SETI Lost and Found

The announcement that the primary instrument in the Search for Extraterrestrial Intelligence (SETI), the Allen Array, is going offline for lack of money is a sobering reminder of the challenges faced by high risk science. It seems to be the result of a confluence of cuts and declines in both federal funding of the observatory site from the National Science Foundation and the drastic belt-tightening of a virtually insolvent state of California.

Others have written about all the great science that the Allen Array can be used for in addition to hunting for artificial signals. Paul Gilster at Centauri Dreams gives a pitch perfect discussion of this. I think it's also important to remember that SETI has helped pioneer voluntary distributed computing with their SETI@Home project. This began in 1999, the veritable stone age of what we might now call "crowdsourcing". Next time you fire up your iPhone or Android for information on local hot-dog stands you should think about that. By most standards SETI has produced very significant spin-offs along the way since it began in modern form in the early 1960's.

What about the root motivation for SETI though? Is there simply insufficient enthusiasm among scientists and the public to sustain an effort like this, even if it's to the tune of a just few million dollars a year? [A note to cynical misers; American Idol rakes in over $1 billion each year for its various interested parties. You could finish building the Allen Array and fund SETI in perpetuity for significantly less than that].

I feel that SETI has always been a hard sell. Spend an hour talking to one of its key proponents like Jill Tarter and one comes away utterly convinced that this is a vital thing for humanity to pursue. But over the following days and weeks then in all honesty doubt does tend to creep back in. This doubt is multi-faceted. There is the high-risk nature of the endeavor, the risk being the perception that as remarkable as a detection would be, the odds of actually getting it are so very small. Then there is the problem that there are so many unconstrained parameters involved, which just exacerbates the risk assessment question.

Things are changing though. Twenty years ago there was essentially no evidence that planets existed around any star other than our Sun. Sure, it would have been pretty bizarre and unsettling if ours were the only planetary system in a galaxy of 200 billion stars, but we just didn't know. Now we are in the happy situation of being able to argue about exactly how many small rocky worlds there should be, and even how many of them might be terrestrial analogs. The general answer is "lots". This really does change a big piece of the SETI equation as it is employed. Suddenly there is not only a way to improve the estimates of the number of potential targets, but to actually identify those targets. Hence the plans that were in place to use the Allen Array to monitor Kepler planets.

The more difficult questions are those that revolve around some of the implicit (and even explicit) assumptions in SETI. These are to do with the nature of "intelligent" or "technological" life, and the presumed or hoped for motivations of other worlds, other species. It's a quagmire. The problem is that we just don't know, not even a teeny bit, whether the example of humanity is a reasonable template or not. Even arguing about the equally sophisticated but non-technological evolution of dolphins or ants ignores the fact that all of life on Earth is the product of intimate co-evolution across 4 billion years. Similarly fraught are arguments about the longevity and placement of civilizations in cosmic time. The risks for success or failure with SETI are almost impossible to compute, one way or the other.

No. I think the real argument for SETI is that no-one can say whether or not there is another species in our galaxy sending out recognizable signals, and that is precisely why we should be listening.

Long before we had modern astrophysics humans looked out into the universe eyes wide open. There was certainly motivation to understand those objects or phenomena that we had already discovered. But there was also motivation to simply gather knowledge by finding, well, by finding whatever was out there. Modern SETI as performed by the Allen Array is well set up to capture all manner of natural signals as well as those of artificial origin. The transient phenomena of the universe represent one of the next great challenges for astronomy. Projects like the $400 million Large Scale Synoptic Telescope are aiming for precisely this regime. Part of the motivation is to simply discover things that we could not have found before.

Regardless of what it finds, or doesn't, the Allen Array needs to keep operating or else we lose out on the beautiful mysteries waiting for us out there in the cosmos. Let's go help it.

Three Billion Years B.C.

The Earth is still forming. Every year our planet accumulates another 40 million kilograms of material, mostly in the form of microscopic interplanetary dust. More sporadically the planet is also hit by larger bodies. Hundred meter diameter asteroids or cometary lumps arrive on average every thousand years, kilometer-sized civilization manglers arrive roughly every million years. This had been going on since the Earth coagulated from the material of the proto-planetary disk around a baby Sun 4.54 billion years ago.

