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Pluto mission spots planet’s twin

New Horizons image of Neptune Triton

New Horizons image of the planet Neptune. Its largest moon, Triton, can be seen be seen just off to one side. The spacecraft was almost 3.5 billion kilometres away from the pair when it took this image!

  • New Horizons spots Neptune and its moon Triton
  • Triton orbits Neptune the “wrong way”
  • Triton is often considered to be a twin of Pluto

NASA’s Pluto-bound New Horizons spacecraft—now a little over halfway there—has turned its attention to the planet Neptune and its largest moon, Triton.

Mission controllers periodically test the spacecraft’s cameras by aiming them at other Solar System bodies.

New Horizons’ Long Range Reconnaissance Imager (LORRI) snapped several images of Neptune during the latest annual systems checkout, which ended July 30. Neptune was 23.2 astronomical units (about 3.48 billion kilometres!) from New Horizons when LORRI took aim at the gas giant planet—and Triton made a cameo appearance.

Because Neptune and Triton were so far away, they are hard to tell apart in the images. But Triton can be seen as a dot or blob just off to one side.

New Horizons

New Horizons spacecraft prior to launch in 2006.

“That we were able to see Triton so close to Neptune, which is approximately 100 times brighter, shows us that the camera is working exactly as designed,” says New Horizons Project Scientist Hal Weaver, of the Johns Hopkins Applied Physics Laboratory. “This was a good test for LORRI.”

“As New Horizons has travelled outward across the Solar System, we’ve been using our imagers to make just such special-purpose studies of the giant planets and their moons because this is a small but completely unique contribution that New Horizons can make—because of our position out among the giant planets,” says New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute.

Triton is often called Pluto’s twin. Only slightly larger than Pluto—2,700 kilometres diameter compared to Pluto’s 2,400 kilometers—both worlds have atmospheres composed mostly of nitrogen gas with a surface pressure only 1/70,000th of Earth’s, and comparably cold surface temperatures approaching minus -240 degrees Celsius.

Pluto’s twin: an enigma

Triton was discovered on October 10, 1846 by English astronomer William Lassell, just 17 days after Neptune itself had been discovered (by German astronomer Johann Gottfried Galle).

Geyser on Triton

Dark streaks show where Triton's ice geysers have been active.

Neptune’s big moon is very unusual, in that it is the only large moon that goes around its planet backwards. That is, Neptune rotates from west to east (as does Earth), but Triton orbits in the planet from east to west (unlike our Moon). This is called a retrograde orbit.

The only plausible explanation is that Triton’s didn’t form along with Neptune, but rather was captured as it wandered past. Given that it is almost a twin of Pluto, it is supposed by most astronomers that Triton was a member of the Kuiper Belt—the swarm of small icy worlds that orbit the Sun beyond Neptune.

Triton also is quite big—its diameter of 2,700 kilometres makes it the seventh largest moon in the Solar System.

Its surface is a frozen crust of mostly nitrogen, underneath which is a core thought to be composed of rock and metals and making up two-thirds the moon’s mass.

One of the amazing things about Triton is that it has active geological features. When NASA’s Voyager 2 probe flew past in 1989, it spotted dark geysers shooting up from the surface, and dark streaks on the surface downwind of the geysers. The only other Solar System bodies confirmed to have volcanic activity are Earth, Jupiter’s moon Io, and Saturn’s moon Enceladus.

Unlike many moons—which are covered in craters—Triton has few impact craters. Scientists put this down to the geological activity, such as tectonic processes and volcanoes, which can reshape the landscape and wipe out any traces of craters. But unlike the volcanoes on Earth, on this frozen world the lava consists of water and ammonia!

Voyager 2 also sensed a thin atmosphere as it went past—observations made from Earth in 1990s indicated that the atmosphere was, at that time, thicker than when Voyager was there.

Pluto, here we come!

New Horizons was launched on January 19, 2006, on a trajectory and with a velocity that to reach Pluto in the minimum possible time. In consequence, New Horizons is the fastest spacecraft to leave Earth, having reached a velocity of 58,536 km/h after launch.

