Exploring Mars from Phobos

There is no doubt that Mars remains a hugely important target in astrobiology, planetary science, and even human exploration. Between our increasingly good understanding of Martian water distribution (from polar regions to lower latitudes), and the extraordinary observations of a summer plume of some 19,000 metric tons of methane, it is a planet full of tantalizing clues.

The right strategy for exploring Mars is tricky, it has to balance scientific goals, technical feasibility, the potentially delicate nature of the Martian system - possibly even its ecosystem - and budgets. I was incredibly fortunate yesterday to have the opportunity to join a conversation with Buzz Aldrin. He brought up the idea that the Martian moon Phobos represents a perfect 'base' for exploration of all kinds. The more I think about it, the better an idea it seems. He made the point that human control over surface rovers is horribly inefficient when the human is sitting on Earth sipping tea. It takes between 10 and 20 minutes for light to travel from the Earth to Mars and back again, depending on where the planets are in their orbits. Controlling a rover on Mars becomes a matter of sending complex 'instructions for the day' and having the rover cautiously inch its way around.

Suppose instead the human operator was perched on the 'underside' of Phobos - which is tidally locked to always face the same way towards Mars. Phobos is about 2000 miles above the surface of the planet and zips around the equator every 7 hours or so. Combined with a couple of communication satellites you could have near instantaneous contact with almost any location. All manner of robotic devices could be sent off to roam Mars, even atmospheric drones - flying the dusty skies - with direct supervision and piloting. In the meantime, the biologically dirty humans sit on Phobos sipping their tea and keeping their microbes off the planet. It's also much, much easier to land and escape from a place like this, there are likely raw materials - even water - places to hide from damaging radiation, and Phobos itself is of immense scientific interest, potentially offering clues to planetary origins in the solar system.

So, perhaps the best way to explore Mars is to go to Phobos.

Retrograde planets

The field of exoplanets is moving so fast it can be hard to keep up. A truly surprising result in the last few days is the apparent misalignment of a significant fraction of planetary orbits with the spin-axis of their parent stars and the observation that 20% of a particular sample of planets are actually orbiting in the opposite sense to their star's spin - in retrograde motion.

The general consensus (actually harking back to Laplace and Kant in the late 1700's) is that planets and stars form together as dense regions of nebula or molecular clouds are contracted by self-gravity. Since angular momentum (spin) is a generally conserved quantity these clouds form rotating disks of gas and dust - with the star forming at the higher density center and planets forming in stages from the disk material. In this picture planets should always orbit in the same sense as the star spins. So what's going on out in the real universe ?

The bottom line is that we don't yet know. A pretty good candidate explanation is that planetary systems can get rearranged over time. Planets within a system exert gravitational pull on each other, and this can result in a variety of behavior - from gradual orbital change to chaotic behavior. Stars are also more often than not part of binary or multiple systems - in orbit about a common center of mass. These distant heavyweights can also, over hundreds of millions of years, cause planetary orbits to vary in ellipticity and in inclination - sometimes causing a planet to pass close enough to the parent star that tidal friction eventually circularizes it. Although snared by the star, the planetary orbital inclination may still reflect that earlier jostling.  The end result is that some fraction of planets look to be askew - possibly even retrograde.

It raises all sorts of interesting questions. I'm still scratching my head over the retrograde planets - in these systems then tidal forces will, eventually, pull the planets into their stars. So either these are young systems, where the planets have not yet suffered orbital decay, or they're still being tugged at by unseen companions. Or the physical model for putting them there is wrong in some way.  I dare say that there will be a flurry of new papers to address this. If the mechanism for setting these planets into their current places is as proposed then it also means that those systems probably couldn't harbor terrestrial type worlds - at least not in their inner regions. It's fabulous stuff, nature is always surprising us.

