  We report on a multi-wavelength observational campaign which followed the
Earth's transit on the Sun as seen from Jupiter on 5 Jan the 2014. Simultaneous
observations of Jupiter's moons Europa and Ganymede obtained with HARPS from La
Silla, Chile, and HARPS-N from La Palma, Canary Islands, were performed to
measure the Rossiter-McLaughlin effect due to the Earth's passage using the
same technique successfully adopted for the 2012 Venus Transit (Molaro et al
2013). The expected modulation in radial velocities was of about 20 cm/s but an
anomalous drift as large as 38 m/s, i.e. more than two orders of magnitude
higher and opposite in sign, was detected instead. The consistent behaviour of
the two spectrographs rules out instrumental origin of the radial velocity
drift and BiSON observations rule out the possible dependence on the Sun's
magnetic activity. We suggest that this anomaly is produced by the Opposition
Surge on Europa's icy surface, which amplifies the intensity of the solar
radiation from a portion of the solar surface centered around the crossing
Earth which can then be observed as a a sort of inverse Rossiter-McLaughling
effect. in fact, a simplified model of this effect can explain in detail most
features of the observed radial velocity anomalies, namely the extensions
before and after the transit, the small differences between the two
observatories and the presence of a secondary peak closer to Earth passage.
This phenomenon, observed here for the first time, should be observed every
time similar Earth alignments occur with rocky bodies without atmospheres. We
predict it should be observed again during the next conjunction of Earth and
Jupiter in 2026.
