Dark Spots in Our Knowledge of Neptune

An image of Neptune taken by the Voyager spacecraft in August 1989, showing the storm, known as the Great Dark Spot, near the left side of the planet.


This occasional column explores topics covered in Science Times 25 years ago to see what has changed — and what has not.

A quarter century ago, NASA’s Voyager 2 spacecraft was closing in on Neptune for its final planetary visit.

In Science Times on Aug. 15, 1989, John Noble Wilford described the anticipation among the scientists about what they were calling the “last picture show.”

Voyager 2, which would turn 12 a few days before its closest approach to Neptune on Aug. 25, was already elderly in spacecraft years — “arthritic and partially deaf, feeble of voice and prone to memory lapses,” Mr. Wilford wrote.

One of the first features scientists saw was a dark spot in the southern hemisphere as wide as Earth — a giant storm like the Great Red Spot on Jupiter. The wind on Neptune blew up to 1,500 miles per hour, the strongest in the solar system; its magnetic field tilted 47 degrees from the rotational axis.

Voyager 2 also discovered new moons and incomplete rings orbiting the planet. And on Triton, the largest moon, it photographed what appeared to be the frozen flows of ice volcanoes.

25 YEARS LATER The Hubble Space Telescope has had a few good looks at Neptune, beginning in 1994. By then, the storm, known as the Great Dark Spot, had disappeared, and a new dark spot emerged in the northern hemisphere. With limited Hubble observations, it is not known whether that spot remains or whether others have come and gone.

Telescopes on Earth cannot make out the dark spots, but they can see bright clouds that were next to them.

“Over the years, we have seen quite bright clouds come and go in both the north and south,” said Heidi B. Hammel, a planetary astronomer at the Space Science Institute. “Right now, there are some very bright clouds in the north, but we have no approved Hubble programs to verify an underlying Great Dark Spot.”

Others have gleaned new results from the old Voyager 2 photographs. Paul M. Schenk of the Lunar and Planetary Institute in Houston and Kevin J. Zahnle of NASA’s Ames Research Center in Mountain View, Calif., took a more careful look at the craters on Triton and argued that they all formed within the past 10 million years — almost yesterday in geological terms. That would suggest not only ice volcanoes, but also an ocean below the surface with liquid breaking through and freezing.

Candice Hansen-Koharcheck, a senior scientist at the Planetary Science Institute in Tucson, said she had pushed for a new spacecraft to head to Neptune to explore these mysteries. Three of the four giant planets — Jupiter, Saturn and Neptune — line up over the next few years. That is not quite the “grand tour” of the Voyager spacecraft, but it would provide the gravitational boost for a quicker trip. “You can get to Neptune in 10 years,” Dr. Hansen-Koharcheck said, and the mission could fit under a billion dollars.

But NASA did not have enough plutonium to provide a power source for an outer solar system mission, and now it is almost certainly too late to start one before the window closes in 2020. So anything that planetary scientists learn about Neptune will come from Hubble and its successor, the James Webb Space Telescope.

“We’re not going to get a close-up look at it for a very long time,” said Carolyn C. Porco, who was a member of the Voyager imaging team. “And there’s so much to learn there.”

Voyager 2 is indeed still operating, 9.75 billion miles away. Its twin, Voyager 1, entered interstellar space in August 2012, but Voyager 2 remains in the region known as the heliosheath, still dominated by the sun’s magnetic field and outward-flowing particles.

“Several more years is a reasonable estimate,” said Edward C. Stone, the project scientist for the Voyagers both then and now.

Even though it will be second out of the solar system, scientists are anxious for the data from one of Voyager 2’s instruments, which measures solar wind particles. The equivalent instrument on Voyager 1 died in 1980.

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