Ice in our Solar System

1. Ice in our Solar System • International Polar Year Web Presentation • Gregory A. Neumann • NASA Goddard Space Flight Center • Greenbelt, MD 20771 • Gregory.A.Neumann@nasa.gov

2. What are ices made of? • Solar system is 99% hydrogen, helium, carbon and oxygen, with >0.1% of neon, iron, and nitrogen • Some elements form compounds (H2O, CO2, CH4, NH3...) • Ices are solidified compounds that are gases at standard (room) temperature and pressure (1 bar), called ‘STP’ • Most familiar is water ice, which is a fluid at STP, but has at least nine polymorphs and several isotopes diffraction pattern from deuterated water at ~ 8 kbar Fortes, A. D.

3. Interior of gas giants? • At Jovian pressures, interior hydrogen becomes metallic! • Uranus and Neptune probably contain water, ammonia, and methane ices, as does Titan

4. Where can ice reside on terrestrial planets? • Distance from sun? • Inclination and shadowing? • Greenhouse atmosphere? • Water concentration? !! Too dry! Too hot! ?! ?

5. We all know what snow and ice looks like, don’t we? • http://emu.arsusda.gov/snowsite/

6. Polar cold traps • Scientists use the Kelvin absolute temperature scale, where ice melts at 273.16 K. • Dry ice forms at Mars atmospheric pressure at 145 K, water ice clouds form at ~180-200 K. • Liquid oxygen (1 bar): 90 K • Liquid nitrogen (1 bar): 77 K • Temperatures in Shackleton Crater: 88 K No surface ices exposed?

7. Ice on Mercury • Earth-based radar observations - 1961 to present • 900 kW transmitted from the 306-m Arecibo telescope Volume backscattering in ice inverts circular polarization

8. High-Resolution Radar Imaging of Mercury’s North Pole J. K. Harmon, P. J. Perillat, and M. A. Slade, 2000. The case for the existence of polar ice on Mercury • Radar Circular Polarization Ratios • μc=σsc/σoc • specular: no depolarization (μc=0), • rough: μc<1 • ice: polarization inversion (μc>1). • observed:μc= 1.25 • Orbit 0.37 AU, but near-zero inclination to sun • Radar scattering characteristics similar to those of the icy Galilean satellites and Mars’southern ice cap • Strong reflections located in permanently shaded floors of polar craters. Arecibo S-band radar map

9. Moon’s inclination to the Sun is only 1.5°, allowing permanently shadowed regions inside craters Ice on the Moon Lunar Prospector Neutron Spectrometer looks for "slow" (or thermal) and "intermediate" (or epithermal) neutrons which result from collisions of normal "fast" neutrons with hydrogen atoms. The ice was thought to be spread over 10,000 to 50,000 square km and amount to 6 billion metric tons. A significant amount of hydrogen would indicate the existence of water - 4.6% over the north polar region and 3% over the south, at a depth of about 40 centimeters beneath dry regolith. 1) Fluxes of fast and epithermal neutrons from Lunar Prospector: Evidence for water ice at the lunar poles, Feldman et al., Science, v. 281, p. 1496, 1998 No water (as OH-) was detected from the July 31, 1999 crash of Lunar Prospector into the Moon.Possible reasons: might have missed the target area; might have hit a rock; crash had too little energy to separate water from minerals; plume hidden from telescopes by crater walls; telescopes mispointed; or hydrogen simply may not be in the form of water ice.

10. Clementine bistatic radar experiment • S-band radar, 6 W, right circular polarized, transmitted by Clementine • Signals received by 70-m Deep Space Networkantenna showed reflection characteristics suggestive of water ice in permanently shadowed areas near the south pole • Arecibo radio telescope studies using the same radio frequency as Clementine showed similar reflection patterns from areas which are not permanently shadowed. This experiment’s conclusion remains controversial. The Clementine bistatic radar experiment, Nozette et al., Science, v. 274, p. 1495, 1996

11. Lunar Reconnaissance Orbiter / Chandrayaan-1 • Two most recent (late 2008-2009) spacecraft orbiting Moon • Powerful, modern radar experiments (Forerunner, Mini-RF) will image PSR’s in detail • Lyman-Alpha Mapping Project (LAMP) will see interiors of PSRs by galactic far-ultraviolet light (hydrogen emissions) • Lunar Orbiter Laser Altimeter will measure albedo on 5-m footprint scales, detecting 4% ice • Neutron spectrometer (LEND) will resolve hydrogen atoms • DIVINER will measure surface temperatures and rock environments

12. Polar ice on Mars • The seasonal polar caps of Mars were once thought to melt and cause the seasonal changes in appearance (dust, really) • Present-day caps are too cold to melt • Altimetry and radar shows caps are nearly pure water ice, covered by seasonal CO2 frost MOLA Science Team MARSIS Team

13. Water ice on Mars • Ice is buried in regolith and glaciers at temperate latitudes • Phoenix lander scooped it up! • Could Mars once have been wet?

14. Outer Icy Moons • Exploration has only begun • Enceladus, Titan, Europa are prime targets • Distance, time, power, radiation...

15. Jupiter’s Moons - Europa • Europa is in synchronous rotation with Jupiter with a period of 3.55 days. • Europa has a diameter of about 3130 km and is nearly spherica • The exterior surface is believed to be ice of unknown thickness. • Europa is suspected of having a water ocean beneath its ice crust. • If so, a potentially habitable environment in the outer solar system

16. Saturn’s smaller moons • Enceladus is known to have water ice geysers • Energy source is likely tidal, but interior structure is poorly understood • Another habitable environment?