The sensitivity of an immature vestibular system to altered gravity.

Stimulus deprivation or stimulus augmentation can induce long-lasting modifications to sensory and motor systems. If deprivation is effective only during a limited period of life this phase is called "critical period." A critical period was described for the development of the roll-induced vestibuloocular reflex (rVOR) of Xenopus laevis using spaceflights. Spaceflight durations and basic conditions of Xenopus' development did not make it possible to answer the question whether exposure of the immature vestibular organ to weightlessness affects rVOR development. The embryonic development of Pleurodeles waltl is slow enough to solve this problem because the rVOR cannot be induced before 15 dpf. Stage 20-21 embryos (4 dpf) were exposed to microgravity during a 10-day spaceflight, or to 3g hypergravity following the same time schedule. After termination of altered gravity, the rVOR was recorded twice in most animals. The main observations were as follows: (1) after the first rVOR appearance at stage 37 (16 dpf), both rVOR gain and amplitude increased steadily up to saturation levels of 0.22 and 20°, respectively. (2) Three days after termination of microgravity, flight and ground larvae showed no rVOR; 1 day later, the rVOR could be induced only in ground larvae. Differences disappeared after 3 weeks. (3) For 10 days after 3g exposure, rVOR development was similar to that of 1g-controls but 3 weeks later, 3g-larvae showed a larger rVOR than 1g-controls. These observations indicate that the immature vestibular system is transiently sensitive to microgravity exposure and that exposure of the immature vestibular system to hypergravity leads to a slowly growing vestibular sensitization.

Key science questions from the second conference on early Mars: geologic, hydrologic, and climatic evolution and the implications for life.

In October 2004, more than 130 terrestrial and planetary scientists met in Jackson Hole, WY, to discuss early Mars. The first billion years of martian geologic history is of particular interest because it is a period during which the planet was most active, after which a less dynamic period ensued that extends to the present day. The early activity left a fascinating geological record, which we are only beginning to unravel through direct observation and modeling. In considering this time period, questions outnumber answers, and one of the purposes of the meeting was to gather some of the best experts in the field to consider the current state of knowledge, ascertain which questions remain to be addressed, and identify the most promising approaches to addressing those questions. The purpose of this report is to document that discussion. Throughout the planet's first billion years, planetary-scale processes-including differentiation, hydrodynamic escape, volcanism, large impacts, erosion, and sedimentation-rapidly modified the atmosphere and crust. How did these processes operate, and what were their rates and interdependencies? The early environment was also characterized by both abundant liquid water and plentiful sources of energy, two of the most important conditions considered necessary for the origin of life. Where and when did the most habitable environments occur? Did life actually occupy them, and if so, has life persisted on Mars to the present? Our understanding of early Mars is critical to understanding how the planet we see today came to be.

New perspectives on ancient Mars.

Mars was most active during its first billion years. The core, mantle, and crust formed within approximately 50 million years of solar system formation. A magnetic dynamo in a convecting fluid core magnetized the crust, and the global field shielded a more massive early atmosphere against solar wind stripping. The Tharsis province became a focus for volcanism, deformation, and outgassing of water and carbon dioxide in quantities possibly sufficient to induce episodes of climate warming. Surficial and near-surface water contributed to regionally extensive erosion, sediment transport, and chemical alteration. Deep hydrothermal circulation accelerated crustal cooling, preserved variations in crustal thickness, and modified patterns of crustal magnetization.