Following such eruptions, methane rain could produce the dendritic dark structures seen by Cassini–Huygens. The VIMS team hypothesized that the dry channels observed on Titan are related to upwelling “hot ice” and contaminated by hydrocarbons that vaporize as they get close to the surface (to account for the methane gas in the atmosphere), which are similar to those mechanisms operating for silicate volcanism on Earth (using tidal heating as an energy source) and which may lead to flows of non-H 2O ices on Titan's surface. The exact ice constituent that can satisfy the constraints imposed by all the observations is not easy to determine, hydrocarbon ice has been invoked on the basis of Xanadu appearing bright at all the near-infrared wavelengths observed to date.Ī bright circular structure (about 30 km in diameter) found in the VIMS hyperspectral images is interpreted as a cryovolcanic dome in an area dominated by extension. It has been hypothesized that they could be associated with some topography and more exposed ice content, and this tends to be in agreement with findings by the Huygens/DISR instrument whose stereoscopic imaging revealed that the brighter terrain was also more elevated than the darker, smoother, and lower ice regions. The reason is that the Cassini camera observing at 0.94 μm cannot see shadows and also Titan's icy bulk does not plead for high topographic structures on the surface (mountains should not exceed 3 km or so).įor the brighter regions, the task of interpreting the data is more difficult. These variations are more readily attributed to the presence on the surface of constituents with different albedos rather than topography, although contribution from the latter is also expected. A plausible candidate for the darker regions could be accumulations of hydrocarbons (in liquid or solid form), precipitating down from the atmosphere. What exactly is causing the albedo variations is still uncertain. The midlatitude regions around the equator on Titan were found to be rather uniformly bright, while the southern pole is relatively dark. It is centered at 10° S and 100°W and officially named Xanadu Regio. The large bright area around the equator first observed by the HST and the adaptive optics in 1994 was resolved and finely observed by Cassini instruments. The best resolution achieved by ISS was of a few kilometers on Titan's surface. It is notable how well the distribution of bright and dark areas agrees among these three maps. The ISS and VIMS cameras confirmed these results and showed that the borders of these regions were linear but not smooth and that dramatic changes in surface albedo could be noted in the maps produced by these measurements ( Fig. To compare these values to what was measured with ALTEA during a quiet period (0.223 ± 0.003 mSv/day, Narici et al., 2015), which is about one third of what is mentioned above, all these must be taken into account: the solar cycle, the position of the detector, relative different shielding, and, most importantly, the limited energy window for protons.Īthena Coustenis, in Encyclopedia of the Solar System (Second Edition), 2007 3.2 The View from the Orbiter During the trip to Mars ( Hassler et al., 2014), 1.84 mSv/day have been measured for the GCRs (a factor three times that on ISS or the Martian surface), with the dose equivalent per SPE ranging from 1.2 to 19.5 mSv, corresponding to two-thirds of a day to more than 10 days of GCR radiation exposure, or about 2–30 years on the Earth's surface.Īs a consequence of the values reported above, during an 860-day mission to Mars (180 days trip + 500 days permanence + 180 days trip, based on the NASA design reference mission ( Drake et al., 2010)) an astronaut would be exposed to about 1 Sv. The situation is worst during the Mars transfer due to the lack of the protective actions of Mars and its thin atmosphere. The only SPE measured on the Martian surface, a mild one, provided an additional dose equivalent of 0.025 mSv. This is due to the thin Martian atmosphere, to the shield provided by the planet itself, and to the presence of the radiation trapped in the SAA in the ISS measurements. The average radiation environment on the Martian surface is measured to be about the same (0.64 ± 0.12 mSv/day) as that on the ISS ( Hassler et al., 2014). Of course, during extravehicular activity the exposure is several times that inside the spacecraft, and the radiation risk increases accordingly. During an SPE, this value can increase about one order of magnitude ( Semkova et al., 2013). Onboard ISS, under quiet conditions (no SPEs), the dose equivalent rate was about 0.647 mSv/day at the end of May 2016 ( Berger et al., 2016). Let us briefly compare the radiation measured on Mars with what is measured on the ISS. Dario Del Moro, in Extreme Events in Geospace, 2018 6.3 Radiation on Mars
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