From Mercury to Centauri B

Centauri Dreams‘ rarely spends time close to the Sun, preferring to focus on stars other than our own, and their planets. But the MESSENGER spacecraft’s close pass by Mercury, leading eventually to orbit, does have an interstellar connection in the person of project scientist Ralph McNutt, who is prominent not only in exploring the closest planet to Sol but also in planning a mission that would be our farthest yet, the Innovative Interstellar Explorer attempt to study nearby interstellar space.

New imagery of Mercury

Fire and ice. McNutt (Johns Hopkins University Applied Physics Laboratory) obviously enjoys working at the extremes, and one hopes for an outcome for IIE just as successful as MESSENGER has enjoyed thus far. Meanwhile, Mercury looks more or less as expected, but don’t let that fool you. As Greg Laughlin points out at his systemic site, we’re looking at vast stretches of terrain that have never before been seen, our earlier views of Mercury having been delivered by Mariner 10 flybys that saw only parts of this world.

Image (click to enlarge): Just nine minutes after the MESSENGER spacecraft passed 200 kilometers (124 miles) above the surface of Mercury, its closest distance to the planet during the January 14, 2008 flyby, the Wide Angle Camera (WAC) on the Mercury Dual Imaging System (MDIS) snapped this image. The WAC is equipped with 11 different narrow-band filters, and this image was taken in filter 7, which is sensitive to light near the red end of the visible spectrum (750 nm). This view, also imaged through the remaining 10 WAC filters, is from the first set of images taken following MESSENGER’s closest approach to Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

Does any of this remind you of the poignancy that accompanied the Voyager images from Neptune? In that case, we were looking at the last planetary discoveries the Voyager mission would make before the two spacecraft set forth into the outer reaches, where they still perform helpful science. In this case, we’re looking at the last rocky, ‘terrestrial’ class world we haven’t visited in our Solar System. Laughlin, always an elegant writer, catches the excitement of such discovery perfectly:

Nevertheless, we do gain something extraordinary whenever a new vista onto a terrestrial world is opened up. Galileo was the first to achieve this, when he turned his telescope to the Moon and saw its three-dimensional relief for the first time. Mariner 4 and Mariner 9 accomplished a similar feat for Mars. The Magellan spacecraft revealed the Venusian topography. And once Messenger has photographed the full surface of Mercury, there will be a profoundly significant interval before we get our next up-close view of an unmapped terrestrial planet. My guess is that it’ll be Alpha Centauri B b.

Just how significant that interval will be is, of course, unknown, for by ‘up-close,’ I assume Laughlin is talking about imagery from a probe within the Centauri system, a feat we are many decades (at least) away from achieving. Meanwhile, we do have New Horizons, taking us out to the icy worlds of Pluto and Charon and on into the Edgeworth/Kuiper belt.

Doubtless there are still surprises in the deep ranges of that belt, perhaps even a few rocky worlds displaced from inner orbits during the earliest stages of our system’s formation (see the comments on the systemic post for more). But if they are there, New Horizons is unlikely to stumble upon one, nor would it be able to view such a world closely if it did. I think Laughlin is right: Our next detailed look at an unmapped terrestrial planet will take place among the Centauri stars, unless they defy the growing odds and turn out to have no planets at all.

Enceladus: Making the Case for Life

Thoughts on Enceladus as a home to life have kept astrobiological debate lively, an unexpected but welcome development from the Cassini mission. The interest is understandable: Cassini has shown us plumes that seem to be the result of some kind of geothermal venting, with liquid water and geothermal energy sources all possible drivers for the formation of life. We don’t exactly know what’s going on here, but the possibility of a hydrological cycle — liquid, solid, gas — has kept theorists active, as witness a research note by Christopher Parkinson (Caltech) and team.

The early Earth serves as a possible model for life elsewhere. With photosynthesis not available, life would depend on abiotic sources of chemical energy. It’s believed this would have come in the form of oxidation-reduction processes driven by factors like hydrothermal activity, impacts, electrical discharges, or solar ultraviolet radiation. Organics may have been synthesized from inorganic molecules near submarine hydrothermal vents. In similar ways, the authors believe, Enceladus may offer energy-generating reactions that create conditions favorable for life.

