the solar heating at that season was strongest.
Some intriguing dark patches on the ground were
also seen, but couldn’t be unambiguously
interpreted.
When Cassini’s radar viewed TItan’s northern
polar regions for the first time in 2006, however,
there was no doubt. While these areas were still in
winter darkness, the radar found hundreds of
lakes — literally pitch-black to the instrument —
consistent with a hydrocarbon composition. Many
were joined by small channels, and soon three
larger bodies were identified, which earned the
Latin name Maria (seas, like on the Moon). The
International Astronomical Union (IAU) Committee
on Planetary Nomenclature (we can’t just call
them anything) agreed that Titan’s seas should be
named after mythical sea monsters. In order of
decreasing size, they are Kraken Mare, Ligeia
Mare, and Punga Mare. Kraken sprawls over some
1,000 km; Ligeia is about 400 km across; and
Punga is a little smaller (Figure 6).
As the Cassini mission marched on, so did
Titan’s seasons, and in 2009 (northern spring
equinox on Titan), the Sun began to rise on Titan’s arctic.
Cassini’s near-infrared spectrometer — which can peer
through Titan’s hazy atmosphere a little better than
Cassini’s camera (Figure 7), but only over small areas at a
time — caught a remarkable glint of the Sun reflected in the
mirror-smooth surface of one of Titan’s lakes (Figure 8).
Also by the IAU convention, Titan lakes are named after
terrestrial ones — this one got named Jingpo Lacus, after a
lake in China whose name (Jingpo) means ‘mirror.’ Who
says scientists don’t have any imagination!?
This dead flatness had also been indicated by the dark
radar appearance of the lakes and seas, and by special
radar signal processing analyses of echoes from the one
large lake in Titan’s south, Ontario Lacus. Ontario Lacus is
named after its terrestrial counterpart because of its rather
similar size (about 250 x 70 km), and the radar data
showed that its surface was flat to within 3 mm.
That raised an interesting question — how big did the
winds have to be to form waves in Titan’s seas? With
Titan’s thick atmosphere, it should be simpler than on
Earth, plus the low gravity should make it easier. If the
liquid was really ethane and methane, then the lower
density and viscosity of the liquid should also make it easier
to make waves than with water. In fact, back in 2003, I did
an experiment (Figure 9) at the Mars Wind Tunnel at
NASA’s Ames Research Center in Mountain View, CA which
showed that waves start to form in kerosene (the closest
one could get to Titan liquids with relatively safe and easy
handling) at lower windspeeds than they do in water.
I used a BasicX- 24 microcontroller to interrogate SRF-04
ultrasonic ranges and Sharp near-infrared distance gauges
to measure wave heights — straight out of the amateur
robotics catalog. (It is worth noting that all this stuff —
except for the ultrasound — worked fine at Mars’ surface
pressures of 10 mbar. I was also interested in how hard it
would be to make waves in thin atmospheres like that of
Mars.)
We already knew that winds sometimes got high
enough (~1 m/s) to blow sand around. So, where were the
waves? Tune in next month. SV
SERVO 05.2015 47
Figure 9. Waves in liquid hydrocarbon. A tray containing a non-volatile
hydrocarbon (kerosene) is mounted in a wind tunnel at NASA Ames Research
measured with amateur robotics acoustic and near-infrared distance sensors
mounted on stands. The ramp at the left keeps the airflow somewhat smooth at
the edge of the tray. Until this experiment, nobody had ever really looked at wave
generation in anything other than water.
Ralph Lorenz is a planetary scientist at the Johns Hopkins
University Applied Physics Lab in Laurel, MD. He has written
several books, including Titan Unveiled, Dune Worlds, Spinning
Flight, and Space Systems Failures.
