March 12, 2013
Looking Sideways
image by NASA/GSFC/Arizona State University
Two days ago we saw an overhead view of the Gruithuisen domes and now here is an oblique shot of the same area.
The orbiting LRO spacecraft has been rotating over for its cameras to acquire oblique views that graphically provide
information. What we see is that the Gruithuisen Gamma dome has a flat top and relatively steep slopes - with this
perspective it is about 25°. Remember that the other features called domes are the low dark-hued mare mounds such
as those near Hortensius. Those classic mare domes have slopes of just a few degrees. Based on these observations
about the slopes and brightness of domes it has long been (maybe 25 years) considered that the stubby Gruithuisen
domes were made of more silicic magmas than mare domes are made of. There was one more piece of evidence -
these domes have a different spectral reflectivity than other materials, they are relatively bright in the red end of the
spectrum and hence are called red spots. Now the LRO Diviner instrument, which measures brightness in the infrared,
has determined that these domes are indeed made of volcanic rocks such as rhyolite that are full of silica. Notice the
other two red spots - Gruithuisen Delta and the feature informally named Northwest. These are volcanic domes too but
they don't have the stubby dome look. They could be older and more eroded, but probably they are the same age and
simply formed as more irregular mounds of lava.
Chuck Wood
Related Links
Rükl plate 9
21st Century Atlas chart 21.
Yesterday's LPOD: The Moon or a Paellera?
Tomorrow's LPOD: A Miss This Time
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