Last week NASA announced four finalists for its budget-friendly series of Discovery class missions. The first is called VERITAS (stand for Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy), which would orbit Venus and map the surface of this hot world. Another possible Venus mission called DAVINCI+ (Deep Atmospheric Venus Investigation of Noble gases, Chemistry, and Imaging Plus) would drop a sphere-shaped spacecraft down through the atmosphere of Venus to the surface. A possible mission to Jupiter’s moon Io, called IVO, would swoop in to study the surface and its many active volcanoes up close. And, finally, a mission called TRIDENT would travel to Neptune’s moon Triton, an icy place with a possible subsurface ocean; plumes of water even spew out from its surface into space.

In honor of these last two potential missions, we’ll focus this week on Io and Triton. Both of these moons have been observed obliquely during flybys conducted as part of other missions (Voyager 2 took a peep at Triton in 1989) but this is the first time they’d get probes devoted to them. For the next year or so each mission team will develop their proposals before moving onto the next round. Get ready for delayed gratification though: It doesn’t take too long to get to Venus, but it takes about seven years to get to Jupiter, and if TRIDENT is selected a spacecraft wouldn’t get to Neptune and Triton until 2038.

NASA’s Voyager 2 spacecraft flew past the planet Neptune in 1989 and took one of its final photos: a rearview mirror shot of Neptune and its moon Triton, seen as a tiny dot in the lower right corner.Photograph: NASA/JPL
Voyager 2 captured this photo of Triton from 330,000 miles away. Scientists actually expected to see a lot more cratering on the surface of Triton, and the fact that they hardly saw any craters suggests that the surface is being renewed by geological processes.Photograph: NASA
Triton has rippled terrain like Pluto, plumes of water ice that shoot out into space like Saturn’s ocean world Enceladus, and an interior ocean like Europa, only it exists in a much colder part of our solar system. The slight pink and red surface might be because methane in the atmosphere and on the surface is being irradiated and forming a tar-like substance called “tholins.”Photograph: NASA
Io, one of Jupiter’s 79 moons is really special: This tiny rocky body orbits so close to Jupiter that it is constantly getting tugged on and pushed and tugged again. As a result its surface is pocked with volcanoes. Scientists think Io once had an ocean and an ice crust like Jupiter’s other icy moons, but the force of Jupiter’s push and pull stripped the little moon, leaving behind this burping surface.Photograph: NASA/JPL/University of Arizona
This is the highest resolution picture of Io ever taken, courtesy of NASA’s Galileo spacecraft. On Feb 22, 2000, the Jupiter orbiter snapped some close-up photos of the volcanic moon revealing unknown surface processes. Scientists think the surface texture could be the result of some sort of erosion, or perhaps the result of solid ice evaporating.Photograph: NASA/JPL/University of Arizona
This photograph of the southern hemisphere of Jupiter was obtained by NASA’s Voyager 2 on June 25, 1979. Seen in front of the turbulent clouds of the planet is Io, the innermost of the large Galilean satellites of Jupiter.Photograph: NASA/JPL

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