This section is based on the mission scenario currently envisioned by the IMEWG. It is a plan for launching probes to Mars at every launch opportunity from 1996 to 2003. The plan assumes multiple launches, with the U.S. component being two launches at each opportunity so that failure or delay of one element will not result in a total failure for that opportunity. The plan also assumes that missions currently in their implementation phase will be launched as planned. The early missions are predominantly U.S. and Russian, so the scenario is strongly dependent on sustained funding for Mars exploration in both the U.S. and Russia.

THE 1996 OPPORTUNITY.

Three missions are approved for launch in 1996: the U.S. Mars Global Surveyor (MGS) and Pathfinder missions, and the Russian mission, Mars-96 (formerly Mars-94). The first of these is designed to recover part of the Mars Observer objectives.

Mars Global Surveyor
The U.S. MGS, the first element of the proposed multimission Mars Surveyor Program, is an orbital mission intended to recover part of the Mars Observer science objectives. However, the spacecraft, to be launched on a Delta, is considerably smaller than the Mars Observer spacecraft and the original payload cannot be fully accommodated. MGS will carry the following instruments of relevance to exobiology:

Camera. A camera incorporating both wide-angle and narrow-angle capability will provide planet-wide surface imagery and monitor global atmospheric and surface changes at 7.5 km resolution, partial coverage of the surface at a few tens of meters resolution, and selected targets at 1.4 m resolution. These cameras will provide some of the necessary information for geomorphology, geologic mapping (including establishment of relative chronologies), site selection and mission planning.

IR spectrometer. A thermal emission (mid-IR) spectrometer will map variations in the mineralogy of the surface and obtain temperature profiles of the atmosphere. This will permit identification of important surface lithologies of interest to exobiology, including evaporites and those characteristic of hydrothermal activity, mapping their global distribution at about 3 km spatial resolution.

Altimeter. A laser altimeter will determine surface topography. These data will permit definition of drainage divides and basins, necessary for evaluating the potential for aqueous sedimentary deposits. These data are also of crucial importance for planning of future landed missions, because topography places constraints on engineering and systems for landing.

Radio science. A multi-purpose radio-science experiment is expected to yield a greatly improved determination of the martian gravity field, needed for accurate interpretation of the altimeter data. It is also envisioned that the spacecraft would carry a communication antenna to relay back to Earth data from the Mars-96 penetrators and small landers (see below).

Pathfinder (Lander)
The U.S. Pathfinder mission will be launched on a Delta. It is primarily an engineering demonstration. Its main objectives are to develop and demonstrate a low-cost entry, descent and landing system that could be used for subsequent missions. It does, however, carry some instruments of scientific, and even exobiological, interest and will be landing in exobiologically interesting terrain. The primary landing site for Pathfinder is located on Chryse Planitia near the terminus of Ares Vallis (Latitude 20 deg. N Longitude 34 deg. W). The periodic and catastrophic outflows that created these channels could have transported detrital thermal-spring materials downstream from the thermokarst sources located in areas of chaos near their head reaches, depositing them at the lander site. This type of "grab bag" site has the advantage of providing access to a wide variety of lithologies from the surrounding regional geologic terrain, and in the case of Ares Vallis, may also provide samples of aqueous minerals that have a high priority for exopaleontology.

The instruments carried on Pathfinder include the following:

Camera. This will provide color stereo images of rocks near the lander, at about the same resolution as the Viking Lander cameras, but with 12 color filters for each camera rather than the 6 filters of Viking. These filters will permit a limited assessment of mineralogy, especially for iron oxides and pyroxene. In addition, there will be two filters for water-vapor detection and one for atmospheric dust.

Magnetic susceptibility experiment. In principle, this could supply some information about the mineralogy of the regolith close to the lander.

Pathfinder (Rover)
Mars Pathfinder will also carry a small, solar-powered rover that will be able to move within several to tens of meters from the lander, limited mainly by line-of-sight communications. The rover will carry the following instruments:

Camera. The camera used for navigation of the rover will also be used for scientific imaging. This will permit examination of local lithologies at a resolution of about 3 mm at 1 m range, giving some information about rock textures and fabric.

Alpha-proton-x-ray spectrometer (APX). This instrument can be placed in close contact with rocks and soils reachable by the rover, permitting analysis of the surface regions of those lithologies for the major rock- forming elements (O, Na, Mg, Al, Si, S, K, Ca, Ti and Fe) and carbon.

