The majority of scientists have no answer for this.
(click link)
http://www.amperefitz.com/dark.m.e.htm
This is simply incredible!
Fitz
For all we know, our all-inclusive NASA and their various cabals of
special interest groups (including DARPA) doesn't actually know squat,
just like they don't seem to know or much less share public-funded
science data about our physically dark moon/Selene, or into sharing as
to why their interpretation of Newtonian laws say that we supposedly
are not unavoidably associated with the still massive Sirius star/
solar system, or even why the planet Venus has been losing 20.5 w/m2
and otherwise hosting what looks as though it were populated by
something rather intelligent and quite rational.
NASA Magellan:
http://www2.jpl.nasa.gov/magellan/
http://nssdc.gsfc.nasa.gov/imgcat/html/mission_page/VN_Magellan_page1.html
http://nssdc.gsfc.nasa.gov/planetary/magellan.html
http://en.wikipedia.org/wiki/Magellan_probe
Brad Guth / Blog and Google document pages:
http://bradguth.blogspot.com/
http://docs.google.com/View?id=ddsdxhv_0hrm5bdfj