today in astronomy 111: earth and mars - department of physics and
TRANSCRIPT
Slide 1Today in Astronomy 111: Earth and Mars
Earth, life, water and ice • Global temperature
measurements for the last few hundred thousand years
Mars and its basic features • Mars is similar to Earth • Mars is different from
Earth Mars, life, water and ice
Unretouched picture of the “face on Mars,” from Mars Global Surveyor (JPL/NASA). The modern version of Martian canals…
Earth, water and ice
Earth, of course, has a lot of surface water. It is thought that most of what’s here was brought here by
meteorites, after Earth had cooled off enough that parts of it were cooler than the 1 atm boiling point, T = 373 K.
We estimate that 90% of the mass of such meteorites came from inner solar-system planetesimals (like the main-belt asteroids), and 10% came from comets. • …because deuterium abundance in the oceans is a
better match for asteroids than for cometary water. 97% of Earth’s water is in the oceans, and about 2% in the
ice caps (mostly Antarctica); most of the rest is in lakes, rivers, and the ground.
27 September 2011 Astronomy 111, Fall 2011 3
Ice ages
For the last two million years or so, in the era geologists call the Pleistocene, the fraction of Earth’s water in the form of ice has fluctuated wildly, by factors of 2-3 – that is, from less than 1% to about 6 % of the total – in synch with annual average ocean temperature fluctuations of ±3 C. We know this because we can see widespread evidence of glacial action
requiring enormously-extended ice caps in both hemispheres. The place you now live, for instance (the Great Lakes, the Finger Lakes, drumlins, the moraine-like appearance of the hills south of the Finger Lakes….).
we can measure the ocean temperatures through the ages from the heavy oxygen and hydrogen relative abundances
in core samples of seabed and icecaps. 18 16 2 1( O O , H H )
Why this works: heavy water in icecaps and plants, and the temperature of the oceans
There are significant vapor-pressure differences between the isotopologues of water due to differences in molecular mass. Easiest to measure: D/H from HDO is heavier than normal water, and thus compared to
normal water has a lower vapor pressure at any T, that depends more sensitively on T. (All verified in lab.)
Less D/H in icecaps or plants = more D/H in ocean water = lower temperature of ocean. So more-negative values of
means lower global temperature. 27 September 2011 Astronomy 111, Fall 2011 4
16 18 17 2 2 2H O, HDO, H O, H O, .etc− −
2 2HDO H O. (D H.)=
( ) ( )
D H δ
N H
m ea
n oc
ea n
te m
pe ra
tu re
-1 6
C
Year
Why this works: heavy water and oceanic T through the ages.
Combined with timing from annual growth, a.k.a. dendro- chronology, measurements of T via δD can give oceanic T(t) . Most familiar example: tree rings. δD
and thus T can be measured in indivi- dual tree rings, and the year that a tree ring formed can often be determined exactly. This is how T(t) is best measured for the last few thousand years, as at right.
In ice cores, the annual layers of snow and ice are distinguished by different purity of ice deposited in summer and winter, as below. (GISP2/NICL/USGS)
27 September 2011 Astronomy 111, Fall 2011 5
AST 111, 22 September 2011
Ice ages (continued)
Temperatures for the past 420,000 years, from deuterium abundance δ D (ppm = 10-6, relative to SMOW) in a 3.4 km (!) core sample taken at Vostok, Antarctica (Petit et al. 1999, Jouzel et al. 1987, 1993, 1996).
-10
-8
-6
-4
-2
0
2
4
Years before present
Ice ages (continued)
Many scientists have searched for evidence of periodic, oscillating behaviour in the temperature record. And they generally find a
weak tendency for the temperature to oscillate at periods of 23000 years, 40000 years, and 100000 years.
And that’s why we mention ice ages in an astronomy class… Figure by Ken Carslaw, Leeds U.
Ice ages (continued)
As was first pointed out in connection with climate by Milankovitch (1920s), there are periodic changes in Earth’s orientation and orbit that influence how much sunlight is received at various parts of the Earth: Axis precession, which has a
synodic period of 22000 years. Nutation (oscillation of the tilt
of the axis between 21.5º and 24.5º), which has a period of 41000 years.
Oscillation of the eccentricity of Earth’s orbit (due to forcing by the Sun and the outer giant planets), which has periods of 100000 and 400000 years.
