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South polar region, imaged by Mariner 10 on second flyby. Frame 166902. Mercury’s Mysterious Polar Deposits. Sarah Mattson PTYS 395A 2/6/2008. Introduction. RADAR-bright deposits were discovered at Mercury’s polar regions in early 1990’s

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Presentation Transcript
introduction
Introduction
  • RADAR-brightdeposits were discovered at Mercury’s polar regions in early 1990’s
  • Thought to be volatile ice in permanently shadowed craters – but other explanations possible
  • Current MESSENGER mission will investigate

Mariner 10 mosaic

why observe mercury with radar
Why Observe Mercury with RADAR?
  • Mariner 10 did not image the entire surface
  • Orbital geometry allows for full disk imaging of Mercury from Earth ground-based Radio telescopes
    • Arecibo Observatory, Puerto Rico
    • Goldstone-VLA, Mojave Desert, California
  • RADAR not affected by Earth’s atmospheric absorption, or by target’s atmosphere
  • Proximity to Sun makes other types of imaging challenging
  • Can’t image with cameras where there is no light!

Mariner 10 mosaic

radar observation from earth
Radar Observation from Earth
  • Soviet scientists first to use RADAR to image Mercury in 1962 (Evans et al. 1965)
  • Radio telescopes at low latitudes and high altitudes, southern hemisphere (of Earth) best
  • Advances in radar imaging techniques allow better resolution
  • RADAR brightness means
    • Surface or subsurface ice or sulfur
    • Rough surface
    • Instrument-facing surface

Mercury’s orbital inclination to the solar ecliptic

(image from Wikipedia)

Color-coded RADAR image of Mercury from the Goldstone-VLA, 1992; North is up

current questions surrounding mercury s polar deposits
Current questions surrounding Mercury’s Polar Deposits
  • Are they volatile ices?
  • What is the nature of regolith cover?
  • How pure is the ice?
  • RADAR-bright non-ice material such as sulfur
  • RADAR reflective surfaces
  • How do removal processes affect the lifespan of polar crater deposits?
  • How did they get there?

Crater Chao Meng-Fu adjacent to South Pole

Imaged by Mariner 10

Diameter 150km

theoretical basis for existence of ices at mercury s poles
Theoretical basis for existence of icesat Mercury’s Poles
  • Axis of rotation very small: no seasons
  • Temperature in polar craters ~100K, while on rest of planet, temperature varies widely
    • Max. temp. ~700K, day
    • Min. temp. ~100K, night
  • Comparison to Moon – possible ices at poles in permanently shadowed craters

Modeling of maximum (top) and average (bottom) diurnal temperatures of Crater C at the North Pole. Max. solar angle 2.3 degrees above crater rim. (Vasavada et al. 1999)

radar imagery

Anomalous bright areas at the poles

  • 15km resolution.
  • Features are roughly circular, correspond to known craters.
RADAR imagery

North pole

South pole

Harmon, et al. 1994

higher resolution imagery
Higher Resolution Imagery
  • Used RADAR brightness of Galilean icy satellites and Martian polar caps to calibrate Mercury readings and verify signal strength
  • RADAR images of Mercury north polar region from Arecibo, taken in 1998 and 1999
  • Top image resolution is 3 km
  • Bottom image resolution is 1.5 km
  • Arrows indicate direction of signal
  • Brightness indicates stronger signal

From Harmon et al. 2001

volatile ices
Volatile Ices
  • Water ice the most favored
  • Delivery methods
    • Comet or meteorite impact
    • Interaction with exosphere
    • Outgassing and entrapment
  • Must be buried by thin layer of regolith (several cm’s deep) to protect against sublimation (Vasavada et al., 19990
  • Understand abundance of volatiles during planetary formation, delivery processes
other possible materials
Other possible materials
  • Sulfur
    • Accumulated in dark craters after sublimating from surface rocks via random walk
    • Rate of accumulation up to 35m/Ga (Sprague et al. 1995)
    • Stable at higher temperatures than water ice, but no similar bright signals seen at other favorable polar craters within this range (Encyclopedia of the Solar System, 2007)
  • Some highly RADAR reflective material either facing beam direction, or rough textured surface
possible sources of deposits
Possible sources of deposits
  • Cometary or asteroid impacts
    • Dust accumulates as volatiles sublimate, eventually covering ice (Vasavada et al. 1999)
  • Volcanism
    • Mercury known to have experienced massive volcanism in the past
  • Degassing from interior
    • Unlikely due to loss of most volatiles from early megaimpact (Kozlova 2004)
size and lifespan of deposits
Size and Lifespan of Deposits
  • Temperature in polar craters steady for the past 3 Ga
  • Vilas et al. (2005) found d/D ratios for RADAR-bright craters lower than other craters on Mercury
  • Volume estimated >630 km3 for a 30 km diameter crater (Vilas et al. 2005)
  • Area covers 30-50 thousand km2, or <0.1% Mercury’s surface (Butler et al. 2001)
  • Sulfur stable for long lifespan down to 82o lat. (Sprague et al. 1995)
  • Continuous process – many questions
    • Adds to other materials (ices)?
    • Offsets ablation of ices by adding sulfur?
    • Ice lost faster than sulfur accumulates?

Mariner 10 image used to measure crater depth (Vilas et al. 2005); Numbers match Harmon, 2001.

messenger goals
MESSENGER goals
  • Gamma-Ray and Neutron Spectrometer (GRNS) that will look for Hydrogen signature at poles
  • Energetic Particle Plasma Spectrometer (EPPS) to look for signs of sulfur at poles
  • Did not image poles on 1st flyby

Planned 2nd flyby and final orbit (at left) will provide opportunities for observations of north polar deposits

(images JHU/APL from MESSENGER website)