The planet Mercury is often referred to as the ''Iron'' Planet. This is because its iron core is large compared to other terrestrial planets and extends more than half way out to its surface. One of several theories for this anomaly is that strong, dense, strong winds and very high X-ray-FUV irradiances of the young Sun (during the first 500 Myr of its life) eroded (swept) away its early atmosphere and much of its outer mantle. Even today (with a much weaker Sun), ground based observations of heavy constituents like Na+, K+ and O+ in Mercury's present exosphere implicate a strong exosphere-surface interaction related to the particle and radiation environment of the close Sun. Recent studies of isotope anomalies in planetary atmospheres and meteorites indicate that our early Sun underwent a highly active phase after its origin, including continuous flare events where the particle and radiation environment was several hundred times higher than today. Since Mercury is the closest planet to the Sun its surface was exposed more than all other solar system bodies by such an enhanced solar wind particle and radiation flux. Recently Guinan and Ribas (Univ. of Barcelona) have been working with Helmut Lammer and the Astrobiology group at Graz (Austria) on this problem. Guinan and Ribas have provided them with preliminary irradiance (X-ray/FUV) values determined from the ''Sun in Time'' program. Also, they have estimated the winds of the young Sun from the work of Brian Wood (CASA). Initial calculations indicate that energetic flares and large CME events (combined with the steady enhanced winds and strong high energy solar emission) could be sufficient to explain the present large iron core of Mercury.
This research is partially supported by NASA, FUSE and HST grants.