That these numbers match the peaks in the previous period, has convinced many that these orbital fluctuations are the origin of ice ages.
Precession and nutation animation by Michael Gallis, Penn State U.
Ice ages (continued)
Here’s why the Milankovitch cycles do not explain ice ages. The variation in solar illumination from these cycles is
tiny compared to the temperature variation from other sources (like the solar cycle), and to other climate- changing effects (like ocean-current configuration).
The 23000-year precession oscillation should have opposite effects in the northern and southern hemisphere, contrary to the global nature of ice ages.
The 100000-year eccentricity oscillation should be the smallest and the 400000-year oscillation much larger. Instead the 100000-year oscillation is largest, and no 400000-year oscillation is seen.
It’s an odd coincidence of numbers, but such coincidences are not that rare. Read on...
27 September 2011 Astronomy 111, Fall 2011 10
Mars’s vital statistics
Equatorial radius 3.397 10 cm (0.533 )
Average density 3.933 gm cm
Moment of inertia 0.366 Albedo 0.2 Effective temperature 210.1 K
2.2792 10 cmOrbital semimajor axis (1.524
M
R
MR
686.980 days revolution period Sidereal
24.6229 hours rotation period Length of day 24.6597 hours
°
Mars in 2003, by Jim Bell (Cornell) with the HST
Mars’s atmosphere’s vital statistics
Surface pressure: 0.006 earth atmospheres Average temperature: 210 K (-60 C) Diurnal (day-night) temperature range: 184-242 K Surface wind speeds: 2 - 30 m/s Atmospheric composition (near surface, by volume): 95.32% CO2, 2.7% N2, 1.6%Ar, 0.13% O2, 0.08% CO, 0.02% H2O
Mars’s moons
Interesting facts about Phobos and Deimos
Phobos (“fear”) and Deimos (“terror”) are henchmen of the god of war, Ares (=Mars) in ancient Greek mythology.
They are very difficult to observe without photography. • Discovered in 1877 by American astronomer Asaph
Hall, during a close approach of Mars to the Earth. • More on those close approaches: see Homework #4.
But they were first written about a century and a half earlier: by Jonathan Swift, in Gulliver’s Travels (1726). • Swift probably knew about Kepler’s numerological
“prediction” that, since Earth had one moon and Jupiter had four (known to Kepler), Mars must have two, Saturn eight, etc.
27 September 2011 Astronomy 111, Fall 2011 13
Phobos and Deimos (continued)
• “Discovery” of the moons was a feat by the astronomers of the advanced society of Laputa. Swift reported their orbital periods as 10 and 21.5 hours -- they’re really 7.7 and 30.3 hours. (Amazingly close.)
They were next discussed by Voltaire – who had certainly read Swift and knew Kepler’s work (see quote below) – in his short story Micromegas (1752). • “…I am well aware that Father Castel will write, and
pleasantly enough too, against the existence of these two moons, but I believe those who reason from analogy.”
• Micromegas marks the beginning of science fiction as a literary genre.
An even odder coincidence of numbers, but clearly an accident. 27 September 2011 Astronomy 111, Fall 2011 14
The similarities between Earth and Mars
For the past century, ever since it was first appreciated that Mars has an atmosphere, this planet has been the focus of the search for life outside Earth. Mars has: an atmosphere and reasonable surface gravity. a day length and an obliquity (seasons) almost the same
as Earth. terrestrial composition, even terrestrial appearance. not much in the way of surface impact cratering. strong evidence of past volcanism and some faulting and
other geological activity (though no plate tectonics). surface color variegation that, for a time, was thought
possibly to reveal vegetated areas, fancifully connected by “canali” in the view of early observers.
27 September 2011 Astronomy 111, Fall 2011 16
The similarities between Earth and Mars (cont’d)
One is of southern Morocco, the other of Mars. Which is which? (Morocco by Filipe Alves, Mars by the Spirit rover, MER/JPL/NASA.)
Martian volcanism
On the same scale: the largest volcanoes on Earth and Mars.
Olympus Mons: 24 km high, 550 km across (Viking 2 Orbiter/NASA)
The Big Island of Hawai’i, with Mauna Loa, Mauna Kea, Kilauea: 10.6 km high from base (4 km from sea level), 350 km across (140 km on coast)
Mars, then and now
HST images and maps drawn by Eugene Antonaldi, rendered and scaled by Tom Ruen.
