THE AXIAL TILT OF MARS SHIFTS CHAOTICALLY , new computer simulations show. Last week at the AAAS Meeting in Boston, Jack Wisdom of MIT (617-253-7730) described simulations, performed with his colleague Jihad Touma, which demonstrate that the tilt angle of Mars fluctuates over a 100 million year period between 10 and 50 degrees with respect to a line perpendicular to the plane of its orbit. This finding is likely to provoke new insights into atmospheric processes and the evolution of surface features on the planet. In particular, the axial tilt of Mars has a direct effect on atmospheric pressure and surface temperature; and the latter plays a significant role in the appearance and disappearance of polar ice caps on Mars. LEP 200 is the name for the upgraded version of the Large Electron Positron collider at CERN. Although the total collision energy will be something more like 190 GeV rather than 200 GeV, this will be enough to produce pairs of W bosons, which along with the Z boson are the carriers of the weak nuclear force. The upgrade, which should be finished in 1994, adds superconducting rf cavities to existing room-temperature cavities for accelerating the beams and for replenishing the energy lost to synchrotron radiation, a loss which amounts to a not-inconsiderable 2.3 GeV per turn. The mass of the W (about 82 GeV) is currently known from studies at Fermilab and CERN's own proton-antiproton collider to within an uncertainty of 300 MeV. LEP 200 should reduce this to about 60 MeV, which will permit a more thorough test of the standard model of particle interactions. LEP 200's other big quarry will be the Higgs boson, the particle that supposedly endows the W and the Z bosons with mass. (SLAC Beamline, Fall 1992.)