Mars-96 (Orbiter)
The Mars-96 spacecraft is to be launched on a Proton. It consists of an orbiter, two penetrators and two small landers. The penetrators and landers will be released to the surface shortly before the orbiter is injected into Mars orbit. The orbiter will carry a wide array of instruments of exobiological relevance:

Cameras. Three separate cameras are designed, respectively, for navigation, for low-resolution surface and atmospheric monitoring, and for high-resolution color and lower-resolution stereo surface mapping. As with Mars Global Surveyor, these images will provide basic information about martian geology, and will be invaluable for site selection and mission planning. Optimum spatial resolution will be ~10 m/pixel on the ground, with stereo "pixel height" resolution of ~20 m.

UV, visible and IR spectrometers. A variety of such instruments will provide information on the global distribution of major lithologic units, including aqueous mineral deposits. Spatial resolution of the visible and IR imaging spectrometer will be between 0.4 and 4.0 m/pixel over the spectral range 0.32 to 5.2 *m.

IR radiometer. The mapping radiometer will operate over a spectral range of 8.5 to 12 *m at 0.1 km/pixel and will map thermal anomalies at a resolution of 0.5 km/pixel.

Long-wave radar. This experiment will probe beneath the martian surface in a global search for subsurface water ice. It will operate with a range measurement uncertainty of 3.5 km. Neutron and gamma-ray spectrometers. These instruments will map variations in the surface distribution of a number of key elements (H, C, Na, Mg, Al, Si, S, Cl, K, Ca, Ti, Fe, Th and U). Although the spatial resolution of this experiment is poor, essentially equal to the height of the spacecraft above the planets surface, information on the global distribution of water is of fundamental importance to exobiology, as well as to many other disciplines. The data for the rock- forming elements will usefully complement the mineralogical mapping performed by the UV/Vis/IR spectrometers as well as being necessary for accurate derivation of water abundance from the spectrometer data.

Neutral mass spectrometer. This is designed to determine the elemental and isotopic composition of hydrogen, oxygen neon and argon in the upper atmosphere. These data, in conjunction with a variety of plasma experiments designed to characterize the solar wind and its interaction with the planet, will aid in reconstruction of the long- term evolution of the light-element inventories on Mars.

Mars-96 (Landers)
The objectives of the small landers are to determine the vertical structure of the atmosphere at the landing sites, to determine the chemistry of the materials at the sites, and to make prolonged magnetic, seismic, and meteorological measurements. To meet these objectives each lander carries a variety of instruments, of which the following will generate data of exobiological relevance:

Alpha-proton-x-ray (APX) spectrometer. Similarly to the case for the Mars Pathfinder rover, this instrument will yield the abundances of the rock-forming elements, and carbon, in the close vicinity of the lander. However, on Mars-96, resulting data will be mainly derived from regolith samples rather than from individual rocks.

Mars oxidant experiment (MOx). This experiment, which is being supplied by a US team, is designed to study the oxidant(s) apparently responsible for elimination of organic matter from the upper martian regolith. The MOx employs a series of fiber-optical detectors, coated with reagents sensitive to a wide range of different oxidants, which are exposed to the regolith close to the lander.

Mars-96 (Penetrators)
Each penetrator consists of two parts. A forebody will penetrate the surface to a depth of a few meters carrying with it several instruments of interest to exobiology:

Gamma-ray spectrometer. This will measure the concentrations of a number of elements (H, Na, Mg, Al, Si, K, Cl, Ca, Ti, Mn, Fe, Th and U) in the regolith within a meter or so of the penetrator.

Neutron spectrometer. This is designed to monitor the water content of the regolith within about 30 cm of the penetrator.

APX. This instrument will measure the composition of the regolith adjacent to the penetrator for the elements listed above for the lander APX.
The aft body, which remains at the surface, contains several instruments of which two are of particular exobiological relevance:

Camera. A camera operating in the visible range will image the terrain surrounding the impact site and local lithologies exposed at the surface near the penetrator.

Gamma-ray spectrometer. This will analyze the martian surface adjacent to the penetrator for the elements listed above in connection with the forebody instrument.

In conclusion, Mars-96 is a very comprehensive mission addressing a broad range of science questions, and making a variety of pilot measurements on the surface that will be useful for the design of subsequent more definitive experiments. It is also an international mission, the instruments being provided by a large number of nations.