As we turn back the cosmic clock the rate of accumulation of material increases. The pockmarked lunar surface has served as a proxy for reconstructing the history of asteroidal and cometary impact on the Earth. Without an atmosphere or significant geophysical activity the Moon has an excellent memory of impacts, while the Earth had eroded and resurfaced itself in continual reinvention. This record has indicated that during a period between about 4.1 and 3.8 billion years ago the Earth must have been subject to a particularly brutal pummeling. A substantial fraction of the outer shell of our planet could have been laid down during what has become known as the Late Heavy Bombardment.
It's a fascinating time in the history of our world. The first indications that microbial life might have been at work come not so very long after this quite cataclysmic episode ended.

The reason for this infall of material seems likely to be connected to a period of dynamical evolution in the outer planets. Models suggest that both Neptune and Uranus could have migrated outwards and dug into a rich belt of outer, Kuiper or trans-Neptunian objects. Many of those distant small bodies would have been pushed into orbital paths that would eventually lead to passage through the inner solar system and collision with the Earth. At the same time, Jupiter and Saturn would have migrated inwards and could have scattered material from the asteroid belt onto inbound trajectories. Once the dynamical reorganization of the giant planets was finished the Late Heavy Bombardment would have tailed off. A settling planet Earth then gave rise to the tentative steps of biochemistry and single-celled organisms.

Or so we thought. New evidence is emerging from the terrestrial rock record that the Earth actually continued to be pounded by very significant impacts from 3.8 billion years ago all the way up to around 2.5 billion years ago. "Life Killer" type asteroid impacts seem to have happened roughly every 40 million years during this timespan, rather than every 500 million years as had previously been thought. 

So what gives? Where did these chunks of material come from? W. Bottke and colleagues have studied the gravitational dynamics of the teenage solar system and suggest that a now-depleted inner belt of material between Mars and Jupiter could have been scattered onto an inclined set of orbits - out of the plane of the planets. This population would then slowly "leak" into Earth-crossing paths, thereby greatly extending the tail of the Late Heavy Bombardment over another billion years or so. The leftovers of these bodies are still there, known as the Hungaria asteroids.

It all looks to fit rather well. The dynamics are believable, and provide a mechanism for the impacts that littered the planet with the molten globs of rock that geologists find in layers of ancient strata. There's just one teensy question. What are the implications for the evolution of life on Earth? While evidence of microbe-built structures like stromatolites from 3.5 to 3.8 billion years ago remain a little controversial, the presence of a diverse planet-wide biosphere is pretty incontrovertible in the 3 to 2.5 billion year ago span. Apparently microbial life not only dealt with continual destructive asteroid impacts but really did rather well for itself.

This raises another intriguing issue. As W. Bottke and colleagues point out, this prolonged period of heavy impacts does effectively stop around 2.5 billion years ago. That is suspiciously coincident with the first signs of a rising oxygen content in the Earth's atmosphere (the "Great Oxidation Event"), and the eventual emergence of multi-cellular life somewhere around 1.6 to 2 billion years ago. Is there a connection? Could the continual accumulation of planetary material have held back the full-on evolutionary party of early life? It's highly speculative, but one is tempted to think that this might be further evidence for the incredible resilience of life and its near-relentless nature once it becomes entrenched on a planet.  

Rotten Eggs

Posts here have been a bit more threadbare than usual. Mea culpa. Can be blamed on a number of projects, including a fun writing one that you will be able to find in the May 7th issue of New Scientist magazine as their "Instant Expert" piece on Astrobiology - a lavishly illustrated 8 page spread. Hats off to the editorial and graphics staff there.

It's also been a hard time to summon the courage to move on after the kind of dreadful, gray news coming recently from NASA as missions are abandoned left, right, and center. Not good. The trickle down from this kind of mass slaughter is going to be significant.  NASA's entire budget is a miniscule 0.5% or so of the federal budget of the USA, and the projects being cut are an even smaller fraction of that. Between this and the shutdown of the Tevatron it would seem that the shining city upon a hill is getting a little dilapidated, when it comes to fundamental science.

