Strange New Worlds


John Merck

12-2-2008

The "Wow" Factor:

  • 2008 - Ice and permafrost features in the Martian arctic.

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  • 2008 - The first volcano discovered on Mercury.

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  • 2006 - Standing on the edge of Victoria Crater on Mars.

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  • 2005 - The view from the top of Husband Hill on Mars.

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  • 2004 - The surface of Titan.

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  • 2005 - The inside of comet Temple I.

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  • 2005 - The celestial gravel-pile asteroid Itokawa.

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    Recent Breakthroughs:

    These images are the products of a decade that has seen remarkable advances in our knowledge of ths Solar System - mostly thanks to the Hubble Space Telescope and a fleet of robot spacecraft. Some highlights include:

    Indeed, only one of the traditional nine planets - Pluto - has not ever been visited by a robot spacecraft (although the New Horizons spacecraft is due to fly past in 2115). With all of this new data to look at, it would be a very strange thing if interesting patterns didn't emerge. In fact, although every planetary body is unique, there are distinct patterns of similarity and differences.

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    Demotions

    And yet, the biggest planetary news of 2006 had nothing to do with science. Instead, it was the International Astronomical Union's decision to "demote" the nineth planet Pluto to "dwarf planet" status, joining Ceres, Eris, and a list of other small worlds. These days, what is a planet?

    According to the IAU, "a "planet" is defined as a celestial body that:

    It is this last criterion that worlds like Pluto, Eris, and Ceres fail. Alas, having been demoted, are they destined to be ignored as well? And, what happens when we discover a Kuiper Belt object this size of Mars that has also not cleared its orbit? (I'm just sayin' is all.)

    So, to hell with "planets" per se. This lecture is on worlds. Merck's practical definition of a "world:" Any primarily solid object that exerts enough gravitational force to pull itself into a roughly spherical shape. By this criterion we can identify a world anywhere that we encounter one, regardless of what it is orbiting. Thus:

    This leaves us plenty to work with - At least 24 proper worlds in the Solar System spanning a great size range. In addition, there are smaller bodies such as asteroids and comets.

    The worlds we are coming to know: Let's look at a few of these "new worlds" and for each, say what's new and what scientific principle it represents.

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    Mars:

    We've known for a generation that some sort of liquid - probably water - once flowed across Mars. Now robot orbiters and rovers are giving us a sense of how recently it flowed and what it was like.

    What's new

    • Analysis of the minerals found in non-volcanic Martian rocks confirm that liquid water was present, at least as ground water.
    • the rovers have sampled extensive outcrops, such as those in the walls of Endurance Crater.
    • One mineral that is common - jarosite forms on Earth only in the presence of highly acidic water. Mars' ancient water may have had a pH as low as 2.
    • Does water still flow? Possibly. We know that on Mars, the wind actively moves sediments. Indeed, the Mars rovers have witnessed dust devils and constantly have to avoid dunes. From orbit, however, we have seen water deposited features like alluvial fans superposed over dunes. These can't be terribly old.
    • And just for variety, we've seen that meteorites continue to fall onto the surface.
    • In 2008, the Phoenix lander revealed permafrost related features and exposed subsurface ice deposits.


    Possible cross-beds and hematite nodules at Eagle Crater

    Principles

    To hear the popular press, the only interesting question about Mars is whether there was ever life there. If there were, that would be interesting. One hint is the presence of methane in the atmosphere. This gas breaks down quickly, so it must be being replenished. It comes from two sources - volcanoes and life. Which is the culprit on Mars?

    • Unlike Earth and Venus, Mars lost its internal heat rather quickly because of its greater surface to volume ratio. In its first two billion years, Mars was volcanically very active. Whether volcanoes ever erupt today is a pressing question. Advocates of Martian life usually assert that Mars is geologcially dead.
    • Unlike Earth, no region within Mars has ever had the right combination of pressure and temperature to give it an asthensophere. You could say that it's thick lithosphere is one big plate. Mars did have one large hot-spot that resulted in a group of fantastically huge volcanoes - the largest in the Solar System - on the Tharsis Bulge. They grew so large because they were not on a moving plate that carried them away from the hot spot.
    • All of this bulging caused fissures to form, the largest of which, the Vallis Marineris is roughly 3000 miles long.
    • Yet we have seen no unambiguous evidence either of life or of active volcanoes.

    Tharsis bulge volcanoes and Vallis Marineris

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    Jupiter's moon Io: a world turning inside-out

    What's new

    • This place still holds the record as the "extreme" world. Io has never been observed up close when there has not been a gigantic volcanic eruption in progress somewhere.
    • Indeed, Io continues to resurface itself significantly on a time scale of months.
    • Io's volcano Tvashtar Catena treated us to a fissure eruption creating a curtain of lava 40 km. long and 1.5 km. high.
    • What's news is how extreme this volcanism is. When first discovered, speculation was that the eruptions on the otherwise frigid surface of Io might be of material that, if we encountered it on Earth, would not seem so hot - just hot by comparison with the rest of Io. In fact, Io magmas are typically much hotter than those of Earth volcanoes.

    Io with volcano Pele

    Principles

    Researchers weren't expecting recent geological activity in any of the cold worlds of the outer solar system, not to mention the volcanic contortions of Io, but the reason for these soon became clear.