The Martian dichotomy
The northern hemisphere of Mars looks different – far fewer mountains and craters – than the southern. Lower elevation, too, spawning
many theories that an ocean used to occupy most of the north.
Wilhelms & Squyres proposed in 1984 that this was due to the impact of a Earth’s-Moon-size object, about 4 billion years ago.
27 September 2011 Astronomy 111, Fall 2011 19
Martian topography with (top) and without (bottom) its volcanoes, by Jeff Andrews-Hanna et al. 2008.
The Martian dichotomy (continued)
This idea seems to be validated by new (2008) simulations and results on Martian topography and magnetism: Elliptical shape covering 40% of
surface consistent with oblique impact (Andrews-Hanna et al. 2008).
Can do with 0.1-0.3 Lunar-mass object (Marinova et al. 2008), of which there were enough, 4 Gyr ago.
Could explain southern concentration of Mars’ small magnetic field (Stanley 2008).
27 September 2011 Astronomy 111, Fall 2011 20
Jeff Andrews-Hanna/ NASA
The differences between Earth and Mars
The differences outweigh the similarities; Mars is in almost every sense intermediate between Venus/Earth and Mercury/Moon. It’s low in mass and relatively undifferentiated (low
density, high moment of inertia). The atmosphere is thin and dominated by heavy
molecules, probably because of its small mass. It’s cold; too cold for liquid water on the surface.
So it has not been terribly surprising that the Viking landers (and Pathfinder and MER rovers) found no evidence of life, nor that the claims of fossil microorganisms in Martian meteorites have not held up.
27 September 2011 Astronomy 111, Fall 2011 22
Water on Mars
Still, it is clear that Mars has a little bit of water on the surface in the form of ice near the poles, and evidence is emerging that in the distant past (a few Gyr ago) it may have had liquid water oceans and a denser atmosphere: Mars is remarkably free of impact craters, especially small ones and
especially in the low plains, but even in the southern highlands. Thus the surface used to have more protection from impacts than it does now (Helfer 1990).
Erosion features can be seen scattered over the planet (albeit rarely) that resemble terrestrial gullies and canyons in exact detail.
Hematite – a mineral that forms in water on Earth – is very abundant on the Martian surface.
So Mars may once have had the appropriate conditions for life, and may still, beneath the surface.
Water on Mars (continued)
Water ice on the floor of a deep, high-latitude, Martian crater (G. Neukum, Mars Express/ESA).
Water on Mars (continued)
Canyons in Gorgonum Chaos, from Mars Global Surveyor (Malin Space Science Systems/ JPL/NASA)
Water on Mars (continued)
Sedimentation (?) in Schiaparelli Crater, from Mars Global Surveyor (Malin Space Science Systems/ JPL/NASA)
Water on Mars (continued)
Canyon walls in Gorgonum Chaos, from Mars Global Surveyor (Malin Space Science Systems/ JPL/NASA)
Water on Mars (continued)
Walls in a high- latitude pit, from Mars Global Surveyor (Malin Space Science Systems/ JPL/NASA)
Water on Mars (continued)
Nirgal Vallis, south- facing wall, from Mars Global Surveyor (Malin Space Science Systems/ JPL/NASA)
Water on Mars (continued)
Best evidence for surface water: wadis on Mars? Here are recurring slope lineae – dark-looking channels – which darken in seasons expected to be wet, and fade in dry seasons.
(McEwen et al. 2011)
Water on Mars (continued)
Spherules: blueberry-size pebbles on the Martian surface, which geologists think may have been shaped by liquid water (Mars Exploration Rovers/JPL/NASA).
And, finally, water under the Martian surface
Long suspected, this was finally confirmed in 2008 by the NASA Phoenix lander, which dug several cm into the dirt, found ice, melted it, and chemically analyzed the results.
27 September 2011 Astronomy 111, Fall 2011 31
NASA/JPL/ U. Arizona
Earth, water and ice
Ice ages
Why this works: heavy water in icecaps and plants, and the temperature of the oceans
Why this works: heavy water and oceanic T through the ages.
