The Earth-based astronomers first observed Methane on Pluto in 1976.
"We already knew there was methane on Pluto but these are our first detections," said Will Grundy, team leader with the Lowell Observatory in Flagstaff, Arizona.
Soon, we will know if there are differences in the presence of methane ice from one part of Pluto to another, he added in a statement.
New Horizons is now about 16 million km from the Pluto system - around 4.75 billion km from the Earth.
Methane was detected by a team of ground-based astronomers led by New Horizons team member Dale Cruikshank of Nasa's Ames Research Center, Mountain View, California.
The detection was made possible by the help from the infrared spectrometer on New Horizons spacecraft.
Methane is an odourless, colourless gas that is present underground and in the atmosphere on the Earth.
On Pluto, methane may be primordial, inherited from the solar nebula from which the solar system formed 4.5 billion years ago.
Just hours after its flyby of Pluto on July 14, the spacecraft will observe sunlight passing through the planet's atmosphere, to help scientists determine the atmosphere's composition.
"It will be as if Pluto were illuminated from behind by a trillion-watt light bulb," noted New Horizons scientist Randy Gladstone.
The spacecraft is healthy and all systems are operating normally.
"We are really on the final path. It just gets better and more exciting every day," said project manager Glen Fountain.
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