In the general area of upper-atmosphere, ionosphere, and solar wind interaction studies of Mars, the main priorities for research are these:

1. The dynamics of the upper atmosphere. Absolutely no direct information exists on neutral gas velocities, but it is badly needed in order to obtain a basic understanding of the dynamical processes and coupling to the lower atmosphere.

2. Hot atom abundances and escape fluxes. Pioneer Venus’s ultraviolet spectrometer established the presence of hot oxygen and carbon atoms at Venus, and the earlier Mariner observations indicated the presence of hot hydrogen. There is no information on these hot atoms at Mars, but in light of the similarities with Venus and of theoretical models, it is clear that they must be present. The low escape energy at Mars also implies that hot atom escape fluxes may be important.

3. Ion escape from Mars. Both theory and observations indicate that there are significant ion escape fluxes at Mars. These escape flows, whether due to tailward flow from the ionosphere or to scavenging, are related to the solar wind interaction processes. A meaningful understanding of the escape processes, mentioned under priorities 2 and 3 above, are of great importance in understanding how the martian atmosphere evolved during the last few billion years. If there was life on Mars some time ago, how did an atmosphere that was capable of supporting life evolve into the one that remains today?

4. Mini-magnetospheres and reconnection at Mars. The discovery of remnant crustal magnetic fields at Mars means that small, localized “magnetospheres” are likely to be present. There have been suggestions that such mini-magnetospheres are present around the Moon. These small magnetospheric regions may undergo reconnection with the compressed interplanetary magnetic field in the magnetosheath. These reconnection events must depend on the specific location of the crustal field with respect to the subsolar location and solar wind parameters, such as magnetic field angle.

5. The energetics of the ionosphere. There are major unexplained issues concerning the mechanisms that determine the electron and ion temperatures in the ionospheres of both Venus and Mars. The question is why the electron and ion temperatures are as high as the measurements indicate. There are two possible answers to this question, but no clear resolution.

The five priorities listed above are embraced by recommendations COMPLEX has made in the past (Appendix B: [1.13, 1.14, 4.6]). Two of the secondary recommendations put forward in Chapter 12 are consistent with the priorities outlined here.

There are no existing plans in the current U.S. Mars Exploration Program to address any of the scientific priorities outlined in the previous section. The Nozomi mission would address items 2, 3, 4, and 5 to some extent. However, that spacecraft has already lost one of its two transmitters, and while one hopes for its success, the mission was not designed for its current 5-year interplanetary cruise phase. At best, it will provide some initial answers to these areas of research, but much more is needed to meaningfully elucidate these open issues. The instruments aboard Mars Express will address issues listed under items 2 and 3 above, but here again, much will be left unanswered. It is also noted that both spacecraft will arrive at Mars during solar minimum, and as indicated above, data from solar maximum are imperative in order to answer some of the outstanding questions (e.g., regarding nonthermal escape).

The instruments needed for a meaningful attack on the five scientific questions listed above would require no new, basic instrument development, and could be installed as a partial payload complement on an orbiting spacecraft. The neutral winds can be measured by either a “baffled” neutral mass spectrometer or a Fabry-Perot interferometer. The latter instrument, along with a good ultraviolet spectrometer, could address in a meaningful way the hot atom and neutral escape flux questions. The neutral mass spectrometer would also provide neutral composition and temperature information. A plasma instrument complement consisting of a magnetometer, a low-

Front Matter (R1-R12)

Executive Summary (1-4)

1. Introduction (5-8)

2. Interior and Crustal Structure and Activity (9-17)

3. Geochemistry and Petrology (18-25)

4. Stratigraphy and Chronology (26-33)

5. Surface Processes and Geomorphology (34-47)

6. Ground Ice, Groundwater, and Hydrology (48-53)

7. Life, Fossils, and Reduced Carbon (54-63)

8. Lower Atmosphere and Meteorology (64-70)

9. Climate Change (71-76)

10. Upper Atmosphere, Ionosphere, and Solar Wind Interactions (77-82)

11. Rationale for Sample Return (83-88)

12. Assessment of the Mars Exploration Program (89-104)

13. Conclusions (105-106)

Appendix A: The NASA Mars Exploration Program (107-117)

Appendix B: Compilation of Recommendations Concerning Mars Exploration Made by COMPLEX and Other Advisory Groups (118-130)

Appendix C: Acronyms (131-132)