Onto better stuff. Hydrogen sulphide, in fact. An interesting result appeared recently in the Proceedings of the National Academies, by Parker et al. that re-analyzes the products of an experiment performed half a century ago. Back in the early 1950's Stanley Miller and Harold Urey performed a number of experiments on the chemistry of a mix of 'raw' ingredients of water, methane, ammonia and hydrogen gas. By subjecting this gaseous stew to electrical discharges they attempted to reproduce conditions that might have existed on a very young planet Earth. The basic idea was to see if the rudimentary molecules for life, the prebiotic organics, might arise. Some indeed did, a smattering of amino acids for example, along with a lot of chemical muck.  There were big uncertainties though, not least of which was the true composition of the young Earth's atmosphere.

Quite recently some of Miller's later experiments came to light, by way of the analysis of sealed glass vials stored in his lab from the 1950's. Modern chemical analysis techniques are far more sensitive and precise than those that had been available to Miller, and it became clear that many more amino acids had formed inside his experiments than he had realized at the time.

Now Parker and colleagues have taken a look at a particular batch of vials from 1958 where Miller included hydrogen sulphide (H2S) in his starter mix. He had never reported the results. Hydrogen sulphide is a widespread compound, from volcanoes on Earth to extraterrestrial environments. Remarkably, these experiments seem to have produced the highest yields of amino compounds in any such conditions. The finger points to the presence of sulfur. A whiff of that could be truly magic for building bio-molecules on a primordial world.

Quite incredibly, the mix of amino acids is also a very close match to that of a number of carbonaceous chondrite meteorites. Given that hydrogen sulphide is certainly found in these meteorites the indication is that the same kind of chemistry could have taken place off-world in the proto-planetary environment. So, that wonderfully malodorous substance hydrogen sulphide could provide a critical boost to the abiotic chemical synthesis of some of life's building blocks - both here on Earth, and further afield.

Thank goodness Stanley Miller didn't like to throw things away.

Paradox Earth: III

Understanding the climate and overall environment of the very young Earth continues to be an extremely tricky business. Previous posts on several issues (I, II) surrounding the so-called Faint Young Sun paradox have discussed some of the sticking points. In a nutshell; 4 billion years ago the Sun was about 30% fainter than it is today, a direct consequence of the fundamentals of stellar evolution. So the puzzle is that as far as we can tell the surface environment harbored liquid water, yet today's atmospheric composition would have resulted in a vastly colder climate. Boosts to greenhouse gases might solve the problem, but it remains at the hairy edge of plausibility.

Now a new study by Court and Sephton casts an even murkier pall over the problem, literally. We have high confidence (from the record of lunar cratering, as well as the orbital evolution of the outer planets) that some 4.1 to 3.8 billion years ago the Earth was subjected to period of sustained impact over about 100 million years by asteroidal-type material. The so-called Late Heavy Bombardment (LHB) was quite a pounding. It likely provided the major constituents of the juvenile Earth's outer layers. Court and Sephton have studied the effect of the sand-grain sized components of material that may have poured into the Earth's atmosphere as micrometeorites during this era. Atmospheric friction as these tiny particles raced into the upper atmosphere produces high temperatures and the grains ablate, releasing sulfur dioxide - among other gases.

Sulfur dioxide is great for making particulates in a planetary atmosphere. This increases reflectivity, and can dramatically lower the solar radiation reaching the surface. Net result; planet cools. During the LHB roughly 20 million tonnes of sulfur dioxide a year may have been dumped into the atmosphere by this flux of tiny meteorites. That's equivalent to having a massive volcano erupt into the stratosphere every year for a hundred million years. The problem of keeping the Earth warm is greatly exacerbated. Court and Sephton also point out that Mars would have received a significant flux of these sulfur-bearing micrometeorites, seemingly creating an even bigger problem for an early temperate martian climate.

There are still a lot of questions. Was the sulfur content of these particles really as high as claimed? Do we really know the rate at which such tiny grains hit the Earth? Could the atmospheric chemistry of the young Earth have mitigated the production of sulfate aerosols?

Understanding what happened on the young Earth is a major issue. It seems for every solution to keeping the planetary surface warm there is an opposing mechanism that will plunge it into deep freeze. Yet the evidence remains for the presence of substantial liquid surface water during at least the tail end of the LHB and likely much earlier. Clearly somewhere we're missing a piece of the equation, or perhaps several pieces. Being able to study the deep geological history of Mars could help enormously, since it would allow us to separate out some of the planet-specific mechanisms at play. It may also be time to think a little more radically. Putting aside the mineralogical evidence for an early aqueous environment then perhaps a deep-frozen young Earth offers some advantage for the subsequently rapid emergence of life?