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_at_nasa.gov

2
What are ices made of?

• Solar system is 99 hydrogen, helium, carbon and
  oxygen, with gt0.1 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, dont
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
The case for the existence of polar ice on Mercury
High-Resolution Radar Imaging of Mercurys North
Pole J. K. Harmon, P. J. Perillat, and M. A.
Slade, 2000.

• Radar Circular Polarization Ratios
• µc ssc /soc
• specular no depolarization (µc 0),
• rough µc lt 1
• ice polarization inversion (µc gt 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 Marssouthern
  ice cap
• Strong reflections located in permanently shaded
  floors of polar craters.

Arecibo S-band radar map
9
Ice on the Moon
Moons inclination to the Sun is only 1.5,
allowing permanently shadowed regions inside
craters
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 Network
  antenna 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 experiments
  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 PSRs 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
Jupiters 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
Saturns smaller moons

• Enceladus is known to have water ice geysers
• Energy source is likely tidal, but interior
  structure is poorly understood
• Another habitable environment?