The International Space Station made a short pass last night (Mon.) at 9:21 p.m. The 70-second-long time exposure records it as the streak of light at right. The top of the streak is where the ISS entered the Earth’s shadow and faded from view. Tonight (Tues.), the ISS with the shuttle Discovery in tow, will make a very bright pass starting at 8:13 p.m. across the southern sky. It will first appear a fist below Orion’s Belt. Photo: Bob King
I like puddles. What surer sign of spring is there? Most of us slow down when driving through them or gingerly walk around their edges, hardly stopping to appreciate how rare a thing they are. Across the vastness of the solar system there is only one planet that has water puddles. Mercury’s hot and dry, ditto for Venus, and while liquid water may still appear from time to time on Mars, it never sits around. The atmospheric pressure there is less than one percent what it is here on Earth. Any water that might appear would quickly boil away.
The star Sirius is reflected in a puddle on a muddy
road last night. Photo: Bob King
The outer gaseous planets Jupiter, Saturn, Uranus and Neptune, don’t even have solid surfaces on the outside to support a puddle. Their atmospheres are thousands of miles deep. Water’s a component of the air on those distant worlds, but you’re not likely to find any water pooling on their pressure-heated rocky centers. We’ll never know anyway since getting to the core of one of these planets would crush us to mush.
That leaves the moons of the various planets. Jupiter’s volcanic moon Io gushes enormous pools of hot, sulfurous lava, but that’s not exactly what we’re looking for. Enceladus sprays a fine icy mist through cracks in its surface. It, along with Jupiter’s moon Europa, may have water, but it’s locked inside beneath the crust. If a little H2O did somehow bubble up to the surface, it would freeze as hard as rock almost instantly. Temperatures on the surfaces of these moons are in the 250-300 below zero range, cold enough to freeze thought itself.
Images taken a year apart of the south polar region of Titan show what may be lakes filled with seasonal rains of liquid hydrocarbons. It’s likely that rain from a large storm created the new dark areas. Ontario Lacus (Lake Ontario) is similar in size to the more familiar earthly version. The cross marks the south pole. Credit: NASA/JPL/Space Science Institute
There is however the curious case of Saturn’s moon Titan. It’s large enough to hold onto a significant atmosphere, and the Cassini spacecraft has discovered hundreds of lakes on its surface over the last few years. Where there are lakes, I’ve got a strong hunch there must be puddles too.
One of Titan’s biggest lakes, located near its north pole, is similar in size to our own Lake Superior. Credit: NASA/JPL/Space Science Institute
So have we found another place to muddy our boots? Maybe not just yet. The temperature on Titan is around 300 below, and the puddles and lakes are filled with liquid hydrocarbons like ethane and methane. At Earthlike temperatures and pressures, these substances are normally gases but in the bitter cold of Titan, they’re liquid. Scientists now have convincing evidence that fresh lakes form after clouds in Titan’s atmosphere unleash downpours of liquid methane. There’s water on Titan too, but it’s locked up as ice.
Maybe someday an astronaut will break waves on the surface of a Titan puddle, but for the present we’ll enjoy Earth’s more temperate variety. Spring arrives this Friday — time to find a puddle near you.