Sun-Scorched Mercury

5.430 g/cm 3 MESSENGER's suite of instruments have provided insight into the mineral makeup of the surface terrain and detected ultraviolet emissions from sodium, calcium and hydrogen in Mercury's exosphere.

Physical Properties MoonMercuryEarth Radius1700 km2440 km6380 km Mass7.3 × kg3.3 × kg6.0 × kg Density3.300 g/cm g/cm g/cm 3 Escape Speed 2.4 km/s4.3 km/s11.2 km/s

Guiding Questions 1.What makes Mercury such a difficult planet to see? 2.What is unique about Mercury’s rotation? 3.How do the surface features on Mercury differ from those on the Moon? 4.Is Mercury’s internal structure more like that of the Earth or the Moon?

88 days

Orbital Properties Phases of Mercury can be seen best when Mercury is at its maximum elongation:

Earth-based optical observations of Mercury are difficult At its greatest eastern and western elongation, Mercury is never more than 28° from the sun It can be seen for only brief periods just after sunset or before sunrise

Apparition Date Elongation Magnitude Morning 2012 Dec °W -0.3 Evening 2013 Feb °E -0.2 Morning 2013 Mar °W +0.5 Evening 2013 Jun °E +0.7 Morning 2013 Jul °W +0.4 Evening 2013 Oct °E +0.2 Morning 2013 Nov °W -0.3 Evening 2014 Jan °E -0.3 Morning 2014 Mar °W +0.4 Evening 2014 May °E +0.7 Morning 2014 Jul °W +0.6 Evening 2014 Sep °E +0.3 Morning 2014 Nov °W -0.3 Greatest Elongations

Solar Transit There will be a transit on May 9, 2016 Transits occur about twelve times a century when the sun, Earth and Mercury are aligned

Best Earth-based Views of Mercury Difficulties observing Mercury from Earth led early astronomers to incorrectly decide that Mercury always kept the same face towards the sun in synchronous orbit Note phases like the moon

Rotation and Revolution Like Earth’s moon (tidally locked to revolution around Earth), Mercury’s rotation has been altered by the sun’s tidal forces, but not completely tidally locked: Revolution period = 3/2 times rotation period Revolution: ≈ 88 days Rotation: ≈ 59 days Mercury was long thought to be tidally locked to the Sun; measurements in 1965 showed this to be false.

Rotation of Mercury 3:2 Spin-Orbit Coupling Because of the highly elliptical orbit, during perihelion the sun appears to stop in the sky, move backwards (for several Earth days), and then return to normal motion.

Strong tidal effects, Mercury’s slightly elongated shape and its very eccentric orbit cause this strange 3- to-2 orbit A “day” of solar light on Mercury would be 88 earth days

Images from Mariner 10 reveal Mercury’s heavily cratered surface Most of our detailed information about Mercury’s surface is from this fly-by mission in 1974/1975. Mariner only saw one side of the planet. Presently, Messenger is in orbit. –Volcanics at surface are mostly basaltic –Silicate rocks are low in Fe BepiColombo mission, scheduled to launch to Mercury in 2014.

Mercury Very similar to Earth’s moon in several ways: Small; no atmosphere lowlands flooded by ancient lava flows heavily cratered surfaces (Less dense cratering than moon) Gently rolling plains Scarps No evidence of tectonics View from Earth

Mercury Note the density of craters. Note the lava flows. Note the similarity in color.

Note how much more densely the craters occur on the moon’s surface.

At the terminator, shadows give us crater wall heights.

The previously unseen side of Mercury. (From Messenger)

Note how much more densely the craters occur on the moon’s surface.

