluxes, microbial colonization of hot springs at the sites of fresh craters depends on a source of propagules from non-impact hydrothermal systems, such as those associated with volcanoes or subsurface geothermally-heated habitats. The exception to this statement may be when multiple impacts occur, as might happen after fragmentation of the bolide. In this case, cross-contamination of the craters could potentially occur. Also, although the mean interval may be greater than the duration of hydrothermal systems, statistically it is still possible today to have two impacts occur whose hydrothermal systems overlap in time, although this would be rare.

During the Archaean (3.8 to 2.5 Ga ago) impact fluxes were much higher than today, possibly two to three orders of magnitude more [13]. At this period in Earth's history the interval between large impact events would be very similar to the period of their hydrothermal phases (perhaps one million years between 10-km-sized bolides with a similar duration of their hydrothermal systems). Cross-contamination of post-impact hydrothermal systems could conceivably have occurred for craters formed from completely different bolides. Concomitantly, however, volcanic hydrothermal systems were probably much more common on early Earth. Although Archaean Earth may have been a place of interaction between post-impact hydrothermal systems, volcanic hydrothermal systems, like today, were the dominant and most abundant habitats for heat-loving microorganisms.

Hydrothermal Systems

Fig 11.2 During the Archaean it is plausible that the mean interval between impact events was less than the duration of hydrothermal systems, in which case cross-inoculation of impact hydrothermal systems might have been common. Today, most new impact crater hydrothermal systems require a source of thermophiles and hyperthermophiles from nonimpact hydrothermal systems such as volcanoes. Exceptions would be multiple impact events caused by fragmentation of the bolide and rare occasions when the impact interval was less than the duration of the hydrothermal system.

Fig 11.2 During the Archaean it is plausible that the mean interval between impact events was less than the duration of hydrothermal systems, in which case cross-inoculation of impact hydrothermal systems might have been common. Today, most new impact crater hydrothermal systems require a source of thermophiles and hyperthermophiles from nonimpact hydrothermal systems such as volcanoes. Exceptions would be multiple impact events caused by fragmentation of the bolide and rare occasions when the impact interval was less than the duration of the hydrothermal system.

As the history of the impact flux on Mars is similar to that on the Earth we can postulate the same constraints on the cross-contamination of post-impact hydrothermal systems there.

In summary, impact fluxes today do not provide for hydrothermal systems that offer a consistent planetary habitat for heat-loving organisms. The maintenance of such forms of life on planets with impact fluxes similar to those of the present-day requires the sustained presence of other hydrothermal habitats such as those formed by geothermal or volcanic processes (Fig. 11.2).

Was this article helpful?

0 0

Post a comment