Getting Ready to Fingerprint a Comet
The Submillimeter Array (SMA) will be ready and watching when NASA's Deep Impact probe strikes the nucleus of Comet Tempel 1 on July 4th. The impact is expected to excavate material from the comet's interior - material left over from the earliest days of our solar system.
"The SMA will be the only operational millimeter or submillimeter array observing Tempel 1 at the time of the impact" said astronomer Charlie Qi of the Harvard-Smithsonian Center for Astrophysics (CfA). "As such, there's no telling what we may see!"
Only a few observatories can study comets in the same wavelength regime as the SMA, but none match SMA's exquisite resolution. "The SMA will be the only submillimeter telescope in the world to make an image of the molecules released by the impact," said Glen Petitpas (CfA).
By observing submillimeter radiation, the SMA can "fingerprint" comets and determine their molecular composition. "SMA will observe mostly cool gas and dust with relative chemical abundances that may or may not be typical of other comets. We hope to see something NOT typical, because that would be most exciting to us scientifically," said Qi.
In the 1950s, the late Dr. Fred Whipple (CfA) developed the famous "dirty snowball" model for comets. Whipple hypothesized that comets consisted of ice with some dirt and rock mixed in. Modern astronomers believe that comets are better described as "icy dirtballs," containing more dirt and less ice than previously thought.
Deep Impact's mission is to test these new models by excavating a crater more than 80 feet deep and 300 feet in diameter, revealing the comet's pristine interior. Scientists will compare the freshly excavated material to the more easily visible cometary surface.
Ernst Tempel discovered Comet Tempel 1 in 1867. The comet has made many passages through the inner solar system orbiting the Sun every 5.5 years. This makes Tempel 1 a good target to study evolutionary change in the mantle, or upper crust, of the comet.
While the impending SMA observations of Deep Impact were a hot topic of discussion at this week's meeting on submillimeter astronomy held in Cambridge, Mass., Comet Tempel 1 is only one of the solar system objects that the SMA has studied.
Monitoring the Weather on Mars
The planet Mars also has been a target of particular interest. As rovers and orbiters study the Red Planet close-up, the SMA takes in the big picture from a distance.
"Our research is very complementary to the spacecraft exploring Mars," said astronomer Mark Gurwell (CfA). "We get a large-scale picture in a different way from the Mars Global Surveyor, for example."
While missions to Mars have focused mainly on the planet's surface, the SMA is sensitive to conditions within the planet's atmosphere. As such, it serves as an Earth-based weather station, able to monitor atmospheric conditions on Mars from millions of miles away.
"The SMA enables us to study abundances of molecules and thus atmospheric chemistry. We can study atmospheric dynamics of an entire hemisphere at a time," said Gurwell. "Our measurements give us a 3-d map of atmospheric temperatures to greater heights than any Mars-orbiting spacecraft can measure. We can go higher, and we can monitor the planet throughout the Martian day."
Investigating the Atmosphere of Titan
Gurwell also uses the SMA to measure the upper atmospheric temperature of Saturn's moon Titan. Before the Cassini spacecraft reached the Saturnian system, Gurwell was studying Titan to learn more about this enigmatic moon, whose surface is shrouded by its thick, hazy atmosphere.
"We saw large gradients of hydrogen cyanide and other chemicals between the northern and southern hemispheres," said Gurwell. "These gradients tell us about the climate and dynamics of the atmosphere. Plus, since Titan is an analog to the early Earth, we can use our measurements of Titan to start to extrapolate what conditions were like on the Earth four billion years ago, just before life developed."
Continued SMA observations will provide useful information about seasonal weather patterns on Titan, where a `year' is nearly 30 Earth years long.
"If we develop an understanding of Titan's climate and seasonal changes, we may be able to better understand how other atmospheres, including the Earth's, are affected by changes in seasons," Gurwell explained. "This could become very important when we begin to study planets around other stars."
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.