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Mercury's hidden splash.

ONLY ONE SPACECRAFT, MARINER 10, HAS ever been to Mercury. In the mid-1970s lucky geometry allowed it to fly by the planet three times in 12 months. But that same orbital geometry kept it from seeing almost half of the innermost planet's surface--a "terra incognita" that remains virtually unknown nearly three decades later.

Radar astronomers on Earth, however, are probing the unseen territory with ever-improving resolution, and Mercury is slowly giving up some of its secrets. Several days of observations with the 305-meter Arecibo dish in Puerto Rico last June and July recorded details only a few kilometers across and turned up a number of surprises.

Nearly straddling the equator on Mercury's unseen hemisphere is a prominent, 90-kilometer-wide crater sitting in a splash of bright rays some 900 km across, nearly a fifth of the planet's diameter. According to John K. Harmon (National Astronomy and Ionosphere Center) and Donald B. Campbell (Cornell University), the rays are extremely rough jumbles of rock thrown out by the impact, which makes them excellent reflectors of Arecibo's radar pulses. The rays look so fresh, in fact, that the crater may be much younger than similar-sized Tycho, which was blasted out of the Moon's landscape just 109 million years ago. Harmon notes that the still-unnamed Mercurian crater was seen in previous radar campaigns but with only a tenth as much resolution.

Fortunately, two new spacecraft should be en route to mysterious Mercury before the decade is out. NASA's mission, named Messenger, calls for a heavily instrumented craft to slip into orbit around the planet in April 2009 and conduct comprehensive mapping surveys and compositional assays. That same year the European Space Agency plans to dispatch its BepiColombo spacecraft (which honors Italian space scientist Giuseppe "Bepi" Colombo, 1920-1984). Upon arrival at Mercury after a 212-year flight, it will separate into one orbiter to study the globe, a second orbiter to monitor the magnetic field, and a small lander to analyze the surface rocks.--J. Kelly Beatty

Follow That Story

The Universe Isn't Turquoise. When Karl Glazebrook and Ivan Baldry (Johns Hopkins University) announced "the color of the universe" at the American Astronomical Society meeting in January, they didn't expect it to become front-page news (April issue, page 21). In what they admit was "something of a gimmick," they averaged the light of 200,000 galaxies, and then displayed the result as a vivid turquoise-green. The press ate it up, but astronomers (who know that galaxies aren't bright green) challenged the color as unrealistic. Color scientists soon pinpointed the problem. "We found a bug in our code!" Glazebrook and Baldry admit on their Web site. They had accidentally matched the galaxies' average spectrum against a "white" standard that was too red.

"So what color is the universe?" they continue." Really the answer is so close to white, it is difficult to say." By choosing a more realistic white point they produced the very pale yellowish white tint here. By no coincidence, visual observers will recognize this as a lot like the average color of stars.

The revised cosmic color got almost as much news play as the original. But the story didn't end there. Michael H. Brill, a color scientist for McClendon Automation Inc., was in the news in late March declaring that the universe is actually salmon pink. He chose a white point matching full daylight illumination.

Astronomers, who know that galaxies are no more pink than turquoise, called the whole debate increasingly ridiculous --based on near-universal confusion between objective color (wavelengths and intensities) and subjective human perceptions, which depend on surrounding lighting conditions. The point is how galaxies look in space, not how they would look to the eye in broad daylight where the eye can never see them.

Lensed Galaxy Sets New Redshift Record. More astronomers have started using the gravitational fields of distant galaxy clusters as, in effect, giant zoom-lens attachments on their telescopes, allowing them to hunt for objects extremely far in the background (January issue, page 17). Using the 10-meter Keck I telescope--boosted by an estimated 4.5x lensing effect from the cluster Abell 370 in Cetus--Esther M. Hu (University of Hawaii) and her colleagues have found the farthest object ever identified. It is a galaxy with a redshift of 6.56, enough to shift its Lyman-alpha hydrogen emission from the far ultraviolet to the near infrared. The galaxy's visibility indicates that the cosmic "Dark Age"--the era before the first stars and galaxies lit up and reionized the intergalactic medium (S&T: November 2001, page 19)--was already ending by about 780 million years after the Big Bang.--A. M.
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Title Annotation:previously unseen side of Mercury being viewed with radar telescopes
Author:Beatty, J. Kelly
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jun 1, 2002
Words:770
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