The spacecraft reached the orbit of Jupiter in February 2007, passed the orbit of Saturn in June 2008, and is not far away from the orbital distance of Uranus.

New Horizons is due to reach Pluto on July 14, 2015, and conduct a fly-by. It is not equipped with a rocket system to slow down and go into orbit around Pluto; instead, it will go sailing past.

But for around 200 days leading up to the encounter, it will start taking images that are better than best images we currently have of the icy world, so there will be plenty of time to make new discoveries.

Following the encounter, New Horizons will continue into deep space. There is a strong chance that mission controllers will be able to target the spacecraft to do a subsequent fly-by of one of the other icy worlds that inhabit the Kuiper Belt.

Adapted from information issued by NASA / JHU APL.

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Neptune’s big moon is heating up

Artist’s impression of how Triton, Neptune’s largest moon, might look from high above its surface

Artist’s impression of how Triton, Neptune’s largest moon, might look from high above its surface. The distant Sun appears at the upper-left and the blue crescent of Neptune right of centre.

Summer is in full swing in the southern hemisphere of Neptune’s moon Triton, say astronomers who performed the first-ever infrared analysis of its atmosphere.

The European observing team used the European Southern Observatory’s (ESO) Very Large Telescope to detect carbon monoxide and make the first ground-based detection of methane in Triton’s thin atmosphere.

These observations revealed that the thin atmosphere varies seasonally, thickening when warmed.

“We have found real evidence that the Sun still makes its presence felt on Triton, even from so far away. This icy moon actually has seasons just as we do on Earth, but they change far more slowly,” says Emmanuel Lellouch, the lead author of the scientific paper reporting the results.

On Triton, where the average surface temperature is about minus 235 degrees Celsius, it is currently summer in the southern hemisphere and winter in the northern.

A Voyager 2 image of Triton, with Neptune in the distance

Combined images of Neptune's moon Triton (foreground), and Neptune.

As Triton’s southern hemisphere warms up, a thin layer of frozen nitrogen, methane, and carbon monoxide on Triton’s surface turns into gas, thickening the icy atmosphere as the season progresses during Neptune’s 165-year orbit around the Sun.

A season on Triton lasts a little over 40 years, and Triton passed the southern summer solstice in 2000.

Triton’s thickening atmosphere

Based on the amount of gas measured, Lellouch and his colleagues estimate that Triton’s atmospheric pressure may have risen by a factor of four compared to the measurements made by Voyager 2 in 1989, when it was still spring on the giant moon.

But despite that increase, the atmospheric pressure on Triton is still only between 40 and 65 microbars — that’s 20,000 times less than on Earth.

Carbon monoxide was known to be present as ice on the surface, but Lellouch and his team discovered that Triton’s upper surface layer is enriched with carbon monoxide ice by about a factor of ten compared to the deeper layers, and that it is this upper “film” that feeds the atmosphere.

While the majority of Triton’s atmosphere is nitrogen (much like on Earth), the methane in the atmosphere, first detected by Voyager 2, and only now confirmed in this study from Earth, plays an important role as well.

Is Triton a twin of Pluto?

Of Neptune’s 13 moons, Triton is by far the largest, and, at 2,700 kilometres in diameter (or three quarters the Earth’s Moon), is the seventh largest moon in the Solar System.

Since its discovery in 1846, Triton has fascinated astronomers thanks to its geologic activity, and its many different types of surface ices, such as frozen nitrogen as well as water and dry ice (frozen carbon dioxide).

Triton is also the only large moon in the Solar System with a retrograde motion, which is an orbital direction in the opposite direction to its planet’s rotation. This is one of the reasons why Triton is thought to have been captured from the Kuiper Belt, and thus shares many features with the dwarf planets, such as Pluto.

Pluto, often considered a cousin of Triton and with similar conditions, is receiving renewed interest in the light of the carbon monoxide discovery, and astronomers are racing to find this chemical on the even more distant dwarf planet.

Adapted from information issued by ESO / L. Calçada.