1) There are few maria on Mercury and they are small. No large impact era like the Moon. Therefore, Mercury must have cooled faster. 2) Cratering is less heavy, more plain region between craters. Due to the higher surface gravity on Mercury. This means impacts did not throw debris as far, fewer secondary craters and more concentrated around primary crater. 3) Long scarps or wrinkles are found on the crust and the tops of craters (i.e. after cratering epoch). After Mercury cooled, its crust solidified first. Scarps are believed to have been formed as Mercury cooled. Moon vs. Mercury

The Plains of Mercury No large maria, but intercrater plains: Marked by smaller craters (< 15 km) and secondary impacts Smooth plains: Even younger than intercrater plains

The Caloris Basin is evidence of a large impact ~960 miles in diameter (from new estimates of Messenger images)

Caloris Basin = 1300 km

The sprawling Caloris basin on Mercury is one of the solar system's largest impact basins. Created during the early history of the solar system by the impact of a large asteroid-sized body, the basin spans about 1,500 kilometers and is seen in yellowish hues in this enhanced color mosaic. The image data is from the January 14th flyby of the MESSENGER spacecraft, captured with the MDIS instrument. Orange splotches around the basin's perimeter are now thought to be volcanic vents, new evidence that Mercury's smooth plains are indeed lava flows. Other discoveries at Mercury by NASA's MESSENGER mission include evidence that Mercury, like planet Earth, has a global magnetic field generated by a dynamo process in its large core, and that Mercury's surface has contracted significantly as its core cooled. (APOD, Credit: NASA, Johns Hopkins Univ. APL, Arizona State U., CIW)on Mercuryenhanced color mosaicflyby MESSENGERMDISnew evidence discoveries at Mercurymagnetic fielddynamoNASAJohns Hopkins Univ. APL

Caloris Basin Floor

“The spider" appears to be an impact crater surrounded by more than 50 cracks in the surface radiating from its center. It is on the Caloris Basin.

The seismic waves from the impact that caused the Caloris Basin caused this deformation on the opposite side of Mercury. This terrain covers 500,000 km 2 (twice the size of Wyoming)!

The Caloris Impact Caloris Basin, very large impact feature; weird terrain on opposite side of planet:

Orientale Since studying Caloris, we have come to understand that the same process has been at work elsewhere. This is the large crater basin Orientale on the far side of the Moon. It too has jumbled terrain opposite it on the Moon.

Radar of Mercury Dominated by ancient lava flows and heavy meteorite bombardment. Radar image suggests icy polar cap.

Mercury's North Pole. These unusual images of what looks likes ice have sparked the imagination of scientists. It has been suggested that a tiny flow of ice from comets and meteorites could be cold-trapped in these polar deposits over billions of years, or that the polar deposits consist of sulfur that has seeped from minerals in the surface rocks over the eons.

Scarps are cliffs This one is more than a km high They probably formed as the planet cooled and shrank

Scarps Some distinctive features: Scarp (cliff), several hundred km long and up to 3 km high

January 14, Part of an old, large crater occupies most of the lower left portion of the frame. An arrangement of ridges and cliffs in the shape of a "Y" crosses the crater’s floor. The shadows defining the ridges are cast on the floor of the crater by the Sun shining from the right, indicating a descending stair- step of plains. The main, right- hand branch of the "Y" crosses the crater floor, the crater rim, and continues off the top edge of the picture; it appears to be a classic “lobate scarp” (irregularly shaped cliff) common in all areas of Mercury imaged so far. In contrast, the branch of the Y to the left ends at the crater rim and is restricted to the floor of the crater. Both it and the lighter-colored ridge that extends downward from it resemble “wrinkle ridges” that are common on the large volcanic plains, or "maria," on the Moon.

Mercury has an iron core and a surprising magnetic field Most iron-rich planet in the solar system with a core that is 85% of the diameter (New # from Messenger data, March 2012.) The earth’s core is 55% of its diameter and the moon’s core is 20% of its diameter Highest uncompressed density for the planets Weak magnetic field indicating part of the core is liquid Magnetic field causes a magnetosphere similar to Earth’s but weaker

Interiors Mercury is much denser (over 60%) than the Moon, and has a weak magnetic field – not well understood!

