Activity 1:

Observing Mercury

Module 10: Mercury -

Planet of Extremes

Summary:In this Activity, we will investigate

(a) Mercury’s vital statistics, and

(b) Mercury missions - Mariner 10 - transfer orbits & gravity assists.

We will leave the Earth - Moon system for now, and investigate Mercury, a planet with a strong resemblance to the Moon.

Then in the next Activity we will look briefly at the most likely explanations of how Mercury and the Moon were formed.

First we’ll compare Mercury’s vital statistics with those of the Moon and Earth:

(a) Mercury’s Vital Statistics

Mercury follows a rather eccentric orbit (e = 0.2) nearthe Sun. It can be seen just after the Sun sets, orjust before the Sun rises, and shows phases, like Venusand our Moon.

Moon

D

M = M

Earth

core mantle

0.27 D

M = 0.01 M

Core - small, possibly partly molten

mantle

crust

Mercury

0.38 D

M = 0.056 M

mantle

crust

crust

Core - large, probably solid

EarthMoon MercuryAv. Distancefrom Sun 1 AU 0.39 AU1 AU

Being on a eccentric orbit, Mercury’s distance from the Sun varies between 46 million km and 70 million km. Mercury’s proximity to the Sun leads to temperature extremes, as we will see. Careful observations of Mercury’s orbit in the 19th century showed that something was not quite right, and people began searching for an unseen planet dubbed Vulcan. It turned out, however, that the ‘defects’ in Mercury’s orbit were due to its proximity to the Sun - which distort nearby space as predicted by Einstein’s General Relativity theory. Predicting and detecting this effect was one of the first great tests of General Relativity.

EarthMoon MercuryAv. Distancefrom Sun 1 AU 0.39 AU1 AU

Length of “Year”

1 y 1 y 88 d

As we saw in the Activity Solar System Orbits, Mercury, being close to the Sun, has a high orbitalvelocity. The combination of a small orbit and a high velocity makes its orbital period (its “year”) very short indeed.

EarthMoon MercuryAv. Distancefrom Sun 1 AU 0.39 AU1 AU

Length of “Year”

1 y 1 y 88 d

Length of solar day

29.5 d 1 d 176 d

The rotation period of Mercury (time it takes to spin on its axis) is 59 days, while the orbital period (time it takes to orbit the Sun) is 88 days. Thus there is a 3:2 relationship between the spin and orbit, which leads to a solar day of 176 days - twice as long as its year! To find out more, follow this link.

EarthMoon MercuryAv. Distancefrom Sun 1 AU 0.39 AU1 AU

Length of “Year”

1 y 1 y 88 d

Length of solar day

29.5 d 1 d 176 d

Inclinationof axis

1.3° 23.5° 1°

Like the Moon, Mercury does not seasonal effects since there is very little tilt of its rotation axis to the ecliptic .

EarthMoon MercuryAv. Distancefrom Sun 1 AU 0.39 AU1 AU

Length of “Year”

1 y 1 y 88 d

Length of solar day

29.5 d 1 d 176 d

Inclinationof axis

1.3° 23.5° 1°

Accelerationdue to gravity

1.0 g 0.38g0.17g

The acceleration due to gravity on the surface of Mercuryis considerably greater than that on the Moon (0.38 g

compared to 0.17 g).

* Follow this link to see how we estimate the mass of a planet like Mercury.

This is largely because Mercury is 60% denser than the Moon - a number we can calculate from its mass* and radius - which is why we conclude that it probably has a large metal core.

EarthMoon Mercury

av. albedo 0.39 0.120.10

Mercury’s surface reflects very little light - similar to the Moon

EarthMoon Mercury

av. albedo 0.39 0.120.10

atmosphere Almost none 78% N2

20% O2 0.03% CO2

~2% H2O

No permanent atmosphere,

“borrows” solarwind particles

Again, like the Moon, Mercury has almost no atmosphere.Hydrogen and helium particles from the solar wind brieflylinger around Mercury. Sodium and potassium gaseshave been detected on the side of Mercury facing the Sun,“baked out” of the rocks by the Sun’s radiation.

EarthMoon Mercury

av. albedo 0.39 0.120.10

atmosphere none 78% N2

20% O2 0.03% CO2

~2% H2O

No permanent atmosphere,

“borrows” solarwind particles

surface temperature

-170°C +130°C

- 50°C + 50°C

- 180°C + 430°C

With its proximity to the Sun and lack of an insulating atmosphere, Mercury’s surface swings between

extremes of temperature.

