Science Feature

There's Too Much Confusion

SAND dunes fill the gaps in jumbled, chaotic terrain on Mars, as seen by the Mars Reconnaissance Orbiter.
The Iani Chaos region may have resulted from ancient floodwaters that carved the haphazard channels and created irregular patches of rocks. The rocks also have linear features that reflect wind erosion in a certain direction. Other processes that could have helped form the rocks include volcanic eruptions, lake deposition and accumulated wind-blown sand.
Stepped layers that appear in certain places suggest a repeating cycle, which would hint at wind-blown or lake-bed sediments slowly building up. NASA's Phoenix Mars Lander is currently examining the history of water on Mars.

Huge Tunguska Explosion Remains
Mysterious 100 Years Later

Charles Q. Choi

A full century after the mysterious Tunguska explosion in Siberia leveled an area nearly the size of Tokyo, debate continues over what caused it.

Many questions remain as to what crashed into the Earth from above -- how big it was and what it was made of. Some question whether it even came from space at all, or whether it erupted from the ground instead.

And there is always speculation that it was caused by a UFO or famed inventor Nikola Tesla's "death ray."

Death from above?

The explosion near the Podkamennaya Tunguska River on June 30, 1908, flattened some 500,000 acres (2,000 square kilometers) of Siberian forest. Scientists calculated the Tunguska explosion could have been roughly as strong as 10 megatons to 20 megatons of TNT -- 1,000 times more powerful than the atom bomb dropped on Hiroshima.

The longstanding theory regarding the cause of the event is a cosmic impact from an asteroid or comet. In the last decade, researchers have conjectured the event was triggered by an asteroid exploding in Earth's atmosphere and measuring roughly 100 feet wide (30 meters) and 617,300 tons (560,000 metric tons) in mass -- more than 10 times that of the Titanic. But recent supercomputer simulations suggest the asteroid that caused the extensive damage was much smaller.

Specifically, physicist Mark Boslough at Sandia National Laboratory in Albuquerque, N.M., and his colleagues say it would have been a factor of three or four times smaller in mass and perhaps 65 feet (20 meters) in diameter. As the asteroid exploded as it ran into Earth's atmosphere, Boslough and colleagues calculate it would have generated a supersonic jet of expanding superheated gas. This fireball would have caused blast waves that were stronger at the surface than previously thought.

However ...

At the same time, prior estimates may have overstated the devastation the event caused.

The forest back then was unhealthy, according to foresters, so it would not have taken as much energy to blow down such trees. In addition, the winds from the explosion would naturally get amplified above ridgelines, making the explosion seem more powerful than it actually was. What researchers had thought to be an explosion between 10 and 20 megatons was more likely only 3 to 5 megatons, Boslough said.

As to whether the impact was similar to a stony, carbonaceous asteroid or a comet, "while the community has pretty much accepted the view that it was a carbonaceous asteroid, I'm not sure it's a slam dunk," Boslough said. "The main argument against it being a comet is statistical. There are a lot more small Earth-crossing asteroids than comets at least by a couple orders of magnitude. While it's unlikely to be a comet, I'm not convinced it's physically impossible."

Discovering the size and makeup of whatever hit at Tunguska could shed light on how often such a devastating impact might take place, explained NASA Ames Research Center planetary scientist and astrobiologist David Morrison.

"As interesting though Tunguska is, I'm more interested in the next Tunguska," Morrison told SPACE.com. "We know small objects are far more numerous than large ones out there, so we want to see how much damage they might be able to do."

Death from below?

Instead of a cause from above, in the last decade some researchers have suggested the Tunguska explosion actually came from below. Astrophysicist Wolfgang Kundt at the University of Bonn in Germany and others have suggested that an eruption of natural gas from kimberlite, a kind of volcanic rock best known for sometimes holding diamonds, could be to blame.

"It would have come from the molten earth, some 3,000 kilometers deep (1,864 miles)," Kundt said. "The natural gas would be stored as a fluid that deep, and when it reaches the surface it would become a gas and expand by a factor of thousand in volume, for a huge explosion."

For support, he cited the pattern the trees fell in, as well as chemical anomalies.

