Holiday Greetings

Merry Christmas! And for those who celebrate other holidays — or none at all — best wishes for the season. Centauri Dreams will not publish on December 25. Normal publication resumes on the afternoon of the 26th.

Exoplanetary Weather: From a Single Pixel

How much information can you extract from a single pixel? That’s a key question for exoplanet studies as we look to the day when advanced telescopes can actually see a planet orbiting another star. But a single point of light seems to offer scant value, which is where Enric Pallé (Instituto de Astrofísica de Canarias) and colleagues go to work. They’ve been looking at how that single pixel changes over time, and what we might glean from it in terms of planetary details.

A key factor is cloud cover. Using data from Earth’s weather satellites, the scientists have been able to discover consistent patterns associating clouds with arid or rainy landmasses. Pallé explains:

“The trick lies in interpreting the movement of the Earth’s surface and the clouds as periodical signals, just as if we were to observe the spots on a spinning ball appearing and disappearing…[O]n a global scale clouds aren’t as random and chaotic as is generally believed, but instead follow a pattern marked by continental orography and oceanic currents.”

Cloud cover analyzed

The team studied two decades’ worth of data to produce its computerized model of Earth’s brightness. Over time, cloud patterns would help astronomers figure out the rotation period as particular brightening showed up with each rotation. And with the length of the day established, variations during that period would provide some indication of weather changing on the surface.

Image: Clouds are indicative of atmospheric pressure and existing temperature. Terrestrial cloud cover can be seen this three-dimensional representation, obtained from combined measurments from various meteorological satellites. The cloud distribution is represented during the phenomenon known as “El Niño” (1997-98) and shows anomalies in sea surface temperatures. Credit: NASA.

So an extraterrestrial civilization looking at Earth just might be able to say it had detected a living planet, one probably containing clouds and liquid water. Pallé and team had a poster at the Extreme Solar Systems conference last summer at Santorini that explains the latter:

Such variability is likely to be related to the atmospheric temperature and pressure being near a phase transition. Thus, such observations would support the possibility of liquid water on an extrasolar planet.

The assumption is that the repeated appearance and disappearance of clouds indicates active weather, and the contrast to worlds like Venus, whose brightness doesn’t change, is obvious. So are the limitations of the method, but add it to spectroscopic observations identifying elements in the planet’s atmosphere and you begin to piece together a picture of a living world.

How best to put such data to work? From the paper:

…we could learn if dynamic weather is present on an Earth-like exoplanet, from deviations from a fixed phase curve. In contrast, a cloud-free planet with continents and oceans would not show such light curve deviations. With phased light curves we could study local surface or atmospheric properties with follow-up photometry, spectroscopy, and polarimetry, to detect surface and atmospheric inhomogeneities and to improve the sensitivity to localized bio-markers. Finally, we have also provided guidance for the necessary specifications for future space missions.

The paper is Pallé, et al., “Identifying the rotation rate and the presence of dynamic weather on extrasolar Earth-like planets from photometric observations,” slated to appear in The Astrophysical Journal; full reference when available.

2007 WD5: More on the Mars-Crosser

The latest on the asteroid approaching Mars, with potentially Tunguska-like dangers, is that it will likely pass a safe 48,000 kilometers from the surface at about 1100 UTC on January 30. This news release describes the possibility of an impact as ‘unlikely,’ but goes on to say that if it does occur, the best view of the event will come from the Mars Reconnaissance Orbiter, whose High Resolution Imaging Experiment (HiRISE) would provide an unprecedented look at the crater.

While the size of asteroid 2007 WD5 approximates the object whose impact formed Meteor Crater (northern Arizona) some 50,000 years ago, the latter is thought to have been a metallic asteroid, while the one approaching Mars is probably stony. Current estimates of 2007 WD5 make it out to be 50 meters wide, traveling at some 13 kilometers per second. That’s enough to carry quite a punch, as the Tunguska impact proved in 1908, and as we may conceivably see at the end of January.

As we watch for updates, ponder the Catalina Sky Survey, which discovered the asteroid on November 20. The job of the CSS is to add to our inventory of near-Earth objects (NEOs), working under a congressional directive to NASA to identify objects one kilometer or larger to a confidence level of 90 percent or better. A later mandate brings that size down to 140 meters or larger. You can see the problem for objects in what we might call the ‘Tunguska-class.’ 2007 WD5 is almost three times smaller than our current programs are designed to track, though the CSS work on it proves that such identifications can be made.

2007 was a banner year for this particular survey. The Catalina team found 450 NEOs during the year, and that’s not the final count. Moreover, the number is growing: 400 were found in 2006, 310 in 2005. Even so, this is tricky work. The threat an object represents depends upon its impact energy, a quantity that demands knowledge of its size, density and velocity. That makes characterizing the objects that threaten our planet a major goal of observing programs, and an important objective for an early mission to an asteroid to further refine this information.

Asteroid Strike on Mars?

Perhaps it’s fortuitous that an object similar to the asteroid that caused the Tunguska event in Siberia may put on a display for us on Mars. It’s only a one in 75 chance that 2007 WD5 will strike the surface, and those odds may change again as further data are analyzed, but if it does hit, the object could strike with much the same force as the Siberian explosion. This news story reports Tunguska as a 15-megaton explosion, though as we saw on Wednesday, new work at Sandia National Laboratories has re-considered that figure and now opts for the 3-5 megaton range.

The potential impact site is near the Martian equator. If we were to witness such a spectacle, it would bring back memories of the 1994 strike of comet Shoemaker/Levy 9, the famous ‘string of pearls’, on Jupiter. The devastation of that event was stunning, far greater than 2007 WD5 would produce, but the latter might deliver enough fireworks to produce a crater the size of Meteor Crater in Arizona. Is the Solar System trying to tell us something? If so, let’s hope the powers that handle the budgetary purse strings for critical planetary radars are listening.