Mercury, the planet closest the Sun, receives sunlight which is about seven times more intense than does Earth. Added to this, the long solar days on Mercury means that the daytime temperatures reach about 430°C, which is hot enough to melt lead.

In the long Mercury night, temperatures at the equatorplummet to below -180°C, which is cold enough for carbon dioxide to freeze!

(b) Observing Mercury - Mariner 10

We do not know a great deal about Mercury - the last space mission which observed Mercury in any detail was Mariner 10, launched way back in November 1973.

Transfer Orbits & Gravity Assists

Mariner 10 was the first spacecraft to use the gravitational pull of one planet (Venus) to reach another (Mercury). This is an example of the technique called a gravity assist - usually used to speed a spacecraft up, but in this case, needed to slow Mariner 10 down.

To see why this is necessary, imagine again that we are sending the Swinburne executive spaceship from Earth to Mercury, using as little fuel as possible.

Sun

EarthSpaceship

Mercury

The most efficient trajectoryfor the spaceship is tocoast along part of an ellipse,called a transfer ellipse.The aphelion(point furthestfrom the Sun)of the ellipseis at thepoint wherethe spaceshipleaves Earthorbit.

aphelion

*

* coast means engines off

Sun

EarthSpaceship

Mercury

The perihelion(point nearestto the Sun)of the ellipseis at thepoint wherethe spaceshipmeets Mercury.

aphelion

perihelion

Sun

EarthSpaceship

Mercury

To get the spaceship to follow the transfer ellipseinstead of Earth’salmost circular orbit, the spaceshiphas to be slowed downat aphelion.

aphelion

perihelion

This can be achievedby launching the spaceship in the oppositedirection to the Earth’srotation,and then making fineadjustmentsby firing spaceship rockets to slow itdown even more.

Sun

EarthSpaceship

Mercury

aphelion

perihelion

As the spaceship reachesperihelion, it speedsup, and, unlessslowed down,would betravelling toofast to be able to observe Mercury for any length oftime.

Sun

EarthSpaceship

Mercury

aphelion

perihelion

Again, spaceship rocketsmust be fired in theopposite directionto the spaceship’smotion to slowit down even more. Sun

EarthSpaceship

Mercury

aphelion

perihelion

Firing rockets for any length of time at Mercury’sorbit means that a considerable amountof fuel needs tobe carried,increasing the‘payload’(total weight) Sun

EarthSpaceship

Mercury

aphelion

perihelion

- whichincreases theexpense of placing the spaceship in Earthorbit in the first place.

Our spaceship can use another method to slow down, while still on the way to Mercury:a “gravity assist”.

Sun

EarthSpaceship

Mercury

aphelion

perihelion

If the transfer orbit is timed so thatthe spaceship comes upclose behind Venusin its orbit,then the effectof passing inand out of Venus’s gravitationalfield is to slowthe spaceshipdown.

Sun

EarthSpaceship

Mercury

aphelion

perihelion

Venus

This means that less fuelis needed to slow thespaceship downfurther at perihelion.

Sun

EarthSpaceship

Mercury

aphelion

perihelion

Venus

Our spaceship’s encounter with Venus is a little like a car colliding into the back of a van - the car is definitely slowed down.

(The aim of course is not to collide the spaceship withthe surface of Venus! Its encounter with Venus’s gravitational field has the required effect of slowingit down without destroying it.)

The orbit of Mariner 10 was more complicated than that of our hypothetical spaceship,

for example, Mariner 10 made three flybys of Mercury- to closest approach distances of 704km, 47,000km and 327km,

however the basic principles of the orbit were similar to those in our example above.

In the next Activity we will study what information we have about Mercury’s surface and evolution, gleaned largely from the Mariner 10 flybys.

Note though, that Mariner 10 only imaged approximatelyhalf the surface of Mercury - we do not know what the other half looks like.

NASA: Mariner 10 mosaic of one hemisphere of Mercury

http://nssdc.gsfc.nasa.gov/image/planetary/mercury/mercuryglobe1.jpg

Mariner 10 spacecraft

http://nssdc.gsfc.nasa.gov/thumbnail/spacecraft/73-085A.gif

Goldstone/VLA Maps of Mercury

http://wireless.jpl.nasa.gov/RADAR/mercvla.html

Image Credits

Now return to the Module home page, and read more about observing Mercury in the Textbook Readings.