I’ve never seen the case for this moon made quite so emphatically. From the paper (the italics are mine):

The combination of a hydrological cycle, chemical redox gradient and geochemical cycle give favorable conditions for life on Enceladus. To our knowledge, these conditions are not duplicated anywhere else in our solar system except our planet. Compared to Mars, Titan and Europa, Enceledus is the only other object in our solar system that appears to satisfy the conditions for maintaining life at present, even if the ability of life to evolve there is uncertain.

Plumes on Enceladus

Image: Ice geysers erupt on Enceladus, bright and shiny inner moon of Saturn. Shown in this false-color image, a backlit view of the moon’s southern limb, the majestic, icy plumes were discovered by instruments on the Cassini spacecraft during close encounters with Enceladus in November of 2005. Eight source locations for these geysers have now been identified along substantial surface fractures in the moon’s south polar region. Researchers suspect the geysers arise from near-surface pockets of liquid water with temperatures near 273 kelvins (0 degrees C). That’s hot when compared to the distant moon’s surface temperature of 73 kelvins (-200 degrees C). The cryovolcanism is a dramatic sign that tiny, 500km-diameter Enceladus is surprisingly active. Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA.

What about those other habitats we’ve been considering? The paper continues:

Mars may have had a hydrological cycle in its early history, but there is no evidence that one exists today. Titan may be a repository of pre-biotic organic chemicals, but the conditions do not appear favorable for the development of life. Europa currently may have a hydrological cycle, but it may be a closed chemical system that will eliminate any chemical redox gradient in a geologically short time. Presently Enceladus is the most exciting object in the solar system for the search of extant life.

And nowhere else have I seen the suggestion that concludes this interesting paragraph:

We have compelling evidence supporting the view that Enceladus has active hydrological, chemical and geochemical cycles, which are essential ingredients for originating and sustaining life. Planetary protection issues aside, if life does not yet exist on Enceladus, arti?cial introduction of terrestrial life to this environment would be an interesting, and most likely successful experiment.

Here’s interesting fodder for a science fiction story. If you could introduce life into an environment ready for it, what sort of life would you choose? Enceladus, like any other body on which this was attempted, would pose its own formidable constraints, but the broader question of whether life ought to be introduced into such environments is open to considerable philosophical debate. Just what constitutes planetary protection?

Of course, Enceladus offers us plenty of work before we ever reach the stage of attempting such a thing. For one thing, we’d like to know whether those intriguing plumes are transient or long-term. We’ll need information about the presence of oxidants in this environment, along with information on surface properties to study impact erosion and resurfacing. And we need a world of information about the chemical evolution of organics in ice in the presence of energetic particles.

The list could continue, but I turn you over to the paper, which is Parkinson et al., “Enceladus: Cassini observations and implications for the search for life,” in Astronomy & Astrophysics 463 (2007), pp. 353-357 (available online).

More Eyes for the Asteroid Hunt

Centauri Dreams has always advocated a robust asteroid detection program to help us get an accurate census of objects that might endanger Earth. Thus I’m happy to report on promising events at the UK’s sole observatory dedicated to Earth-crossing asteroids. The Spaceguard Center in Wales has been offered a new telescope by the Institute of Astronomy (Cambridge), the light pollution in the latter location having reached the point where observations are seriously compromised.

Fortunately, there are parts of Wales with dark skies indeed. Thus the Schmidt instrument, useful for identifying objects moving against the stellar background, should be useful not only for searching but also tracking comets and asteroids. Absent funding sources in Wales or the UK government itself, the observatory turns to private sponsorship as the potential solution. We’ll keep an eye on how that effort goes — an estimated £54,000 ought to do the trick, and as this BBC report notes, the site’s possibilities as a tourist attraction may boost fundraising.

Looked at from an international perspective, it is stunning that there is so little coordination among scientists trying to identify Earth-crossing objects or deal with the threat once a danger is identified. Yet our growing knowledge of the role impacts have played in the development of life on our planet makes the need to prepare for the worst contingencies an imperative. Located near Knighton in mid-Wales, the Spaceguard Centre offers another option for finding a potential danger and, let’s hope, for educating the public about a threat to life that is all too real.