    • Jupiter has four large moons: Io, Europa, Ganymede, and Callisto, in order from closest to Jupiter to most distant.
    • Io, for instance, is the innermost large moon of Jupiter. It is locked in a two-for-one orbital resonance with Europa, the next moon out. I.e., for every orbit of Europa, Io makes two. Every time Io passes Europa, Europa's gravity tugs at it, stretching it by about 100 m. That's a 100 m. rock tide! This rhythmic stretching generates huge amounts of frictional heat - enough to heat Io's interior and drive its volcanoes.
    • Europa, itself is in a similar resonance with Ganymede, and Ganymede with Callisto, although as you move outward from Jupiter, the effect diminishes because the moons move more slowly in their orbits. Europa may not be pyrotechnically warm like Io, but it is warm enough to support a deep layer of liquid water, and Ganymede has something resembling plate tectonics. Only Callisto is geologically unmoved.


    Grooved terrane of Ganymede preserves active tectonics

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    Jupiter's moon Europa becomes the Holy Grail of planetary exploration:

    What's new


    Europa

    Principles

    The same tidal resonance that heats Io heats Europa, but with some differences.

    • Outside the asterior belt, in the outer solar system, "ices" like water and carbon dioxide exist as solids and make up much of the bulk of the smaller worlds. Among Jupiter's large moons, only Io lacks significant ice because its volcanic heat has evaporated it and driven it off.
    • Europa is not heated as much as Io, so it has retained its ice, but with much of it as a liquid, warmed to its melting point by tidal forces stretching the solid inner parts.
    • Any life on Europa would primarily be driven by this internal heat, like in Earth's hydrothermal vent environments, not the heat of the sun.


    Agenor Linea (5 km. wide) on Europa

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    Cassini continues to reveal information about the geological activity of Saturn's tiny moon Enceladus:

    What's new

    • A moon this size should have long since given up its internal heat and be unable to sustain an atmosphere, yet Enceladus suddenly turns out to have an atmosphere of oxygen ions.
    • Instead of being a crater pock-marked sphere, Eceladus is geologically active, with a surface crossed by ridges, furrows, and conspicuous tiger stripes.
    • The tiger stripes ar a source of anomolous heat
    • A back lit image revealed jets of water vapor and ice crystals being vented up from them.
    • This material makes up Saturn's E-ring.
    • In 2008, Cassini made low passes over the region from which the jets emerge (The image shows a region about 16 km. across).

    Principles

    When Cassini was launched, no one expected much from such a very small body as Enceladus. Indeed, its near twin Mimas is geologically dead and rather boring. But now it's amng the most interesting worlds in the solar system.

    • But apparantly Enceladus is locked in a tidal 2:1 resonance with Dione in the same way that Io is with Europa.
    • Thus, Enceladus may be like a benign small-scale Io.

    Enceladus with geysers

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    Saturn's moon Titan begins to yield its mysteries:

    • Titan's atmosphere is 1.5 times as dense as that of Earth, and at high altitudes is suffused with an opaque smog of complex hydrocarbons. To the Voyager spacecraft, it just looked like an orange pool-ball.
    • The Cassini orbiter can see its surface in infrared light, revealing an odd dichotomy of light and dark regions.
    • Are these continents and liquid methane oceans as some had speculated? Apparantly not, however stereographic images indicate that they do represent highlands and plains.
    • Cumulus clouds of methand vapor were observed.
    • As was a water-ice volcano.
    • Synthetic aperture radar has yielded topographic information, showing a landscape of high ridges and dune-covered (?) lowlands.
    • But the best part: The Huygens lander separated from the Cassini probe, entered Titan's atmosphere and hung on its parachute for several minutes, photographng the landscape below. These photos confirmed the dichotomy between bright and dark areas and revealed what look like extensive streams or gullies.
    • The final images was from the surface where the probe came to rest. The probe recorded that the ground on which it landed was apparantly moist.
    • The sequence of photos taken during the descent have been stitched into an amazing video narration.
    • Apparantly Titan, although "arid" has an active hydrologic cycle based on liquid and gaseous methane. On Titan, water is simply a mineral.
    • Some researchers had hoped to see oceans of liquid methane. For over a year, there was no sign of liquid bodies on the surface. Finally in 2006, the north polar region has been shown to harbor many large (Great- Lakes size) lakes. .
    • In 2007, remote sensing revealed that higher elevation on Titan experience night-time and early morning methane drizzle.

    The surface of Titan (from about 10 cm. height)

    Principles

    Besides Earth, Titan is the only world in the solar system known to have an active hydrologic cycle. Thus, it is strangely similar to Earth and strangely different.

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    Why do we give this presentation in ELT?

    As more planetary exploration is done by robot probes and rovers and less through telescopes, the kind of data available has come to resemble the data the geologists usually process. The study of the planets has, thus, passed from the domain of astronomers to that of "planetary geologists." But don't cry for the astronomers because they own the last big breakthrough of the decade:

    Extrasolar planets:

    By observing the perturbations in the motion of stars caused by the gravity of orbiting planets, the presence of up to two hundred planets outside the solar system - Extrasolar planets has been inferred.

  • These tend to be very large and to orbit very close to their parent stars - so called "hot Jupiters."
  • The big news of 2008 - The first images to be captured of exoplanets circling HR8799 and Fomalhaut
  • The discovery of Earth-like planets in Earth-like orbits awaits the development of a new generation of detectors - the "terrstrial planet finders."

  • Possible exoplanets circle HR8799