Even stranger ideas

Wilder theories have been bandied about over the years regarding what caused the Tunguska explosion, including:

* A UFO crash. Struck by the similarity of Tunguska and Hiroshima decades later, a science fiction writer named Alexander Kazantsev wrote a story in which the Tunguska blast was the exploding nuclear power plant of a spaceship from Mars. A few Russian scientists took up the cause and claimed to find various bits of evidence -- never substantiated -- for a civilized alien explanation.
* The annihilation of a chunk of antimatter from space. This does not account for mineral debris the explosion left behind.
* A black hole zipping through Earth. This also does not account for mineral debris the explosion left behind, and there was no subsequent explosion as such a black hole, having tunneled through the Earth, would have shot back out through the surface of the Atlantic.
* A Nikola Tesla "death ray." The man who pioneered radio and modern alternating current electric power (AC) systems was often seen as a mad scientist. One story alleges he test-fired a death ray on the evening of June 30, 1908, and once he found out about the Tunguska event, he dismantled the weapon, deeming it too dangerous to remain in existence.

All the speculation concerning Tunguska is to be expected, Boslough said.

"Lots of theories are going to pop up -- it's like a crime scene, and everyone wants to have a hand in solving the mystery," he commented. "It's fun to speculate."

Phoenix scientists are now assured they have a complete soil-layer profile in Wonderland's "Snow White" extended trench. Reaching the icy layer confirms that surface soil, subsurface soil and icy soil can be sampled at a single trench.

By rasping to icy soil, the robotic arm on Phoenix proved it could flatten the layer where soil meets ice, exposing the icy flat surface below the soil. Scientists can now proceed with plans to scoop and scrape samples into Phoenix's various analytical instruments. Scientists will test samples to determine if some ice in the soil may have been liquid in the past during warmer climate cycles.

Observations from the lander's Thermal and Evolved-Gas Analyzer made last week indicated that the soil had at some time in the past interacted with water.

Phoenix has also photographed white chunks in trenches that mission scientists think must be ice.

Mars and Saturn Get Together

By Joe Rao
SPACE.com Skywatching Columnist

TWO bright planets will approach each other in our evening sky during the next couple of weeks. They are Mars, which was so brilliant during the Christmas season of 2007 and has since diminished dramatically in brightness, and Saturn, which has adorned our evening sky since midwinter.

Both worlds are now visible about one-third of the way up from the western horizon as darkness falls.

And as a bonus, located between these two planets is the bright star Regulus, in the constellation of Leo, the Lion. Yellow-white Saturn, shining sedately at magnitude +0.4, is located above and to the left of Regulus, while the much dimmer Mars appears below and to the right of Regulus. At magnitude +1.6, Mars has now fallen to the rank of second magnitude and appears only one-half as bright as Saturn. Regulus, meanwhile is roughly midway in brightness between the two planets.

On this scale of brightness, smaller numbers represent brighter objects.

If you watch the sky carefully through the next two weeks, you'll be able to take note of the changing positions of these two planets and nearby star, relative to each other.

Key nights to watch

On the evening of June 30, you'll see Mars just above and to the right of Regulus, while Saturn sits above and to the left; the trio will resemble an arrowhead that night with Saturn making the arrow's tip.

On the evening of July 1, Mars will appear perched almost directly above Regulus. The color contrast will be quite striking, especially in binoculars: Mars appears yellow-orange, while Regulus is bluish.

On July 5 looking low in the western sky about an hour after sundown, you'll see a waxing crescent moon, with Regulus, Mars and Saturn all oriented in a straight line in that order, from lower right to upper left. Mars will be situated almost exactly in between Regulus and Saturn. On the following night, the moon will have shifted eastward, forming a broad triangle with Regulus and Saturn, while Mars will have moved noticeably closer to Saturn.

Saturn-Mars interactions

Saturn, at a mean distance of 886.2 million miles (1.4 billion km.) from the sun, takes 29.46 years to move once around the sun. Mars, at a mean distance of 141.6 million miles (227.9 million km.) from the sun, requires only 1.88 Earth-years to complete one journey around the sun. As a result, Mars' normal eastward motion among the stars is considerably faster than that of Saturn. So from our vantage point here on the Earth, Mars will periodically seem to overtake Saturn in the sky.

During the evenings of July 9 through 11, watch how the faster-moving Mars closely interacts with the much slower Saturn. Despite the difference in brightness, they still should make for a rather eye-catching pair in the western twilight glow about an hour after sundown. On the 9th, Mars is situated about one degree below Saturn. On the 10th, they are closest together, separated by 0.7 degree; Mars now appears just below and to the left of Saturn. On the 11th, Mars has moved noticeably farther away to Saturn's left.

Interestingly, when we combine the motions of Earth, Mars and Saturn, we find that the interval between conjunctions of Mars and Saturn average about every 2 years and 20 days. The last time Mars and Saturn got together was June 17, 2006. Their next meeting is set for July 30, 2010.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.