Where in the Solar System is It?

What planet do we know of that has deserts and crater fields and looks rather reddish?  Where the sands of time have covered up any traces of water that may have flowed across the surface? That has scenes like this one?

Where is this?

One of the most intriguing things about studying the surfaces of other worlds is figuring out just how they came to be the way they are. Planetary scientists know of several processes that shape solid surfaces: cratering (made by incoming projectiles), weathering (caused by liquid or wind erosion (which is itself often termed “aeolian”)), volcanism (molten materials from deep beneath the surface that flow across terrains and cover over what was there before, or disrupt the landscapes with calderas and pits), and tectonism (the processes that fault and fold the surface of a planet or moon (such as earthquakes, mountain-building)).

So, if you look at a surface like the one shown above, you see no water, but you do see dunes and fields of sand and  dust. This tells you that at least wind-blown erosion and deposition are taking place.  You can also see some circular impressions that turn out to be the ancient, eroded remains of impact craters. Erosion takes time, which means that this surface is not  young and fresh. Weathering and deposition are covering up what’s left of these craters and various surface measurements give an estimated age of the craters themselves at about 140 million years old. And so we ask again: what planets in the solar system have evidence of aeolian (wind-blown) weathering and ancient impact cratering?  And, whose sands look some what reddish?

What does scene this tell you about the planet where this terrain lies? What assumptions can you make to help you guess where this scene is?  Think about it before dragging your pointer across the blank-looking area between the ( ) for the answer.

(It’s on Earth — in a desert area in Libya (northern Africa), that sports a pair of ancient impact sites called the Arkenu craters. This image was provided by astronauts aboard the International Space Station.)

Venus Express Looks for Earth Life

In all the excitement about planetariums and U.S. politics (and the insanity that is ensuing), poor little Venus Express hasn’t been getting much attention. This is a mission launched by the European Space Agency to study our neighboring planet. It’s loaded with cameras and heat-sensing spectrometers and other instruments so that it can tell us more about that cloud-shrouded world. Well, as it turns out, those instruments can also look at Earth as if it were an alien planet and figure out if it’s habitable.

Yes, indeed folks, we DO KNOW there IS life on Earth. We know it because we’re here. Live evolved here on Earth beginning some 3.8 billion (or perhaps earlier) years ago, spurred by a mix of chemical elements leftover from the formation of the Sun and planets. Some of that “life stuff” was created inside other stars that died long before our solar system existed. It’s a cosmic thang! But, all that’s in the past. Venus Express is looking at Earth now and helping us ask some kind of importan questions, like “What do life signatures look like on a planet?”

Images of Earth (top, from NASA's solar system simulator) and spectra showing the signatures of water and oxygen in our atmosphere, as seen by Venus Express and its VIRTIS system. It took these spectra between April and August 2007. Courtesy ESA/VIRTIS/NAF-IASF/Obs. de Paris-LESIA.

Here on Earth, there’s life ranging from microbes to us monkey-types, and at each level, it leaves clues to its existence. For example, us humans are putting out huge amounts of carbon dioxide, which can be traced in our atmosphere.

Plants, on the other hand, are bright in infrared light, and very soon we’ll have detectors able to discern the signatures of plant chlorophyll on our planet (and others). However, the biggest clues about whether our planet can sustain life are the signatures of oxygen and water in our atmosphere, which Venus Express can see quite nicely, thank you very much.

Okay, you think, big deal!  We already knew all that about our planet.

True. But, if you saw those same signatures on another planet, you’d get all excited, wouldn’t you?  Such observations would tell you that the planet is capable of sustaining life that relies on water and oxygen.  If we’re lucky, and using such instruments as Venus Express has, we might even be able to detect stuff like molecules of chlorophyll.

If you keep the instruments aimed at a planet over a long period of time, as Venus Express is doing with Earth, you can also learn things about the weather systems on that planet (because atmospheric changes over time can be mapped), and maybe even something about oceans and glaciers, which have their own unique ways of interacting with the atmosphere.

The amazing thing about the Venus Express observations is that, from its point of view, Earth is less than a pixel wide. It appears as a single dot.  Which is a LOT like how planets around other stars look to us right now. Yet, it was able to get detailed spectra of Earth’s atmosphere and figure out that the conditions for life exist here.

Since we’re on the verge of finding Earth-like planets, astronomers using techniques and instruments similar to those of Venus Express will very likely be able to track down any life-bearing (or life-bearing-capable) worlds.  The one thing we won’t be able to tell about that distant life is whether it’s intelligent or not. That will have to wait until we intercept alien signals and can figure out what they’re saying to each other (and the cosmos). (Let’s hope we don’t get the equivalent of THEIR political debates being broadcast to the neighbors — I’d hate to find out that the Greeblethorax Old Party on 55 Cancri IV also doesn’t like planetariums!)