Hit the Esc key (escape) to return to the Module 10 Home Page

To understand the relationship between Mercury’sday and year, we need to remember a few definitions:

• A planet’s “year” (or orbital period) is the time it takes to make one complete orbit around the Sun.• A planet’s “sidereal day” is the time it takes to rotate once, relative to the fixed stars - e.g. the time from when a particular star is at the zenith, to when it is next back in that position.• A planet’s “solar day” is the time it takes to rotate once, relative to the Sun - e.g. the time from when the Sun is at the zenith, to when it is next back in that position.

(If you want to revise these concepts, look again at the Activity Night and Day)

A day on Mercury

In order to understand how Mercury’s solar and siderealdays compare to its year, we will imagine that the Swinburneexecutive branch is travelling on a fact-finding tour to Mercury in the Swinburne executive spaceship, with a view to establishing a campus there.

Welcome to sunny Mercury!

Welcome to sunny Mercury!

(not to scale!)

Sun

Mercury

Spacecraft

When the Swinburne spaceship touches down, the Sun happens to be directly overhead ...

… and so it is midday on Mercury at the landing site.

midday

We’ll follow what happens at the landing site over acomplete solar day, also keeping track of how manysidereal days pass.

We will see that, on Mercury:

1 Mercury solar day = 2 complete Mercury years = 176 Earth days, and

1 Mercury sidereal day = 1/3 of a Mercury solar day = about 58 Earth days (actually 58.65)

Remember that solar days have to do with wherethe Sun is in relation to the zenith,

Whereas sidereal days have to do with where the starsare in relation to the zenith.

It may help if you first go through the following animation,focussing particularly on the length of the solar day,then repeat the animation and focus on the length of the sidereal day.

(not to scale!)

Sun

Mercury

Spacecraft

midday

Earth day: 0

(not to scale!)

early afternoon

Earth day: 14

(not to scale!)

late afternoon

Earth day: 29 Half a Mercurysidereal day

(not to scale!)

sunset

Earth day: 44

(not to scale!)

early night

Earth day: 58 One complete Mercurysidereal day

(not to scale!)

Before midnight

Earth day: 73

(not to scale!)

midnight

1 Mercury year later

Earth day: 88 One and a half Mercurysidereal days

One half a Mercury solar day

(not to scale!)

late night

Earth day: 102

(not to scale!)

before sunrise

Earth day: 117 Two complete Mercurysidereal days

(not to scale!)

Earth day: 132

sunrise

(not to scale!)

early morning

Earth day: 147 Two and a half Mercurysidereal days

(not to scale!)

late morning

Earth day: 161

(not to scale!)

midday again!

2 Mercury years later

Earth day: 176 Repeat Three complete Mercurysidereal days

One compete Mercury solar day

The Swinburne executive branch decides that timetabling classes on a planet where the teaching day is twice the length of the academic year would be too difficult, and heads back to Swinburne.

Have a nice(long) day!

Have a nice(long) day!

We saw in the Activity Time and Tide that the orbital and rotational period of the Moon are tidallylocked in a ratio of 1 to 1 due to tidal interactions between the Moon and the Earth.We will see a number of other cases of tidal interactionsin the Solar System producing a 1 to 1 ratio of orbitaland rotational period.

You will have noticed that two Mercury years correspondto three Mercury sidereal days.

Back to the Activity

1 to 1 isn’t the only ratio possible for tidal interactions,however.

For Mercury, the ratio of 2 to 3 is also a result of tidalinteractions - this time between Mercury and the Sun.

Estimating the mass of another planet isn’t necessarilyeasy - we can’t weigh them!

However Newton’s Law of Gravity tells us that the acceleration of a satellite under gravity depends on the mass of the planet it is orbiting.

For planets with natural satellites (“moons”) - which applies to all but Mercury and Venus - we can observe their orbits,calculate their accelerations, then take advantage of Newton’s Law to estimate their masses.

Calculating Mercury’s Mass

For planets (and other Solar System objects) withouttheir own natural satellites, estimating their mass is difficult unless we send an artificial satellite to visit them.

Mariner 10’s mission to Mercury provided trajectory information which was used to estimate Mercury’s mass.

Back to the Activity