Hostile planet

Sometimes it just seems that the Earth is out to get life. Often, in discussions of what makes a planet habitable - nice liquid water oceans, mild temperatures, gentle seasonal changes - I think we can forget that even a good planet for life may also show an extraordinarily hostile side. I was reminded of this in reading a short piece about an ocean drilling program in the Pacific that may help with our understanding of so-called 'supervolcanoes' (like Yellowstone). Supervolcanoes are a breed apart as volcanoes go - and are possibly due to the buildup of huge amounts of magma that doesn't manage to leak through the Earth's crust politely, and instead goes 'bang' in a big way. The vast amounts of material, in particular gases, that a supervolcano can release have long been considered as a sure fire way to cause mass extinctions and drastic variations to the Earth's climate and environments.

Equally, it seems very clear that without robust geophysical activity a planet like the Earth would be a much less habitable place. The carbon-silicate cycle helps maintain atmospheric CO2 at a level that keeps surface temperatures in the liquid water range, and is critically dependent on geophysical cycles. It also seems likely that the Earth's geophysical magnetic field helps prevent atmospheric loss (and in particular the loss of hydrogen, which comes from dissociated water molecules, and therefore results in the 'drying out' of a planet). Some research even argues that planets somewhat more massive than the Earth, so-called super-earths, may therefore be more amenable to life because all this geophysical activity is more guaranteed than it is on a smaller world.

There's a grand process at play here. While we as a species like stability, the larger phenomenon of life definitely requires planets that are still kicking - geophysically that is. Not only that, but the very process of evolution itself, the natural selection of traits, nature's endless experimentation, seems to ultimately benefit from the occasional disaster. Mass extinctions are bad news for the species getting the pink slip, but terrific news for the thousands of new species that will emerge at a later time. So, supervolcanoes; to use an old adage, can't live with them, can't live without them.

No O2 please, we're multicellular

A long standing piece of received wisdom bit the dust this week. All multicellular animal life on Earth had been thought to require oxygen to survive - be it atmospheric or dissolved in water. Danovaro et al. now report the discovery of three species of multicellular life (metazoans) living happily in the oxygen devoid, highly salty, muck at the bottom of the Mediterranean. They belong to a family known as Loricifera, about 1mm in size.

Unlike creatures like us, these tiny organisms forgo cellular mitochondria (the energy power plants of most multi-cellular life) for other molecular structures than can produce energy carrying molecules without the need for oxygen. This is a trick that had been thought to be the sole province of single-celled microbial life - like bacteria and archaea. In fact, just as the mitochondria are the end result of endosymbiosis - the assimilation of a previously symbiotic organism - it seems that the same is true here, a molecular tool from a useful microbe has become part of the genetic makeup of these teeny animals.

The standard lore - at least the one I generally spout - is that multi-cellular life arose on Earth because the increasingly oxygenated atmosphere 1-2 billion years ago enabled life to exploit a new and rich, energetically favorable biochemistry. What would be really interesting now is to figure out whether these sub-Med creatures are ancient or a later 'backwards' adaptation of multi-cellular life ?

Clearly as well, for astrobiology, this raises some fascinating questions. One that immediately springs to mind is the issue of whether or not a subsurface ocean on Europa could sustain complex life. People have debated how oxygen might find its way into the liquid interior of the moon, but it's tricky without invoking mechanisms that can seem a bit contrived. So, if multi-cellular life can function using a different biochemical trick to do without oxygen I'd say that the bets might need to be revised. [ZPHM5ZUP82SE]

Pictish This

I'm not big on worrying about communicating with life on other planets. While I feel that it's important that someone else worry about it (as for example the people at SETI do), I think that it's such a high-risk enterprise that I'd rather spend my days doing something else. That being said, there are still some intriguing ideas that crop up in relation to the idea of talking to an utterly alien civilization. Not least of these are the questions surrounding what might constitute common ground for communication. I'll confess that as a kid I  chuckled at the Pioneer spacecraft plaque - here on the left - not because I knew better, but because I imagined that nude humans with a raised hand might constitute a declaration of extreme hostility to any decent alien life. Here's what I think of you, alien !