85% 61%

Mercury may have a very Different core: Out solid iron sulfide layer Deeper liquid iron layer Possible solid Fe core

If the core is so big, why does Earth have a higher density? We failed to take account of the mass of the object. What happens is that more massive objects have stronger gravities. As a result, more massive objects get more compressed than less massive objects. This compression (decrease in the size of the object), means that the density of the object will be greater. Thus, even for objects which have the same composition, if one is more massive, then the objects will have different densities. This effect needs to be accounted for when using densities to determine chemical compositions. A quantity known as the Uncompressed Density is defined which is free of this mass dependence. The uncompressed densities of Mercury 5.3 g/cm 3 Venus 3.95 g/cm 3 Earth 4.03 g/cm 3 Moon 3.3 g/cm 3 Mars 3.71 g/cm 3 Mercury is significantly denser than the Earth and thus composed of heavier elements; Mercury has significantly more iron and nickel than the Earth.

How do you get such a big core? Hypotheses: –Close to Sun, so more metals –New Sun blew upper portions off –Impact exploded upper portions off

Magnetic Field Orbital data reveal that Mercury's magnetic field is offset far to the north of the planet's center, by nearly 20% of Mercury's radius. Relative to the planet's size, this offset is much more than in any other planet. Implications: –the magnetic field in the southern hemisphere should be a lot weaker than it is in the north. At the north geographic pole, the magnetic field should be about 3.5 times stronger than it is at the south geographic pole. –The big difference in northern and southern surface field strengths means that energetic particles, solar wind, and high- energy electrons will preferentially impact the surface in the south, and this situation should lead to asymmetries both in sources of atoms, ions, and molecules for Mercury's exosphere and in the discoloration of the surface by charged particle bombardment

The Magnetic Field of Mercury Magnetic field only ~ 0.5 % of Earth’s magnetic field. Difficult to explain at present: Liquid metallic core should produce larger magnetic field. Solid core should produce weaker field.

Exosphere Mercury is surrounded by a tenuous exosphere of gas generated and maintained by the interaction of the space environment with the planet's surface. the major constituents are sodium, magnesium, and calcium atoms Before MESSENGER, the prevailing theory suggested that material was released from the dayside by solar wind and radiation. In this picture, lofted material then was carried to the nightside by solar radiation pressure. MESSENGER measurements show that magnesium and calcium in the tail region are substantially more abundant than would be expected if they were produced in this way New magnetic field models indicate that the planet's intrinsic field can couple with the interplanetary field to direct solar wind ions to the nightside, sputtering material from non-illuminated surfaces. But that source is too weak to explain the observed concentrations. Calcium also exhibits an unexplained enhanced concentration at the equator near dawn, a pattern that appears to be a persistent feature in the exosphere. Such dawn enhancements are not observed for magnesium, which is chemically similar to calcium.

Shallow, flat-bottomed pits on Mercury may be caused by subatomic particles from the sun zapping away at sulfur-laden minerals. Credit: NASA

Evolutionary History of Mercury Formed about 4.6 billion years ago Melted due to bombardment, cooled slowly Extrusion of intercrater plains during heavy bombardment Cooling, contraction (scarps), and Caloris basin formation near end of heavy bombardment Eruption of smooth plains within and surrounding large impact basing (at tail end of heavy bombardment) Continued cooling, lithospheric thickening and contraction (closing off magma sources) Light impact cratering to present

Similarities Moon vs. Mercury Main surface features on Moon: maria, highlands, Mercury has similar features but also a great number of plains Both heavily cratered Both have no atmosphere, and large day–night temperature excursions Tidal interactions responsible for synchronicity of Moon’s orbit, and resonance of Mercury’s Both may have ice in polar regions Regolith Basin Formation Evidence for volcanic activity

Differences Moon vs. Mercury Iron Core (density) Origin Magnetic Field Scarps Mercury has no maria, but does have extensive intercrater plains Similar colors on Mercury (between terrain types), different colors on the Moon