So keep your eyes on this intriguing object. No one wishes Mars ill, but given the equipment we’ve now got orbiting that planet for close-up viewing, the image of what an asteroid strike can do in real time could become a powerful teaching tool. My guess is that as the new observations come in, the chances of an impact will lessen dramatically, but we won’t know for a while. Until we do, this particular game of celestial roulette should make us reflect on the odds as we consider how poorly prepared we would be if 2007 WD5 were coming at us.

Addendum: This asteroid is interesting, and unusual, in being both an Earth crosser and a Mars crosser. Says Steve Chesley (JPL):

“We estimate such impacts occur on Mars every thousand years or so. If 2007 WD5 were to thump Mars on Jan. 30, we calculate it would hit at about 30,000 miles per hour and might create a crater more than half-a-mile wide.”

Opportunity is exploring a crater of approximately this size right now.

Remembering ‘The Cosmic Connection’

You knew as soon as you opened Carl Sagan’s 1973 title The Cosmic Connection that you were leaving an Earth-centric view of the cosmos behind. The title page showed, spread across both it and the facing page, a spiral galaxy. The work of Sagan friend and collaborator Jon Lomberg, the illustration included reference to Type I, II and III civilizations, the Kardashev ranking that few laymen had heard about in those days, but which Sagan’s work would illuminate for an increasingly interested public.

Cosmic Connection

The public would have been drawn first, though, to the cover of that first edition of The Cosmic Connection. A night landscape in black and white, a solitary tree outlined against the sky. But what a sky, filled with what looked like a galaxy — billions and billions of stars — rising. That image encapsulated so much of the book’s message. It juxtaposed our familiar terrain against something so vast, so filled with the potential other stars suggest, that you were forced to speculate on our place in the universe, and ponder all over again how unlikely it was that we might be alone.

Image: A bit dog-eared but still a prize possession, my copy of The Cosmic Connection.

I remember feeling these things when I bought the book in 1973, and find them reawakened as I look at that same copy on my desk this morning. The subtitle of the book is ‘An Extraterrestrial Perspective,’ and Sagan wasted no time in providing it, speculating in an early chapter on using a computer to view the sky as seen from Alpha Centauri:

We now ask the computer to draw us the sky from the nearest star to our own, Alpha Centauri, a triple-star system, about 4.3 light years from Earth. In terms of the scale of our Milky Way Galaxy, this is such a short distance that our perspectives remain almost exactly the same. From α Cen the Big Dipper appears just as it does from Earth. Almost all the other constellations are similarly unchanged. There is one striking exception, however, and that is the constellation Cassiopeia. Cassiopeia, the queen of an ancient kingdom, mother of Andromeda and mother-in-law of Perseus, is mainly a set of five stars arranged as a W or an M, depending on which way the sky has turned. From Alpha Centauri, however, there is one extra jog in the M; a sixth star appears in Cassiopeia, one significantly brighter than the other five. That star is the Sun. From the vantage point of the nearest star, our Sun is a relatively bright but unprepossessing point in the night sky. There is no way to tell by looking at Cassiopeia from the sky of a hypothetical planet of Alpha Centauri that there are planets going around the Sun, that on the third of these planets there are life forms, and that one of these life forms considers itself to be of quite considerable intelligence. If this is the case for the sixth star in Cassiopeia, might it not also be the case for innumerable millions of other stars in the night sky?

Indeed. Back then, speculations about the ‘Great Silence’ weren’t quite so hard-edged as they’ve become, and the belief that if a large-scale SETI search were mounted, it would produce results in short order seemed more than reasonable. It’s certainly what I believed at the time.

Reading Sagan’s book made the prospect of extraterrestrial intelligence not just accessible but thrillingly real. I was a graduate student in 1973, studying arcane medieval languages like Old Icelandic, Gothic and Anglo-Saxon, absolutely in love with the idea of re-creating a vanished past through words. I identified with Sagan not only because of my own long-standing interest in our place in the universe but also because he brought that same kind of passion to the study of space.

Passion was the ticket, and The Cosmic Connection was loaded with it. I can’t say it changed my life, because even as a kid I was obsessed with the questions it considers. But I know people whose life it and the later COSMOS series did change, people who went on to careers in various of the sciences because of the enthusiasm they gathered from Sagan’s work. On a broader scale, I know people who continued at their own careers in entirely other fields, but who retained a sense of the grandeur and awe of the universe that they first acquired by watching COSMOS or reading other Sagan books, particularly the absorbing Contact.

Back in 1973, I took my well-read copy of The Cosmic Connection around to my friends. We were, most of us, laboring in subjects far removed from the celestial, but the book got passed around, read, discussed at our innumerable coffee sessions in UNC-Chapel Hill’s Pine Room. It’s not as if we didn’t have other things to do, all of them on tight deadline. Now that’s skillful — to take subjects as vast as exobiology and astrophysics and render them with such transparent, supple style that amateurs like we all were then found ourselves excitedly weighing the prospects for other life in the cosmos, for missions to other stars. All of this while we were supposed to be parsing scribal errors in the Beowulf manuscript!

Making the technical accessible is one of the most difficult writing jobs there is. Sagan had the gift, but he coupled it with art (the exquisite designs of Lomberg), music, special effects, and photography from our space missions to create the classic COSMOS series. In a sense, his gift was more that of film director than writer, for a director works with a team, and not only with a script but with values of cinematography that transform written words into visual experience. His later books drew on the same values. No one in the cosmology game has equaled what Carl Sagan did with these tools, and it’s both humbling and empowering to remember how he used them on this eleventh anniversary of his death.