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And Climate Change

Sometimes you can’t keep ahead of the news. Especially in science.  As I was working on the climate change exhibits for California Academy of Sciences earlier this year, I’d keep tabs on research and discoveries in Earth sciences, particularly Earth’s atmosphere and oceans. And, as fast as I’d write up something from peer-reviewed science, there’d be more information and discoveries coming in.  Which is great, but when it comes to climate change, it seems like it might be chronicling drastic change that we neither need nor want.  But, that’s the nature of science research — it reports on what’s happening and tries to find out why it’s happening.If we’re smart, we heed what we see and take action.

Tomorrow there’s a peer-reviewed science paper coming out that I wish had come out earlier, since it would make a striking addition to the exhibits. It states that as Earth’s oceans absorb more carbon dioxide generated by human consumption of fossil fuels and other activities (which warms up oceans and causes them to become more acidic), sounds will travel farther underwater. What’s the big deal, you might ask.  Well, noisier oceans affect marine mammal, for one thing. And, there are likely other effects that reverberate throughout the ocean environment.

Image credit: © 2008 MBARI (Base image courtesy of David Fierstein). This illustration shows how increasing carbon dioxide in the atmosphere leads to an increase in the acidity of seawater, which in turn allows sounds (such as whale calls) to travel farther underwater.

This projected impact on ocean sound is the result of calculations by Keith Hester and his colleagues at the Monterey Bay Aquarium Research Institute (MBARI) in Moss Landing, Calif. Their paper is coming out in tomorrow’s (October 1, 2008) issue of  Geophysical Research Letters, a journal of the American Geophysical Union (AGU).

As the oceans become more acidic, sounds will travel farther underwater and the level of underwater noise will rise. This change in chemistry will have the greatest effect on sounds below about 3,000 cycles per second (two and one half octaves above “middle C” on a piano).

This range of sound includes many of the underwater noises generated by industrial and military activity, as well as by boats and ships. Such human-generated underwater noise has increased dramatically over the last 50 years, as human activities in the ocean have increased. For marine mammals that also use this range of sounds to communicate, it’s like having your neighborhood go from one of relative quiet to one where the neighbors are blasting their stereos and revving their engines all the time.

Cassini at Enceladus: More!  More!

These are the infamous tiger stripes in the region on Enceladus where Cassini scientists spotted material coming out of vents.  It’s a false-color mosaic–meaning that several sets of images from the Imaging Subsystem were pieced together and colored to highlight specific units of the surface that scientists want to study. Here’s what the Cassini mission press release has to say about this image:

Areas that are greenish in appearance are believed to represent deposits of coarser grained ice and solid boulders that are too small to be seen at this scale, but which are visible in the higher resolution views, while whitish deposits represent finer grained ice. The mosaic shows that coarse-grained and solid ice are concentrated along valley floors and walls, as well as along the upraised flanks of the “tiger stripe” fractures, which may be covered with plume fallout that landed not far from the sources. Elsewhere on Enceladus, this coarse water ice is concentrated within outcrops along cliff faces and at the top of ridges. The sinuous boundary of scarps and ridges that encircles the south polar terrain at about 55 degrees south latitude is conspicuous. Much of the coarse-grained or solid ice along this boundary may be blocky rubble that has crumbled off of cliff faces as a result of ongoing seismic activity.

Wouldn’t it be fun to hike this area? Perhaps in the future, planetary geologists will bring their equipment here to sample the surface, measure its properties, and give all of us here on Earth the ultimate close-up pictures of this fascinating moon.

Just to give you an idea what Voyager 2 saw, here’s the image we all marvelled over 27 years ago this month. Even then, we were all fascinated with the juxtaposed terrains and mysterious cracks on this icy surface.  What a difference nearly three decades makes!

I’ve written before about the scene at Jet Propulsion Laboratory in Pasadena that night, when Enceladus showed us her stuff. It was a noisy, wonderful experience, made even  more exciting by the fact that when this picture came down from the Deep Space Network and scanned across the screen, it was during a live broadcast of Nightline. A lot of science writers and planetary scientists were standing around watching, and thus we were all together in one big happy family jabbering to each other about what we were seeing on the screens. We made so much noise when we saw the pics that the floor directors for Nightline had to shush us several times, pretty much to no avail!  Hey… we were watching planetary science history unfold before our eyes.  With all due respect to Ted Koppel, Enceladus was far more fetching and mysterious, and we weren’t going to let the chance go by to do instant science interpretation on that amazing image!

What Defines a Planet?

That is still a good question even a couple of years after the big announcement that Pluto is not a planet. In fact, it’s the first of many other questions about the pieces and parts of our solar system and it’s also at the heart of how we classify things that exist here in our little niche of the galaxy. 

The system of classifying planets began as soon as the first planet was discovered moving across the sky. Well, perhaps even earlier than that, when Oogh and her friend Moogh stepped out of their upscale cave in the Dordogne or the Rift Valley (or wherever it was that the first stargazers set up shop) and saw the Sun and classified it as a shiny thing in the sky but different from the shiny thing they saw in the sky at night (and sometimes during the day) that we now know as the Moon).

