Forgotten Planet

October 18, 2009

There’s another dwarf planet to add to the list of solar system bodies that  share minor league status with Pluto.

Newly published Hubble Space Telescope pictures show that the large asteroid Pallas is nearly spherical. In other words the body has enough gravity to pull itself into ball where all surface features are essentially the same distance from the core.

This is one criterion for a planet according to the International Astronomical Union (IAU). Hubble’s sharp view can resolve the disk of Pallas and shows that it is slightly egg-shaped, and roughly the width of West Virginia.

Mushy Mini-Planets, Incubators for Life?

April 07, 2009

If aliens sent a probe to do a flyby of Earth they might be surprised to discover that beauty is only skin deep. I’m talking about our oceans that cover two-thirds of the planet. The probe would measure Earth’s mass, and alien scientists would easily calculate that Earth has the density of rock. In other words the oceans are just a thin veneer over a ball of rock and molten iron.

But we have two true “aqua-planets” under our nose that are at last revealing themselves.  And, when things get damp, life seems to rear its head.

It was recently reported that Saturn’s giant moon Titan is a soggy world. Measurements by the NASA/ESA Cassini probe show that Titan is squashed like an under inflated soccer ball. It bulges at the middle and is flatter at the poles by about 2,000 feet as compared to the equator.

The fact that Titan is not perfectly round strongly suggests Titan may hide vast reserves of liquid methane beneath its surface that make the giant moon "squishy." This would explain Titan’s “Land-O’-Lakes” topology near the poles, where methane or ethane lakes dot the landscape. Simply put, Titan is swampy at the lower elevation poles where the methane seeps out. Large subsurface reservoirs of liquid hydrocarbons have long been suspected of replenishing the methane in Titan's atmosphere, where sunlight breaks apart methane.

But how do you squash a moon? One explanation is that Titan was once closer to Saturn. Titan also would have had to spin faster to be in synchronization with its shorter orbital period so that it could be tide-locked on Saturn (which would be expected of a moon buried deep in Saturn’s gravitational field). An orbit 23% closer than Titan’s present orbit would account for the extra squashing at the poles and bulging at the equator. But what would have caused the moon to change zip code?

The dwarf planet Ceres, biggest member of the asteroid belt, is also fat around the middle. This too means a damp mantle that is at least one-quarter water ice. That would be greater in volume than all the fresh water on Earth.

As with Titan, if there is liquid in the interior, it might have migrated to the surface. The water would have also carried minerals along with it.  This perhaps caused the mottled pattern seen by the Hubble and Keck telescopes.

Infrared spectroscopy of Ceres’ surface suggests it has a crust of carbonates, clays and other water-modified minerals. These would have been deposited long ago when the asteroid was warm enough for near-surface liquid water.

What’s more, there may have been a substantial amount of ammonia that had been mixed with the original water ice chunks that formed Ceres.  There is a chance that the bottom part of its outer ice layer might still be liquid today day, because ammonia makes excellent antifreeze. Titan is also thought to have subsurface liquid water/ammonia ocean.

Even more importantly, an ammonia-water mix might be a workable soup for incubating life. In fact ammonia has some chemical similarities with water. There is a whole system of organic and inorganic chemistry that takes place in ammonia. Ammonia also dissolves most organics as well as or better than water.

So two dwarf planet class bodies are so soggy, they may be homes for microbial life.  NASA’s Dawn spacecraft arrives at Ceres in 2015, and we’ll get more clues to the mini-planet’s history. Meanwhile, Cassini continues sending back surprises from Titan. Cassini's next Titan flyby is only 12 days away.

Shadow Play on a Dwarf Planet

March 22, 2009

Last week a truly ethereal image of several moons parading across the face of Saturn was release by Hubble Space Telescope astronomers.

These observations piqued the interest of far-planet hunters who contacted me for details.  Mike Brown and Jay Pasachoff are collecting data from several telescopes to follow a small icy moon transiting the face of a dwarf planet that is five times farther away than Saturn.

They don’t photograph any magnificent cloud bands or moon and shadow combinations as seen on Saturn. Their target is too far away and small. All they can do is following small changes in the dwarf planet’s brightness as the moon crosses it  (casting a shadow like Titan did on Saturn) and then passes behind it.

The mini-planet, called Haumea (2003 EL61 for you dwarf planet fans, a.k.a. “Santa”), is by far one of the strangest known objects in the solar system. It is as big as Pluto. But that is only along its long axis because the dwarf is shaped like a football (as deduced from the rhythmically varying light from the planet as it rotates like a fumbled football).

That’s  because the dwarf planet spins end over end in a dizzying 4-hour rotation period. If you were standing on its surface, you could actually watch the sun and stars quickly rise and set. The sun (which would be a brilliant star about 40 times brighter than the full moon) would take one minute to move the angular distance of three times the width of the full moon! If you had breakfast at sunrise, it would be noon by the time you finished your orange juice.

The elliptical shape as a result of the fast spin tells us what the moon is made of. If it were ice it would be even more elliptical. So it must be almost entirely composed of rock. But it is so bright it must be glazed with ices. Also, the orbits of its two satellite allow astronomers to use straightforward Newtonian physics to calculate the planet’s mass.

For the next few years the orbit of the satellite Namaka is nearly edge-on to our line-of-sight. This is because Haumea is in a 283-year orbit and so we see the moon's orbit at different inclinations, like the rings of Saturn. The last time the moon’s orbit was observable as edge-on was during the Civil War (if we had telescopes big enough back then).

Over the course of Namaka's 19-day orbit around Haumea, the moon passes in front of and then behind the dwarf planet. During eclipse the total amount of light from the system drops by one percent.

Measurements of the precise timing of eclipses and transits will allow researchers to learn a tremendous amount about a dwarf planet that is otherwise just a blip of light.   For starters, they’ll be able to nail down the size of Haumea to an accuracy of 12 miles, and better define its unusual shape.  They could even synthesize a map of its surface features as was done for Pluto during Pluto-Charon transits in the late 1980s.

The dwarf planet’s weird condition is clearly the result of an ancient collision with another Kuiper belt object. The collision stripped away most of the planet’s icy mantle leaving behind a rocky core.  The oblique impact caused Haumea to spin rapidly, stretching it into its football shape. Some of the debris from the impact coalesced to form companion satellites.

This scenario is similar to what we think happened to the planet Mercury. It is largely an iron ball with a thin veneer of rock. Mercury was thought to once have been much larger but was smashed by a huge object early in the solar system's  history that removed much of the rocky outer layer and left mostly the iron core.

So a very distant and obscure dwarf planet has a lot to tell us about the demolition derby days of the early solar system. And, fortuitously we live at a time when a brief celestial alignment is yielding new secrets about the “undiscovered country” of the vast outer rim of the solar system.