Of course many serious people have applied careful logic and reasoning to figuring out such communications. The problem I see is that it's incredibly difficult to decipher even a deliberate message between humans - when separated by the gulf of time or space. I was reminded of this  when reading about the effort to decode the carvings made by an Iron-Age Celtic people known as the Picts. The Picts were around between 300 and 800 A.D. The rather wonderful stone patterns and images could be either just that - nice pictures with symbolic importance - or possibly a real, complex, written language. No one knows, but a careful statistical analysis of the arrangement of the Pictish pictures now suggests that they are consistent with a real writing system. That only leaves one problem - how to decipher it ! At the risk of sounding like a naysayer, if it's this hard to interpret the handiwork of our own species, just imagine how hard it will be to deal with an unknown alien species.

Of course, the Picts didn't (as far as we know) use complex mathematics, or have a detailed knowledge of the physics of nature. Those could indeed serve as a common ground, although one wonders how easy that would be - does an alien description of atoms or quantum mechanics look like ours ?

The ups and downs of life

We tend to assume that when (optimistically) we finally locate a planet that looks and smells an awful lot like the Earth, it will be in a 'normal' state - not in the throes of global change. This could be a very dangerous assumption - 'normal' may actually be a state of flux. This is a drum that I'm fond of beating; Earth today is just a snapshot of a bewildering array of possible conditions.

A paper by Napier a few weeks ago raises the possibility of an astronomical explanation for a fascinating period of terrestrial global change some 12,000 years ago - known as the Younger Dryas cooling (the Figure here shows this, taken from the GRID-Arendal website). He sets forth an intriguing case for the large scale impact of debris from a still dispersing comet - now known as the Taurid Complex - across much of the northern hemisphere at this time. These impacts could have ignited surface forestation, consistent with evidence of clear carbon deposits in the geological record, among other pointers. Subsequent atmospheric soot, presumably along with cometary dust, could have then helped cool much of the planet by 5-10 Celsius for the next thousand years or so.

What's interesting here is less the mechanism of the cooling - most theories involve not comets but the shutdown of major ocean thermal transport loops in the Atlantic - but the fact that the planet could experience such an abrupt climate change without a colossal smoking gun. Whole biota got bumped and reshuffled, several dozen species of large mammals are seen to vanish during this episode. It has even been suggested that this could have helped push humans to an agriculture based society - fewer juicy bison to hunt, altered local climates, and dramatic changes of forests into tundra.

Suppose an unsuspecting observer chanced across the Earth in the midst of the Younger Dryas. They would still deduce the presence of a biosphere (from atmospheric chemistry among other things), they would measure the global temperature, perhaps even the extent of photosynthetic life and pigmentation. They would then - if like us - construct an elaborate model of this planet, tweaking it to match the observations, and then categorizing it. The problem is that if they had happened to looked a few thousand years earlier or later then their model would be very different. The Younger Dryas was a mild fluctuation, but what about the big ones, like the mass extinction of perhaps 90% of surface life 250 million years ago? Snapshots of planets carry an inherent sampling error, it's not clear to me how we allow for that in our models, but we should definitely be aware of the problem.

Paradox Earth

On the tail of my post about planetary snowballs, the Faint Young Sun Paradox has raised its head again. Three to four billion years ago the Sun was only about 70% as luminous as it is today - it was young and its core temperature was a little lower, so the output of radiation was somewhat less. This has long been a vexing issue for the early Earth. The geological record firmly points to a warm world with plenty of nice liquid water, but if the Sun was heating the planet so much less then something must have offset this to prevent a global freeze. A long postulated solution, and a seemingly good one, has been that the young Earth must have had more atmospheric carbon dioxide, and possibly methane to boost the greenhouse effect and keep things warm.

However, a new study of ancient marine sediments (banded iron formations) by Rosing et al. (and a great discussion by Jim Kasting) adds significantly to other geological evidence that atmospheric carbon dioxide in that young Earth was at about the same level it is today. So how do we solve the paradox ?

Rosing et al. bring up an idea that has been around before - that the young Earth just absorbed more of the sunlight hitting it, by virtue of being less reflective. One way to do this is to alter the amount and type of cloud cover. Because we think a lot of modern clouds are 'seeded' by the muck that life itself (not just us) dumps into the atmosphere, then a young Earth - with a different biosphere - could have have less reflective cloud cover. This is by no means a done deal, but it's certainly interesting to reconsider, and the paradox of the faint young Sun continues to intrigue.