In addition to tidal heating, the interior of Europa could also be heated by the decay of radioactive material (radiogenic heating) within the rocky mantle. But the models and values observed are one hundred times higher than those that could be produced by radiogenic heating alone, thus implying that tidal heating has a leading role in Europa. The Hubble Space Telescope acquired an image of Europa in 2012 that was interpreted to be a plume of water vapour erupting from near its south pole. The image suggests the plume may be high, or more than 20 times the height of Mt. Everest. It has been suggested that if they exist, they are episodic and likely to appear when Europa is at its farthest point from Jupiter, in agreement with tidal force modeling predictions. Additional imaging evidence from the Hubble Space Telescope was presented in September 2016. In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated critical analysis of data obtained from the "Galileo" space probe, which orbited Jupiter between 1995 and 2003. "Galileo" flew by Europa in 1997 within of the moon's surface and the researchers suggest it may have flown through a water plume. Such plume activity could help researchers in a search for life from the subsurface European ocean without having to land on the moon. The tidal forces are about 1,000 times stronger than the Moon's effect on Earth. The only other moon in the Solar System exhibiting water vapor plumes is Enceladus. The estimated eruption rate at Europa is about 7000 kg/s compared to about 200 kg/s for the plumes of Enceladus. If confirmed, it would open the possibility of a flyby through the plume and obtain a sample to analyze "in situ" without having to use a lander and drill through kilometres of ice. Observations with the Goddard High Resolution Spectrograph of the Hubble Space Telescope, first described in 1995, revealed that Europa has a thin atmosphere composed mostly of molecular oxygen (O2), and some water vapor. The surface pressure of Europa's atmosphere is 0.1 μPa, or 10−12 times that of the Earth. In 1997, the "Galileo" spacecraft confirmed the presence of a tenuous ionosphere (an upper-atmospheric layer of charged particles) around Europa created by solar radiation and energetic particles from Jupiter's magnetosphere, providing evidence of an atmosphere. Unlike the oxygen in Earth's atmosphere, Europa's is not of biological origin.