Ice ages (continued)
Ice ages (continued)
Ice ages (continued)
Ice ages (continued)
Mars’s moons
Phobos and Deimos (continued)
The similarities between Earth and Mars (cont’d)
Martian volcanism
Water on Mars
Earth, life, water and ice • Global temperature
measurements for the last few hundred thousand years
Mars and its basic features • Mars is similar to Earth • Mars is different from
Earth Mars, life, water and ice
Unretouched picture of the “face on Mars,” from Mars Global Surveyor (JPL/NASA). The modern version of Martian canals…
Earth, water and ice
Earth, of course, has a lot of surface water. It is thought that most of what’s here was brought here by
meteorites, after Earth had cooled off enough that parts of it were cooler than the 1 atm boiling point, T = 373 K.
We estimate that 90% of the mass of such meteorites came from inner solar-system planetesimals (like the main-belt asteroids), and 10% came from comets. • …because deuterium abundance in the oceans is a
better match for asteroids than for cometary water. 97% of Earth’s water is in the oceans, and about 2% in the
ice caps (mostly Antarctica); most of the rest is in lakes, rivers, and the ground.
27 September 2011 Astronomy 111, Fall 2011 3
Ice ages
For the last two million years or so, in the era geologists call the Pleistocene, the fraction of Earth’s water in the form of ice has fluctuated wildly, by factors of 2-3 – that is, from less than 1% to about 6 % of the total – in synch with annual average ocean temperature fluctuations of ±3 C. We know this because we can see widespread evidence of glacial action
requiring enormously-extended ice caps in both hemispheres. The place you now live, for instance (the Great Lakes, the Finger Lakes, drumlins, the moraine-like appearance of the hills south of the Finger Lakes….).
we can measure the ocean temperatures through the ages from the heavy oxygen and hydrogen relative abundances
in core samples of seabed and icecaps. 18 16 2 1( O O , H H )
Why this works: heavy water in icecaps and plants, and the temperature of the oceans
There are significant vapor-pressure differences between the isotopologues of water due to differences in molecular mass. Easiest to measure: D/H from HDO is heavier than normal water, and thus compared to
normal water has a lower vapor pressure at any T, that depends more sensitively on T. (All verified in lab.)
Less D/H in icecaps or plants = more D/H in ocean water = lower temperature of ocean. So more-negative values of
means lower global temperature. 27 September 2011 Astronomy 111, Fall 2011 4
16 18 17 2 2 2H O, HDO, H O, H O, .etc− −
2 2HDO H O. (D H.)=
( ) ( )
D H δ
N H
m ea
n oc
ea n
te m
pe ra
tu re
-1 6
C
Year
Why this works: heavy water and oceanic T through the ages.
Combined with timing from annual growth, a.k.a. dendro- chronology, measurements of T via δD can give oceanic T(t) . Most familiar example: tree rings. δD
and thus T can be measured in indivi- dual tree rings, and the year that a tree ring formed can often be determined exactly. This is how T(t) is best measured for the last few thousand years, as at right.
In ice cores, the annual layers of snow and ice are distinguished by different purity of ice deposited in summer and winter, as below. (GISP2/NICL/USGS)
27 September 2011 Astronomy 111, Fall 2011 5
AST 111, 22 September 2011
Ice ages (continued)
Temperatures for the past 420,000 years, from deuterium abundance δ D (ppm = 10-6, relative to SMOW) in a 3.4 km (!) core sample taken at Vostok, Antarctica (Petit et al. 1999, Jouzel et al. 1987, 1993, 1996).
-10
-8
-6
-4
-2
0
2
4
Years before present
Ice ages (continued)
Many scientists have searched for evidence of periodic, oscillating behaviour in the temperature record. And they generally find a
weak tendency for the temperature to oscillate at periods of 23000 years, 40000 years, and 100000 years.
And that’s why we mention ice ages in an astronomy class… Figure by Ken Carslaw, Leeds U.
Ice ages (continued)
As was first pointed out in connection with climate by Milankovitch (1920s), there are periodic changes in Earth’s orientation and orbit that influence how much sunlight is received at various parts of the Earth: Axis precession, which has a
synodic period of 22000 years. Nutation (oscillation of the tilt
of the axis between 21.5º and 24.5º), which has a period of 41000 years.
Oscillation of the eccentricity of Earth’s orbit (due to forcing by the Sun and the outer giant planets), which has periods of 100000 and 400000 years.
That these numbers match the peaks in the previous period, has convinced many that these orbital fluctuations are the origin of ice ages.
