Titanic Thinking in Pictures

The grey-eyed morn smiles on the frowning night,
Chequering the eastern clouds with streaks of light,
And flecked darkness like a drunkard reels
From forth day’s path and Titan’s fiery wheels
–Shakespeare’s Romeo and Juliet

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True color and surface infrared images show features resembling clouds and a continental area about the size of Australia Image Credit: NASA/JPL

In the next half year, Titan, Saturn’s largest moon, seems poised to reveal what mysteries lay beneath its thick veil. If the January descent of the Huygens probe succeeds in capturing a close-up image, the event will mark one of the rarest photographic moments in the space age: sites from the surface of another world.

The Russians landed surface cameras on Venus. Five American probes have scanned across the rusted martian expanses and the now familiar lunarscape has become pockmarked with human bootprints. The NEAR mission even planted a robotic lander on the surface of the asteroid, Eros, on Valentine’s Day 2001.

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Hubble surface maps from four global views. Titan’s planned landing ellipse is just to the north and west of the large white-yellow region, thought to be a continent about the size of Australia.
Image Credit: NASA/JPL

But yet in the solar system, surface pictures of interesting places remain relatively rare. Even if Saturn’s largest moon lacked its interesting chemistry of methane, ethane and nitrogen, its surface would attract scientists because of something nearly as rare as surface images: an atmosphere even thicker than Earth’s.

When combined, Titan’s oily surface and smoggy skies bring together a photographer’s most exotic imagination. When the Huygens probe attempts its descent to the surface around the second week of January 2005, the moon may present all the meteorological signals that we have come to associate only with earth’s weather: lightning, rain (oily), clouds and continental shores.

Titan’s name derives from the union between the Greek gods of the sky (Uranus) and land (Gaia)–a fitting combination for what is likely the only other place in our solar system to harbor a potential shoreline.

What awaits Huygens on arrival at Titan’s surface is a guessing game. As highlighted in the banner image, the probe’s landing ellipse is in the northern hemisphere. To make the map, the haze background was subtracted from a series of Hubble Space Telescope images, leaving only topographical features. The larger and brighter features are all "real" and repeatable across multiple images in the series, showing what appears to be topography.

Uncertainties in locating a final target site for Huygens, however, varies depending on local wind conditions. The longitude is uncertain by 11 degrees due to unavoidable guesses about how strong Titan’s winds might blow in mid-January, and (more importantly), which way will these gusts push the tiny Huygens probe.

If the probe strikes land, it will impact at the speed approximated by jumping off a table top (5 meters per second) and register 16 times earth gravity when decelerating from its atmospheric top (Mach 20, or twenty times the speed of sound). It the probe splashes down in what some anticipate as lakes of oil, Huygens will have confirmed the remarkable liquid environment thought to be unique to such a distant, frozen position in the solar system. But if the Huygens descent ends in such a splashdown, the expected lifetime for the probe will likely last only minutes before sinking beneath an ethane or methane pool.

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Simulated descent to Titan, with the Huygens’ heat shield and angle of reentry designed to keep the thick atmosphere’s friction from raising the temperature above lead’s melting point. Image Credit: NASA/JPL

To survive this harrowing descent, much of Huygens’ protection is justifiably thermal. According to the JPL and ESA specifications, the front heat shield for Huygens will protect the probe during initial atmospheric entry. The plasma in the shock, just forward of the shield, will reach a temperature of around 12,000 deg C (21,632 deg F), which is approximately twice the surface temperature of the Sun.

The front shield is covered with Space Shuttle-like tiles made of a material known as AQ60, developed by Aerospatiale. This material is essentially a low-density "mat" of silica fibers. The tile thickness on the front shield is calculated to ensure that the structure will not exceed 150 deg C (302 deg F), which is below the melting temperature of lead. The rear side of the probe will reach much lower temperatures, so a spray-on layer of "Prosial" silica foam material is used on the rear shield. The overall mass of the Thermal Protection System is more than 100 kg (220 lbs), or almost one third of the probe’s entire mass.

After nearly three weeks (22 days) traveling ballistically from its stowage onboard the Cassini probe to its final Titan encounter, Huygens descent through the smog will last about 2.5 hours.


Related Web Pages

Cassini
Saturn Edition, Astrobiology Magaz.
Cassini Closes In on Saturn
Saturn– JPL Cassini Main Page
Lord of the Rings
Space Science Institute, Imaging Team Boulder, Colorado
Saturn: The Closest Pass
Voyager Image Query Form
David Seal’s JPL site -Solar System Simulator
Gregory Benford’s 1970 Essay ‘View From Titan’