Mercury’s Mysterious Polar Deposits Sarah Mattson PTYS 395A 2/6/2008 South polar region, imaged by Mariner 10 on second flyby. Frame

Introduction RADAR-bright deposits 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? 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 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 Color-coded RADAR image of Mercury from the Goldstone-VLA, 1992; North is up Mercury’s orbital inclination to the solar ecliptic (image from Wikipedia)

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 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 Harmon, et al North poleSouth pole Anomalous bright areas at the poles 15km resolution. Features are roughly circular, correspond to known craters.

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 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., Understand abundance of volatiles during planetary formation, delivery processes

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 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 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 km 3 for a 30 km diameter crater (Vilas et al. 2005) Area covers thousand km 2, or <0.1% Mercury’s surface (Butler et al. 2001) Sulfur stable for long lifespan down to 82 o 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 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 1 st flyby Planned 2 nd flyby and final orbit (at left) will provide opportunities for observations of north polar deposits (images JHU/APL from MESSENGER website)