http://nai.arc.nasa.gov/astrobio/index.cfm NAI Ask an Astrobiologist Two things contribute to uncertainty about how often the Earth is struck by an impactor the size of Tunguska. One is the impact rate itself, which is derived primarily from astronomical surveys. Since Gene Shoemaker's work, the surveys have progressed significantly, and the current estimate for impact frequency for a 60m asteroid is less than once in a millennium. There is an additional uncertainty about the size of the Tunguska impactor, however. If it was much smaller (say 30-40m diameter), then the impact frequency goes back up to once every few hundred years. Thus we don't know the answer any better than to say we expect a Tunguska-size impact at an average interval between a few centuries and a couple of millennia.<br /> <i>David Morrison<br> NAI Senior Scientist</i> http://nai.arc.nasa.gov/astrobio/astrobio_detail.cfm?ID=3542 At the Phoenix landing site in the martian Arctic, we expected to find ice-cemented ground beneath dry permafrost. This kind of ice can form from condensing vapor just as do the polar caps on Mars. However, the small chunks of what appear to be pure ice (segregated ice) were a surprise. On Earth segregated ice is found as ice crystals in soil formed from freezing of a liquid phase. The liquid need not be pure water but can be a eutectic brine, such MgCl2 which freezes at -30ºC. Thus the significance is that this form of ice suggests recent liquid water (probably salt water) at the location where Phoenix landed. Chris McKay (NASA Ames) and David Morrison (NAI) http://nai.arc.nasa.gov/astrobio/astrobio_detail.cfm?ID=3530 No, there is no indication in the rock records of mass extinctions at times when the Earth's magnetic field changed polarity. However, the last polarity change took place nearly a million years ago, so we don't have much data on any possible smaller environmental effects. Probably the change takes place over thousands of years, and the effects are minimal on the Earth's surface, but we don't know for sure.<br /> <i>David Morrison<br> NAI Senior Scientist</i> http://nai.arc.nasa.gov/astrobio/astrobio_detail.cfm?ID=3553 Good luck with your book! There is lots of information available on extremophiles (life that flourishes under conditions that we would consider extreme in terms of temperature, salinity, chemical environment, radiation, etc.). Do a search on Ask an Astrobiologist, or on Wikipedia, or look at any of the books that pop up on the first page of an Amazon.com search on "astrobiology". Of course, this study is now limited to life on Earth, which is the only kind of life we know about, but study of extremophiles on Earth helps inform our ideas about potentially habitable environments on other planets. <br /> <i>David Morrison<br> NAI Senior Scientist</i> http://nai.arc.nasa.gov/astrobio/astrobio_detail.cfm?ID=3580 You seem to be very confused. Eros is not Eris, any more than the letter i is the same as the letter o. Eris is a dwarf planet, larger than Pluto, in the outer solar system, with a moon Dysnomia. Eros is an asteroid, a thousand times smaller, in the inner solar system. Neither one is Nibiru, and neither one is coming close to the Earth.<br /> <i>David Morrison<br> NAI Senior Scientist</i> http://nai.arc.nasa.gov/astrobio/astrobio_detail.cfm?ID=3559