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Ad Astra
Volume 15, Number 3 June/July/August 2003

Mission Control
Spacebeat

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Sun Sets Sultry Season
Global warming might have less to do with hot air than sunburn according to new sun-centered research. Since the late 1970s, the amount of solar radiation has increased by nearly .05% per decade during times of quiet sunspot activity. Although the inferred increase of Total Solar Irradiance (TSI) – the radiant energy received by the Earth from the Sun – in the last quarter century is not enough to cause notable climate change, the trend would be important if maintained for a century or more. “This trend is important because, if sustained over many decades, it could cause significant climate change,” said Richard Willson, a researcher affiliated with NASA’s Goddard Institute for Space Studies.

To investigate the possibility of a solar trend, Willson carefully pieced together long-term solar irradiance data from overlapping records of six satellites in orbit since late 1978. The data showed a significant positive trend (.05% per decade) in TSI between the solar minima of solar cycles from 1978 to the present. If that trend “persisted throughout the 20th century, it would have provided a significant component of the global warming…over the past 100 years,” Willson said. While sultry days are ahead if the trend continues, climatologists looking on the sunny side predict the new data can help distinguish between solar and man-made influences on climate.

Hot Body Disappears
For the first time, astronomers using NASA’s Hubble Space Telescope have observed the atmosphere of an extrasolar planet evaporating into space. Like a moth drawn to a flame, the scorched planet, called HD 209458b, orbits only 7 million kilometers (4 million miles) from its yellow, Sun-like star. Although HD 209458b orbits too close to its parent star to be photographed directly, astronomers were able to observe telltale shadows of the planet as it blocked light during transits across the star’s disk. In addition, starlight passing through the planet’s atmosphere revealed a startling drop in hydrogen emission, which can best be explained by a hot hydrogen atmosphere – resembling a comet tail – trailing behind the singed planet.

“The atmosphere is heated, the hydrogen escapes the planet’s gravitational pull and is pushed away by the starlight, fanning out in a large tail behind the planet – like that of a comet,” explained astronomer Alain Lecavelier des Etangs. Astronomers estimate that at least 10,000 tons of hydrogen gas escape HD 209458b each second. Much of the planet – known as a “hot Jupiter” – may eventually disappear, leaving only a dense core. Such onetime gas giants must have formed in the cold outer reaches of the star system and then spiraled into close orbits, where evaporation of the atmosphere plays a role in setting an inner orbital boundary. HD 209458b’s orbit is only one-eighth the size of Mercury’s.

Stardust In Your Eye
The U.S. space agency, which launched its own Stardust mission in 1999, might have collected the real stuff closer to home. NASA claims to have amassed interplanetary dust particles (IDPs), made from bits of ancient stars, during high-flying U-2 aircraft flights in the Earth’s stratosphere. “The stardust grains…are typical of the kinds of dust that were available at the beginning of our solar system. These were the building blocks of the sun and planets,” said Lindsay Keller, a researcher at NASA’s Johnson Space Center.

The discovery was made possible by using a new kind of ion microprobe, which measures isotopic ratios on scales much smaller than previously possible. This is essential for identifying stardust grains, because, “they have isotopic ratios very different from anything in the solar system,” said Scott Messenger, an astrophysicist at Washington University. Most of the collected IDPs range in size from 5 to 50 millionths of a meter, and often contain crystalline grains clumped together in sizes of 100 to 500 billionths of a meter. “The fact that these IDPs are rich in stardust and molecular cloud material suggests that they have remained essentially unchanged from the time the solar system formed, 4.5 billion years ago,” said Messenger.
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NASA Selects Next Explorer Mission
A swarm of spacecraft, designed to fly through the space storms that cause aurora, has been chosen as the next mission in NASA’s Medium-class Explorer (MIDEX) program. Collectively dubbed the Time History of Events and Macroscale Interactions during Substorms probes, or THEMIS for short, the five-satellite mission will be launched in 2007 to investigate global reconfigurations of the Earth’s magnetosphere. The orbits of the satellite quintet will be carefully coordinated to line up every four days to allow identical suites of electric, magnetic, and particle detectors to track aurora disturbances.

Explorer-class missions are intended to provide frequent, low-cost access to space for physics and astronomy missions with small to mid-sized spacecraft. “The Explorer program allows the science community to identify the most compelling science questions and then design the most effective mission to answer those questions,” said Edward Weiler, NASA Associate Administrator for Space Science. “The mission we’ve selected will directly address the science goals of the NASA strategic plan within a focused, moderate sized project,” he added.
NASA also selected, as a mission-of-opportunity, an instrument for the Extreme Universe Space Observatory (EUSO) mission of the European Space Agency. EUSO will study the most energetic particles in the universe. From its location on the International Space Station, EUSO will look down on the Earth’s atmosphere to observe the characteristic blue light that high-energy cosmic rays generate after hitting the Earth’s atmosphere.