The first five planets (derived from the Greek word “planetes” for “wanderers”) discovered (Mercury, Venus, Mars, Jupiter, and Saturn) were simply classified as bright things that weren’t stars, and they seemed to “wander” against the backdrop of stars. Okay, that’s simple enough. Everybody could buy that without too much disagreement.

That classification worked pretty well (aside from the problem of what the Moon was) for a while, until Galileo used his telescope to spot things that seemed to be orbiting Jupiter. What were those?  And, what about those other weird things he saw poking out from either side of Saturn?  Was it possible that wanderers had their own wanderers?

In the early 1600s, the solar system was already getting complex. From that point on, astronomers had to deal with finding a couple more big planets (Uranus and Neptune), and then the unimaginable discovery that there were moons around those planets. In addition, there were these pesky rings–which turned out to be, in essence, thousands of pieces of ice–orbiting Saturn. When telescopes got better, people started to find asteroids. And, of course, there were the comets. How to classify those?  Like it or not, classifying things in the solar system wasn’t easy anymore.

In recent times, we’ve extended our ability to see a whole class of small worlds that have orbits extending from the orbit of Neptune out to well beyond the orbit of Pluto (which was discovered early in the 20th century) and its companion Charon (discovered in 1970). How do you classify those? They’re bigger than asteroids and, we thought, smaller than moons.

It became pretty clear very fast that, for planets anyway, the old moniker of “planet” was being used to cover a lot of worlds that really needed better definitions. Calling everything of a certain size “planet” is sort of like calling everything that has wheels on it a “car” (which would then include tricycles, unicycles, bicycles, trucks, airplanes, trains, and so on).

Logically, it makes sense to define worlds by their size, for example. Which works to a certain extent, although then you have the specter of trying to figure out just what makes the cut size-wise and what doesn’t. This is where Pluto and its infamous “demotion” come in for attention.  It’s a tiny world — actually, it’s a double world with Charon–and some of the larger moons of the solar system are approaching its size. In fact, the recently discovered object Eris is actually bigger than Pluto.

As I’ve pointed out in my other articles about classification, the way we classify stars and galaxies is closely tied to their intrinsic properties and evolutionary histories (and futures). Those are useful categories that tell us something about the stars and galaxies.  We had to do the same thing for solar system objects.

So, when newer, bigger worlds were found that seemed to approach Pluto’s size, it was clear we needed to figure out what the term “planet” really referred to.  Astronomers did this with stars and galaxies by simply tweaking the categories they already had to reflect the increasing complexity of what they were finding in the sky.  Logically, planetary scientists needed to figure out what cases merited the term “planet” and which merited something else.

The problem is, everybody got attached to Pluto being a “planet.” And, we had these big, emotional arguments about “poor old Pluto” as if that world had feelings and we’d be ruining the character of the solar system by correctly classifying Pluto in its proper place. Those “debates” obscured a really cool fact: that the solar system is an amazingly complex place with structures that we were only just beginning to discover.

The fault, dear readers, was not with Pluto. It was with us, and our all-too-human propensity to turn planets into pets or cartoon characters.  It was also with our definition of “planet” and the way we applied it to certain solar system objects and not others.

And so, the International Astronomical Union took on the task of figuring out what “planet” means these days and which worlds got to be called by that name. It wasn’t easy, but they managed to come up with a more-or-less logical way of applying the word to solar system bodies that met certain criteria.  In particular, they invoked a dynamical basis for classifying planets into sub-categories such as dwarf planet.

Those categories are defined by answers to questions about the evolutionary history of a solar system object:  has it gotten big enough and does it have enough self-gravity to make itself more round?  does it orbit the Sun or some other object? Has it cleared out its own orbit?  Where does it exist? The answers for each world help define whether it’s a planet, a dwarf planet, and so on.

Now, the system is still far from perfect. In fact, it’s more complex and there’s a lot of debate about certain aspects of it. But, it’s a step in the right direction. And, since nothing is set in stone, I predict that there WILL be changes and tweaks to these definitions as time goes by.  This is not a bad thing. It is, in fact, one of the ways in which science works.

If you are truly fascinated with this topic, I refer you again to part one of Mike Brown’s excellent blog entry called “What’s in a Name?” (which I found after I started working on this three-part series). He has some good thoughts about this complex topic from the viewpoint of a scientist who’s been finding and naming worlds lately.

There’s also an event taking place starting tomorrow called the Great Planet Debate. It’s a meeting of planetary scientists who will discuss the latest solar system findings in an effort to further refine the answer to the question I posed above: what is a planet?  Interested folks can sign up to view a debate to be held on Thursday afternoon between Dr. Mark Sykes and Dr. Neil deGrasse Tyson. It should be very interesting!  The debate, moderated by National Public Radio “Science Friday” host Ira Flatow, is free and open to the public, and will be streamed live on the Web.

(NOTE: thanks to VagueofGodalming for pointing out that the debate is Thursday, not Friday. I misread the calendar.)