Precession and nutation animation by Michael Gallis, Penn State U.
Ice ages (continued)
Here’s why the Milankovitch cycles do not explain ice ages. The variation in solar illumination from these cycles is
tiny compared to the temperature variation from other sources (like the solar cycle), and to other climate- changing effects (like ocean-current configuration).
The 23000-year precession oscillation should have opposite effects in the northern and southern hemisphere, contrary to the global nature of ice ages.
The 100000-year eccentricity oscillation should be the smallest and the 400000-year oscillation much larger. Instead the 100000-year oscillation is largest, and no 400000-year oscillation is seen.
It’s an odd coincidence of numbers, but such coincidences are not that rare. Read on...
27 September 2011 Astronomy 111, Fall 2011 10
Mars’s vital statistics
Equatorial radius 3.397 10 cm (0.533 )
Average density 3.933 gm cm
Moment of inertia 0.366 Albedo 0.2 Effective temperature 210.1 K
2.2792 10 cmOrbital semimajor axis (1.524
M
R
MR
686.980 days revolution period Sidereal
24.6229 hours rotation period Length of day 24.6597 hours
°
Mars in 2003, by Jim Bell (Cornell) with the HST
Mars’s atmosphere’s vital statistics
Surface pressure: 0.006 earth atmospheres Average temperature: 210 K (-60 C) Diurnal (day-night) temperature range: 184-242 K Surface wind speeds: 2 - 30 m/s Atmospheric composition (near surface, by volume): 95.32% CO2, 2.7% N2, 1.6%Ar, 0.13% O2, 0.08% CO, 0.02% H2O
Mars’s moons
Interesting facts about Phobos and Deimos
Phobos (“fear”) and Deimos (“terror”) are henchmen of the god of war, Ares (=Mars) in ancient Greek mythology.
They are very difficult to observe without photography. • Discovered in 1877 by American astronomer Asaph
Hall, during a close approach of Mars to the Earth. • More on those close approaches: see Homework #4.
But they were first written about a century and a half earlier: by Jonathan Swift, in Gulliver’s Travels (1726). • Swift probably knew about Kepler’s numerological
“prediction” that, since Earth had one moon and Jupiter had four (known to Kepler), Mars must have two, Saturn eight, etc.
27 September 2011 Astronomy 111, Fall 2011 13
Phobos and Deimos (continued)
• “Discovery” of the moons was a feat by the astronomers of the advanced society of Laputa. Swift reported their orbital periods as 10 and 21.5 hours -- they’re really 7.7 and 30.3 hours. (Amazingly close.)
They were next discussed by Voltaire – who had certainly read Swift and knew Kepler’s work (see quote below) – in his short story Micromegas (1752). • “…I am well aware that Father Castel will write, and
pleasantly enough too, against the existence of these two moons, but I believe those who reason from analogy.”
• Micromegas marks the beginning of science fiction as a literary genre.
An even odder coincidence of numbers, but clearly an accident. 27 September 2011 Astronomy 111, Fall 2011 14
The similarities between Earth and Mars
For the past century, ever since it was first appreciated that Mars has an atmosphere, this planet has been the focus of the search for life outside Earth. Mars has: an atmosphere and reasonable surface gravity. a day length and an obliquity (seasons) almost the same
as Earth. terrestrial composition, even terrestrial appearance. not much in the way of surface impact cratering. strong evidence of past volcanism and some faulting and
other geological activity (though no plate tectonics). surface color variegation that, for a time, was thought
possibly to reveal vegetated areas, fancifully connected by “canali” in the view of early observers.
27 September 2011 Astronomy 111, Fall 2011 16
The similarities between Earth and Mars (cont’d)
One is of southern Morocco, the other of Mars. Which is which? (Morocco by Filipe Alves, Mars by the Spirit rover, MER/JPL/NASA.)
Martian volcanism
On the same scale: the largest volcanoes on Earth and Mars.
Olympus Mons: 24 km high, 550 km across (Viking 2 Orbiter/NASA)
The Big Island of Hawai’i, with Mauna Loa, Mauna Kea, Kilauea: 10.6 km high from base (4 km from sea level), 350 km across (140 km on coast)
Mars, then and now
HST images and maps drawn by Eugene Antonaldi, rendered and scaled by Tom Ruen.
