Introduction: Lunar volcanism was active for an extended period of time (~1.1-4.0 Ga), with the major activity occurring between 3.1 and 3.7 Ga [1-3]. Crater model ages for mare volcanism in Oceanus Procellarum ranges from ~1.2 to 3.93 Ga [2,3] and from 1.33 to 3.4 Ga for northeastern Oceanus Procellarum (Figure 1) [25]. Mare surface ages in these previous studies [2-4] were based on impact crater size-frequency densitybased techniques. The lower end of the age range represents some of the youngest mare surfaces on the Moon, with the possible exception of irregular mare patches [6]. Determining eruption ages for NE Oceanus Procellarum, establishes the timing of mare basalt emplacement and may provide insight into the thermal and eruptive history of this interesting area.

NASA's Office of Space Science is changing its approach to all its missions, both current and future. Budget realities are necessitating that we change the way we do business and the way we look at NASA's role in the U.S. Government. These challenges are being met by a new and innovative approach that focuses on achieving a balanced world-class space science program that requires less U.S. resources while providing an enhanced role for technology and education as integral components of our Research and Development (R&D) programs. Our Mars exploration plans, especially the Mars Surveyor program, are a key feature of this new NASA approach to space science. The Mars Surveyor program will be affordable, engaging to the public with global and close-up images of Mars, have high scientific value, employ a distributed risk strategy (two launches per opportunity), and will use significant advanced technologies.

The most abundant proteins in the lumen of the endoplasmic reticulum (ER) are thought to be molecular chaperones, some of which might also be involved in calcium storage and release. We have purified calreticulin from maize by ion exchange and reverse-phase chromatography. Identity with plant and animal calreticulins was confirmed by N-terminal amino acid sequencing and it was shown to bind calcium with a calcium overlay technique. An antiserum raised to the purified protein was used to screen an expression library and the full coding sequence for maize calreticulin was determined from the clones selected. The sequence shows 96% identity to barley calreticulin and 55% identity to animal calreticulins. The three major functional regions are conserved, as are targeting and retention features. When visualized by indirect immunofluorescence microscopy, calreticulin was found to be confined to the ER and nuclear envelope of maize root cells. It was distributed throughout the ER compartment and we found no evidence of calreticulin-enriched areas of ER, such as might be associated with specialized calcium storage domains. Increasing or decreasing extracellular calcium did not induce measurable changes in calreticulin levels. In addition, maize calreticulin, as well as other recognized chaperones, was shown to bind to denatured protein and could be eluted specifically by nucleoside trisphosphates.

Tethys, Iapetus and Enceladus, satellites of Saturn, display surfaces which indicate that geological processes have been active suggesting a certain degree of internal evolution. On Iapetus activity seems to have been confined to the dark terrain and its timing and extent remain unknown. The widest variation of terrains and crater numbers occurs on Enceladus and indicates the most prolonged geological activity of any satellite. Activity on Tethys seems confined to the first few hundred million years. The satellite Hyperion and the co-orbitals 1980S1 and 1980S3 show no geological activity and apparently are fragments of once larger bodies.

Crater density determinations for parts of the surfaces of several of Saturn's icy satellites including Rhea, Dione, Mimas, Tethys and 1980S3 reveal significant variations. These data, combined with observations of surface morphology, indicate that each satellite has undergone extensive post heavy bombardment geological evolution.

THE recent return of regolith material from Mare Crisium by Luna 24 has provided an opportunity to test our knowledge of lunar impact-flux history. This can be done by comparing crater density data for the landing site with a calibrated impact flux curve to estimate the absolute age of the sample before chemical analysis.

Thirty-five craters and basins larger than 200 km in diameter are recognized on the imaged portion (45%) of Mercury. If the unimaged portion of the planet is similarly cratered, a total of 78 such impact features may be present. Sixty-two craters and basins 200 km in diameter are recognized on the moon, a body with only half the cross-sectional area of Mercury. If surface areas are considered, however, Mercury is cratered only 70% as densely as the moon. The density of impact craters with diameters greater than 400 km on Mercury is only 30% of that on the moon, and for craters with diameters between 400 and 700 km, the density on Mercury is only 21% of the lunar crater density.

Fluviatile and volcanic Martian channels, first discovered on Mariner 9 pictures, have been reexamined by using Viking orbital photography. The superior discrimination of the Viking photographs, resulting from clearer atmospheric conditions and an improved camera system, has permitted us to map additional channels and to estimate their relative ages, using a technique based on crater counting. Broad channels like the Ares and Tiu/Simud Valles are situated along the margin of the southern highlands near Chryse Planitia, the landing site of Viking 1. They originate in areas of collapsed terrain that may have been formed when subsurface water-ice (permafrost) was melted by geothermal heat from deep-seated volcanic centers. When permafrost melting reached an abrupt topographic slope, the interstitially stored meltwater 'lakes' were breached suddenly, releasing the great floods that modified the channels. The volume of material involved in the collapsed terrain is large enough to furnish the water calculated to have filled the broad channels. Conditions are reviewed for persistence of liquid water on Mars under present and more favorable pressures and temperatures. Sinuous channels of intermediate size, like the Ma'adim and Hrad Vailes and other shorter, stubby channels, have multiple tributaries; in the limited coverage available, they appear to result from 'spring sapping,' with the underground permafrost meltwater emerging in box canyons at their heads. The widespread distribution of this type of channel makes their origin by local geothermal heating less likely; climatic warming may be required to explain their formation. The final fluviatile type, dendritic channel networks, has the widest areal distribution and appears to have been formed during at least two episodes. The filamentous channels in their source areas (often the rims of craters) seem to resemble terrestrial river systems; rainfall would seem to be required to form these features. All these channel types debouch onto lowland plains or crater floors, where they disappear in short distances; these abrupt terminations may have resulted from percolation and/or evaporation. Simple and complex lava channels are common; they originate at volcanic centers and are usually morphologically distinct from the aqueous channels. Three types of lava channels are recognized. The wide variation in crater densities implies varying channel ages. Water must have flowed on the Martian surface at many different times in the past, although this would be possible only with great difficulty under the present Martian thermal conditions. Based on a crater flux curve derived by Soderblom et al. (1974) the fluviatile channel ages vary from 3.5 to 0.5 Gy. Lava channel ages range from 3.5 Gy to an age too young to date by the crater counting technique (perhaps 200 m.y.). Methods for dating the channels and volcanic episodes are still insufficiently developed to determine whether episodes of volcanic heating and climatic change are coincident. It is possible that the large floods and volcanic eruptions might trigger a short 'interglacial' interval. Alternatively, the floods may be related to episodic volcanic activity, and the dendritic channels to rainfall that was associated with independent interglacial climatic episodes resulting from variations in solar output or other causes.