37th Carnival of Space

The 37th Carnival of Space is up at Darnell Clayton’s Colony Worlds site. This week I would recommend planetary probe enthusiasts have a look at Music of the Spheres, where the talk is not just about the MESSENGER probe’s visit to Mercury, but about software you can run to simulate various situations in orbital mechanics. Also check Pamela Gay’s look at the Galaxy Zoo project, in which she not only offers tips for using Sloan Digital Sky Survey data but also links to an audio interview with Galaxy Zookeepers Jordan Raddick and Chris Lintott. At advanced nanotechnology, Brian Wang examines Boeing’s ideas for a space gas station, but I also want to turn your attention to his interesting post on the activation of a prototype extending Robert Bussard’s fusion ideas to version WB-7.

Starlight on a Distant Sea

Planets around other stars are too faint to be imaged directly, and although claims have been made for such detections (2M1207b is a case in point), it’s safe to say that our current techniques need significant upgrading to achieve reliable images of such distant worlds. But studying terrestrial planets is a long-term objective and numerous studies have gone into concepts like Terrestrial Planet Finder and Darwin. One day and with some instrument we will indeed be looking at an exoplanet as small as the Earth, working with estimates of surface temperatures and checking its atmosphere for biomarkers that flag the presence of life.

So let’s suppose that in fifteen years or so we’re looking at actual reflected light from a terrestrial world. What else can we learn about the place? The brightness of a planet like this can be affected by many things, including the presence of deserts on the surface or bright clouds above it. An active weather pattern would indicate the presence of a hydrological cycle like the one we see here on Earth. Such changes in brightness would be difficult to detect on planets that rotate in a few days or less, but the presence of oceans on these worlds may well be apparent. Imagine being able to look at starlight reflected off an alien sea.

Darren Williams (Penn State Erie) and Eric Gaidos (University of Hawaii), who address the question in a new paper, say it’s possible. And as we improve our instruments, potentially measurable variations might even include the seasonal blooming of land plants or oceanic algae or the coming and going of snow. All such fluctuations will vary depending on the planet’s obliquity and orbital inclination with respect to the observer. The staggering thing is the amount of potentially recoverable information from a source so dim in relation to its parent star that we cannot see it today.

In terms of liquid surfaces, the chances of detection look quite interesting. From the paper:

A reflected light curve also contains information about the scattering properties of the surface, independent of any seasonal changes. Planets with water will reflect light toward the observer more efficiently in crescent phase than in gibbous phase because of the higher reflectance at low incident angles. This glint from water will make a planet appear anomalously bright in crescent phase compared to diffuse-scattering surfaces observed in the same geometry. Light reflected from water will also impart some polarization to the disk-averaged signal, which might be measurable under idealized (i.e., optically-thin, cloud-free) atmospheric conditions.

The paper goes on to study how starlight reflected off water might be detected, and examines the light curves from a variety of surfaces in the visible and near-infrared spectrum. Half of all extrasolar planets should have the kind of orbital inclinations that would make the glint of existing oceans apparent. But the reflection of starlight from an ocean surface begins to dominate only under certain circumstances:

Specular reflection of starlight from an ocean surface occurs at all phase angles, but only begins to dominate the wholedisk signal when a planet is nearest its star as a thin crescent. Observations at such phase angles can be obtained of planets around G and F stars where they have adequate angular separation and orbit within the habitable zone.

The authors’ figures show that detections within 0.66 AU of a parent star are unlikely. To use these methods, we’ll need to stick with G and F stars, where the angular separation should make such observations possible. Indeed, planets with a surface of continents and oceans, like ours, should polarize the reflected signal by as much as 30-70 percent. Of all the tricky measurements a terrestrial planet finder instrument of this class might make, the identification of reflected water may well be the easiest.

The paper is Williams and Gaidos, “Detecting the Glint of Starlight on the Oceans of Distant Planets,” in press at Icarus and available online.