THE 1998 OPPORTUNITY

Planet-B
In 1998 Japan plans to launch Planet B, its first nonlunar planetary mission. This will be a Mars mission launched on an M-5 rocket. The objectives of the mission are two-fold: (1) to study the martian upper atmosphere and its interaction with the solar wind, and (2) to develop technologies for future planetary missions. The scientific objectives are threefold. The first set of experiments will measure the structure, composition and dynamics of the ionosphere, the effects of interaction of the upper atmosphere with the solar wind, and the escape of atmospheric constituents. The second set of experiments will measure the intrinsic magnetic field, the penetration of the solar-wind magnetic field, and the structure of the magnetosphere. The third set of experiments will measure dust in the upper atmosphere and in orbit around Mars. Numerous instruments, which need not be detailed here, will be employed to perform this wide range of measurements, but of particular exobiological interest is a U.S.-supplied neutral mass spectrometer which will measure the elemental and isotopic composition of the upper martian atmosphere.

Mars Surveyor
The payload of this U.S. orbiter has yet to be determined but there is a strong desire for it to carry the two Mars Observer experiments not included on Global Surveyor:

Infrared radiometer (PMIRR). This instrument will map the three- dimensional structure of the martian atmosphere, including the distribution of water vapor, and will follow the transport of this water throughout the current martian system for a martian year. It has the capability to discover localized sources of water on the martian surface, if such exist.

Gamma-ray/Neutron spectrometer (GRS). As in the case of Mars-96, this instrument will map the chemistry of the surface, including the all- important distribution of near-surface hydrogen, i.e., hydrated minerals or ice. The sensitivity of this spectrometer for near-surface water is likely to be somewhat greater than that of the Mars-96 instrument.

The orbiter will likely also carry a camera, an ultrastable oscillator and a relay antenna to service landers on the surface.

Current NASA plans call for launch of the 1998 Mars Surveyor orbiter on the proposed Med-Lite launch vehicle. The limited throw weight of this vehicle would probably restrict the payload to only one of the remaining Mars Observer instruments. If such an unfortunate circumstance transpires, exobiological priorities would strongly favor selection of the GRS.

Neolander
A Pathfinder-derived Mars lander is currently being considered by NASA for the 1998 opportunity. This "Neolander" would be sufficiently reduced in weight (175 kg landed mass vs. 270 kg for Pathfinder) so that it could be launched on a small launch vehicle, a Med-Lite instead of a Delta. The capabilities and instrumentation have yet to be defined, but the mission theme is planned to be Volatiles and Climatology, with emphasis on the history of water. This could result in measurement of light-element isotope ratios and volatile compounds in the regolith, both topics of significant exobiological interest.

Joint U.S.-Russian Activities
In addition to the above missions, the U.S. and Russian space agencies are exploring possible cooperative arrangements for Mars opportunities from 1998 on. Details of these activities remain to be defined at this time, but exobiology may well be one of the topics of Mars exploration of mutual interest to both parties.

THE 2001 OPPORTUNITY

The possibilities for 2001 are being evaluated. The U.S. Mars Surveyor program calls for at least two launches to Mars at every launch opportunity for the life of the program. The current plan is for the U.S. to use Med-Lite launches to place a network of small meteorological stations on the martian surface.

There is a similar uncertainty with respect to Russian plans. A Proton could be used to launch an as yet undetermined combination of previously developed vehicles.

THE 2003 OPPORTUNITY

2003 is the first opportunity at which an international network of multiple stations could be established on the martian surface. The ESA Intermarsnet concept would use an Ariane 5 booster to launch a Rosetta-derived carrier to Mars. The ESA carrier could deliver an ASI- furnished commun-ications satellite into orbit around the planet and four landers built by an international consortium would be deployed on the surface. At the same time the Russians are exploring the possibility of using a Proton to send four Mars-96-derived small stations and two penetrators to the surface, and the U.S. could send two Mini- geolanders similar to those developed for the 2001 opportunity. Thus the possibility exists of having 12 simultaneously operating stations on the surface, thereby achieving the seismology and meteorology goals of a global network, and adding substantially to the number of sites at which we would have in situ measurements and observations.

A preliminary set of science goals has been established for the ESA-launched landers. The internal structure and dynamics of the planet will be determined by a network of simultaneously operating, broad-band seismo-meters, flux-gate magnetometers and transponders. The dynamics of the atmosphere and the atmospheric boundary layer will be studied using a network of simultaneously operating meteorology stations, and the vertical structure of the atmosphere at each landing site will be determined during descent. The geology and geochemistry of the landing sites will add further to our knowledge of Mars' diversity. All these objectives would be significantly enhanced if the ESA-launched probes were supplemented by or complemented by Russian- and/or U.S.-launched landers.

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