1. MGS was launched in November 1996 and achieved orbit about Mars in September 1997. Pictures taken through September 1998 were obtained from an elliptical orbit. The MOC was turned off from September 1998 to February 1999; it returned to service for the Primary or Mapping Mission in a 2 a.m.–2 p.m. equator-crossing 370 km altitude circular polar orbit in March 1999. The Primary Mission is scheduled to continue through February 2001 with an extension at least through April 2002. The MOC (79) consists of three cameras: a narrow angle system that obtains high–spatial resolution images (1.5 to 12 m/pixel) and red and blue wide-angle cameras to acquire regional and global views (0.24 to 7.5 km/pixel).
2. Early Mars refers to the first 600 to 1000 million years after the planet formed and corresponds to the time of intense impact cratering in the solar system and the emergence of life on Earth. Martian geochronology is divided into three periods: Noachian the earliest is defined as a period dominated by heavy impact cratering and widespread degradation of the cratered terrains; Hesperian generally thought to be a short transitional time as the impact rate and other geomorphic processes acting to modify the heavily cratered terrain tapered off and major ridged plains units formed to cover considerable tracts of cratered terrain in places like Lunae Planum; and Amazonian the recent period that includes Mars as it is seen today in which the last volcanism occurred in Tharsis and Elysium and the polar layered terrains were formed (46). The absolute ages of these periods are not known but attempts have been made to estimate these by assuming that the martian cratering rate is some function of the lunar rate. Hartmann (80) suggested a rate of ∼1.6 times the lunar rate but this has been a matter of ongoing discussion for decades (81–83) and is further complicated by evidence—some of which is presented here as well as in (84)—for burial and exhumation of the craters that are usually counted in attempts to date surfaces. For the purposes of this work and following the discussions of Tanaka (47) the Noachian is regarded here to have occurred before 3.5 billion years ago and the Amazonian after anywhere from ∼3.5 to 1.8 billion years ago.
3. The case for a wet, warm climate on early Mars
4. Early Mars: How Warm and How Wet?
5. J. F. McCauley U.S. Geol. Surv. Misc. Inv. Ser. Map I-897 (1978).