The Martian dichotomy
The northern hemisphere of Mars looks different – far fewer mountains and craters – than the southern. Lower elevation, too, spawning
many theories that an ocean used to occupy most of the north.
Wilhelms & Squyres proposed in 1984 that this was due to the impact of a Earth’s-Moon-size object, about 4 billion years ago.
27 September 2011 Astronomy 111, Fall 2011 19
Martian topography with (top) and without (bottom) its volcanoes, by Jeff Andrews-Hanna et al. 2008.
The Martian dichotomy (continued)
This idea seems to be validated by new (2008) simulations and results on Martian topography and magnetism: Elliptical shape covering 40% of
surface consistent with oblique impact (Andrews-Hanna et al. 2008).
Can do with 0.1-0.3 Lunar-mass object (Marinova et al. 2008), of which there were enough, 4 Gyr ago.
Could explain southern concentration of Mars’ small magnetic field (Stanley 2008).
27 September 2011 Astronomy 111, Fall 2011 20
Jeff Andrews-Hanna/ NASA
The differences between Earth and Mars
The differences outweigh the similarities; Mars is in almost every sense intermediate between Venus/Earth and Mercury/Moon. It’s low in mass and relatively undifferentiated (low
density, high moment of inertia). The atmosphere is thin and dominated by heavy
molecules, probably because of its small mass. It’s cold; too cold for liquid water on the surface.
So it has not been terribly surprising that the Viking landers (and Pathfinder and MER rovers) found no evidence of life, nor that the claims of fossil microorganisms in Martian meteorites have not held up.
27 September 2011 Astronomy 111, Fall 2011 22
Water on Mars
Still, it is clear that Mars has a little bit of water on the surface in the form of ice near the poles, and evidence is emerging that in the distant past (a few Gyr ago) it may have had liquid water oceans and a denser atmosphere: Mars is remarkably free of impact craters, especially small ones and
especially in the low plains, but even in the southern highlands. Thus the surface used to have more protection from impacts than it does now (Helfer 1990).
Erosion features can be seen scattered over the planet (albeit rarely) that resemble terrestrial gullies and canyons in exact detail.
Hematite – a mineral that forms in water on Earth – is very abundant on the Martian surface.
So Mars may once have had the appropriate conditions for life, and may still, beneath the surface.
Water on Mars (continued)
Water ice on the floor of a deep, high-latitude, Martian crater (G. Neukum, Mars Express/ESA).
Water on Mars (continued)
Canyons in Gorgonum Chaos, from Mars Global Surveyor (Malin Space Science Systems/ JPL/NASA)
Water on Mars (continued)
Sedimentation (?) in Schiaparelli Crater, from Mars Global Surveyor (Malin Space Science Systems/ JPL/NASA)
Water on Mars (continued)
Canyon walls in Gorgonum Chaos, from Mars Global Surveyor (Malin Space Science Systems/ JPL/NASA)
Water on Mars (continued)
Walls in a high- latitude pit, from Mars Global Surveyor (Malin Space Science Systems/ JPL/NASA)
Water on Mars (continued)
Nirgal Vallis, south- facing wall, from Mars Global Surveyor (Malin Space Science Systems/ JPL/NASA)
Water on Mars (continued)
Best evidence for surface water: wadis on Mars? Here are recurring slope lineae – dark-looking channels – which darken in seasons expected to be wet, and fade in dry seasons.
(McEwen et al. 2011)
Water on Mars (continued)
Spherules: blueberry-size pebbles on the Martian surface, which geologists think may have been shaped by liquid water (Mars Exploration Rovers/JPL/NASA).
And, finally, water under the Martian surface
Long suspected, this was finally confirmed in 2008 by the NASA Phoenix lander, which dug several cm into the dirt, found ice, melted it, and chemically analyzed the results.
27 September 2011 Astronomy 111, Fall 2011 31
NASA/JPL/ U. Arizona
Earth, water and ice
Ice ages
Why this works: heavy water in icecaps and plants, and the temperature of the oceans
Why this works: heavy water and oceanic T through the ages.
Ice ages (continued)
Ice ages (continued)
Ice ages (continued)
Ice ages (continued)
Mars’s moons
Phobos and Deimos (continued)
The similarities between Earth and Mars (cont’d)
Martian volcanism
Water on Mars