NASA鈥檚 New Horizons spacecraft has found hints that Jupiter鈥檚 tiniest moons have been obliterated. The findings are among a wide variety of observations of Jupiter and its moons that were released today by mission scientists.
Launched in January 2006, the New Horizons probe flew by Jupiter on 28 February 2007 to get a boost from the planet鈥檚 gravity on its way to its main target 鈥� Pluto.
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Now, mission members have announced a raft of new findings from the encounter with the solar system鈥檚 most massive planet, including a puzzling absence of small moons in the planet鈥檚 rings.
The Long Range Reconnaissance Imager (LORRI) camera on New Horizons should have been able to spot moons down to a diameter of about 1 kilometre. But it saw nothing smaller than Adrastea, a 16-kilometre-wide resident of Jupiter鈥檚 faint ring system (see image at right).
Steady rain
This is puzzling, because scientists expected the number of objects to increase at smaller size scales, as they do in the rings of Saturn. The missing moons may have been eroded away by micrometeoroids, say researchers led by Mark Showalter of the SETI Institute in Mountainview, California, US.
A steady rain of small objects 鈥� probably between the sizes of a grain of sand and a pebble 鈥� would destroy small moons while leaving larger ones mostly intact, they say. For example, a 27-kilometre-wide moon could survive having its outer 5 km worn away over time, whereas an object just 5 km across would be eroded away to nothing in the same time period.
鈥淲e鈥檙e probably talking about the same tiny grains that we see as shooting stars in the Earth鈥檚 atmosphere,鈥� Showalter told New 杏吧原创.
But why did this process spare Saturn鈥檚 small moons? Showalter thinks the answer has to do with the fact that Saturn is simply less massive than Jupiter.
Giant blobs
After micrometeoroids knock material off Saturn鈥檚 small moons, the material may be able to fall back down again, replenishing what was lost. But small moons orbiting in Jupiter鈥檚 rings cannot do the same because of interference by Jupiter鈥檚 powerful gravity field, Showalter suggests.
During the Jupiter encounter, New Horizons also became the first spacecraft to travel down the length of Jupiter鈥檚 magnetotail, a stream of charged particles at least 500 million km long that was blown away from Jupiter by the solar wind.
The spacecraft discovered giant blobs of charged particles moving down the tail, and scientists think they originated in a layer of Jupiter鈥檚 atmosphere called the ionosphere.
Alan Stern, the mission鈥檚 chief scientist and head of science at NASA, says the successful observations at Jupiter bode well for the probe鈥檚 2015 encounter with Pluto.
鈥楬ome run鈥�
鈥淚f Jupiter is any guide, then we鈥檒l have a home run at Pluto,鈥� he told New 杏吧原创. 鈥淏ut there鈥檚 a lot of work between here and there.鈥� The mission team will have to take care to keep the spacecraft healthy during its 7.5-year cruise to the distant world, he says.
Other results from the New Horizons Jupiter encounter include:
聲 The first movies of a plume spewing from a volcano on Jupiter鈥檚 moon Io, and the discovery of a new 240-kilometre-long lava flow 鈥� the longest seen since 1979 鈥� from another volcano on the moon
聲 The first observations of lightning near Jupiter鈥檚 poles
聲 Observations of aurorae in the atmospheres of Jupiter and Io
聲 The discovery that clouds near Jupiter鈥檚 Great Red Spot have thinned since the Galileo and Cassini spacecraft observed the region in 1996 and 2000, respectively
Journal reference: (vol 318, pp 216 鈥� 243)