Another mission called the Widefield Infrared Survey Explorer (WISE) also remains on NASA’s wish list. If built, WISE would train a four-channel, super-cooled infrared telescope to survey the entire sky with 1,000 times more sensitivity than previous infrared missions. A decision on proceeding to flight development will be made in 2004. Two MIDEX missions are already airborne, the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE), launched in 2000, and the Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001. A third MIDEX mission, the Swift Gamma Ray Burst Explorer, is slated for a rocket ride in December 2003.

Mars is a Softy Inside
New research suggests that Mars, the blood-red planet named after the Roman god of war, may really be a softy inside. Researchers at NASA’s Jet Propulsion Laboratory analyzing three years of radio tracking data from the Mars Global Surveyor spacecraft, concluded that the Red Planet does not have a completely solid iron core, but instead is made up of either a completely liquid iron core or a liquid outer core with a solid inner core, much like Earth or Venus. “Earth has an outer liquid iron core and solid inner core. This may be the case for Mars as well,” said Charles Yoder, a planetary scientist at JPL.

The JPL team used Doppler tracking of radio signals emitted by the Global Surveyor spacecraft to determine the precise orbit of the spacecraft around Mars, and, by inference, tidal bulges on Mars due to the gravitational pull of the Sun. “By measuring this bulge in the Mars gravity field we can determine how flexible Mars is,” explained Yoder. “The size of the measured tide is large enough to indicate the core of Mars cannot be solid iron but must be at least partially liquid.” Combining this data with information on Mars’ precession rate indicated the core is about one-half the size of the planet, as is the case for Earth and Venus, and has a significant fraction of a lighter element such as sulfur.

In addition to measuring the Martian interior, Global Surveyor has also been able to estimate the amount of ice sublimated, changed directly into a gaseous state, from one pole into the atmosphere and then accreted onto the opposite pole. “Our results indicate the mass change for the southern carbon dioxide ice cap is 30 to 40 percent larger than the northern ice cap, which agrees well with the predictions of the global atmosphere models of Mars,” said Yoder.
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Snow Melt Blamed for Martian Gullies
Just as winter weather fosters pothole hell here on Earth, new images from NASA’s Mars Odyssey and Global Surveyor spacecrafts suggest that melting snow is the cause of eroded gullies on Mars. According to Philip Christensen, the principal investigator for Odyssey’s camera system, at least some Martian gullies are created by trickling water from melting snow packs, not underground springs or pressurized flows, as had been previously suggested. Christensen proposes that gullies are carved by water melting and flowing beneath snow packs, where it is sheltered from rapid evaporation in the planet’s thin atmosphere. “The Odyssey image shows a crater on the pole-facing side has this ‘pasted-on’ terrain, and as you come around to the west there are all these gullies,” said Christensen. “I saw it and said ‘Ah-ha!’ It looks for all the world like these gullies are being exposed as this terrain is being removed through melting and evaporation.”

Christensen points out his melting snow hypothesis explains why gullies occur preferentially on the cold face of the slope at mid-latitudes. “Snow on Mars is most likely to accumulate on the pole-facing slopes, the coldest areas. It accumulates and drapes the landscape in these areas during one climate period, and then it melts during a warmer one. Melting begins first in the most exposed area right at the crest of the ridge. This explains why gullies start so high up.” Once he started to think about snow, Christensen began finding other images showing a similar relationship between “pasted on” snow deposits and gullies. NASA’s tandem of science orbiters are currently laying the groundwork for locating interesting areas for surface exploration by roving laboratories, such as the Mars Exploration Rovers, scheduled for launch in June of this year.

Far-Flung Supernovae Shed Light on Dark Matter
Astronomers using the Hubble Space Telescope’s Advanced Camera for Surveys (ACS) have found two supernovae that exploded so long ago they provide new clues about the accelerating universe and its mysterious “dark energy.” Coupled with Hubble’s powerful vision, the ACS can pick out the faint glow of the distant supernovae and dissect their light to determine if they are a special type of exploding star – called Type Ia supernovae – that are reliable distance indicators.
Previous studies of Type Ia supernovae revealed that galaxies are moving away from each other at an ever-increasing speed under the influence of dark energy permeating the universe. However, astronomers had very little data on the transition period between this repulsion phase and the earlier epoch characterized by the tug of gravity. Fortunately, one of the newly discovered supernovae discovered by ACS exploded so long ago the universe may still have been decelerating under its own gravity. “We’re trying to fill in a blank region where the universe’s rate of expansion switched from deceleration due to gravity to acceleration due to the repulsive force of dark energy,” explained Blakeslee. “The sharper images, wider viewing area, and keener sensitivity of ACS should allow astronomers to discover roughly 10 times as many of these cosmic beacons as was possible with Hubble’s previous main imaging camera.”

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