<p>One part of your question could be paraphrased "why do we measure altitude on Mars the way we do". As you observe we could redefine the ellipsoid of zero altitude to much more closely fit the planet by moving it South a bit. The <a href="https://www.quora.com/How-do-we-calculate-altitude-on-Mars-since-theres-no-large-body-of-water-to-use-as-reference" rel="noreferrer">answer</a> is based on the gravity. 0 altitude is on a surface of constant gravitational potential energy, which was originally determined in terms of atmospheric pressure, but more recently by direct gravity measurement.</p>

<p>The other part might ask why this is in terms of the history of Mars. I don't think there is a definitive accepted answer, but I'll leave that for someone else.</p>

CC BY-SA 4.0
2018-05-21T14:08:38.797Z
I believe it's fairly well accepted that Mars "lopsidedness" is a result of a very large impact some time ago.   http://discovermagazine.com/2009/jan/060  If nobody else covers this I can make it into an answer later.
userLTK
CC BY-SA 4.0
2020-07-23T04:23:23.727Z
Does "...by moving it South a bit" mean moving the center of the ellipsoid away from Mars' center of mass? That doesn't sound like a good idea, does it?
uhoh
CC BY-SA 4.0
2020-07-23T09:15:29.417Z
@uhoh Yes. and No.
Steve Linton
CC BY-SA 4.0
2018-05-21T11:36:56.870Z
2018-05-21T11:36:56.870Z
Steve Linton
<p>The placement of the center of the map is messed up, but it's nobody's mistake. From chapter 4 of The Surface of Mars:</p>
<blockquote>
<p>The elevation difference between the two hemispheres
offsets the planet’s center of figure from its center of
mass by 2.986 km as measured along the polar axis</p>
</blockquote>
<p>As explained in another answer, the altitude is defined based on Mars' center of mass.</p>
<p>Regarding the question that is in the title (&quot;Why are the hemispheres so different?&quot;), this sharp contrast between the Northern and Southern hemispheres is known as the Martian dichotomy.</p>
<p>The elevation on Mars follows a bimodal distribution. That is most of the land is either 4 km under the mean altitude or 1.5 km above the mean altitude.
But not only is the altitude different between the two hemispheres, but the crustal thickness and the number of impact craters are also very different.</p>
<p>There are several competing hypotheses to explain the dichotomy. Here are a few.</p>
<h1>Geological phenomena</h1>
<h2>Part of the crust was overturned</h2>
<p>Very early in the history of Mars, there was a magma ocean that crystallized. <a href="https://science.sciencemag.org/content/307/5713/1214" rel="noreferrer">Solomon et al. (2005)</a> suggest that the top the mantle ended up denser than what was underneath, and it flipped.</p>
<h2>There used to be active margins</h2>
<p>By analogy with the dichotomy seen on Earth between oceans and continents, <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/94JE00216" rel="noreferrer">Sleep (1994)</a> suggests that the northern hemisphere may be the result of new crust being generated and spreading from the center. The limit between the two areas would have been an active margin, similar to the Andes where the oceanic crust drops under the continental crust.</p>
<p><a href="https://i.sstatic.net/qulKr.png" rel="noreferrer"><img src="https://i.sstatic.net/qulKr.png" alt="Figure 2.15, Mysteries of Mars" /></a></p>
<p>However, this would result in the formation of magnetic stripes that are observed in the southern rather than in the northern hemisphere.</p>
<h2>Thermal convection in the mantle</h2>
<p>As suggested by <a href="https://www.sciencedirect.com/science/article/pii/S0012821X01003454" rel="noreferrer">Zhong and Zuber (2001)</a>, there might have been strong convection in the mantle with a single convection cell.</p>
<p>These models succeed in reproducing some differences between the two hemispheres.</p>
<h1>Impacts</h1>
<h2>A giant impact in the northern hemisphere</h2>
<p><a href="https://www.nature.com/articles/309138a0.pdf?origin=ppub" rel="noreferrer">Wilhelms and Squyres (1984)</a> suggest that a giant impact very early in the history of Mars might have caused the northern depression. They also indicate the location of the impact.</p>
<p>However, no thinning of the crust has been found there, like what is observed for younger and smaller craters. The ring deposits that such an impact would have created are missing. The edge of the northern plains are very irregular, when impact craters tend to be somewhat circular.</p>
<h2>A giant impact in the southern hemisphere</h2>
<p><a href="https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014GL062261" rel="noreferrer">Leone et al. (2016)</a> propose that the dichotomy is the result of an impact in the southern hemisphere and that the result of the impact in more crust, rather than missing crust.</p>
<h2>A few not quite as giant impacts</h2>
<p>Explaining the irregular boundary between the two areas is rather difficult with a single impact, so <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/GL015i003p00229" rel="noreferrer">Frey and Schultz (1988)</a> suggest that a few big impacts might have caused the peculiar geography of Mars.</p>
<p>However, this hypothesis doesn't completely explain why the impact basin isn't circular.</p>
<hr />
<p>So the question of the martian dichotomy is still open and doesn't have a definitive answer yet.</p>
<h3>Further reading:</h3>
<ul>
<li><a href="https://link.springer.com/book/10.1007%2F978-3-319-74784-2" rel="noreferrer"><em>Mysteries of Mars</em> by De Blasio</a>, chapter 2</li>
<li><a href="https://www.cambridge.org/us/academic/subjects/earth-and-environmental-science/planetary-science-and-astrobiology/surface-mars?format=HB" rel="noreferrer"><em>The Surface of Mars</em> by Carr</a>, chapter 4</li>
<li><a href="https://www.sciencedirect.com/science/article/abs/pii/001910359190221E" rel="noreferrer">Origin of the Martian crustal dichotomy, George E.McGill and Steven W.Squyres</a></li>
</ul>

CC BY-SA 4.0
2020-07-16T10:58:55.923Z
2021-05-06T08:12:21.990Z
2021-05-06T08:12:21.990Z
usernumber
<p>I'm looking at the Mars topography map: 
<a href="https://i.sstatic.net/LP81v.jpg" rel="noreferrer"><img src="https://i.sstatic.net/LP81v.jpg" alt="enter image description here"></a>
<a href="https://commons.wikimedia.org/wiki/File:Mars_topography_(MOLA_dataset)_with_poles_HiRes.jpg" rel="noreferrer">original link</a></p>

<p>...and I can't shake the impression that someone just messed up the placement of the center point of the planet, shifting it about 8km north from where it should be. If the center point was ~8km further south, the north and south altitudes would be roughly the same. Moreover, it's even hard to argue about placement of equator, as the northern pole is about 16km closer to the equatorial plane than the southern - due to the altitude difference. Move the equator and it all evens out.</p>

<p>It's fairly hard to believe such a mistake to have persisted so long, so... is there any deeper reason behind this strong asymmetry of the planet, other than just a topographical snafu? </p>

CC BY-SA 4.0
2018-05-21T10:56:30.623Z
@Kozaky: No, I haven't. I was quite excited to find one - I did look for a good detailed map of Mars in the past, and it was a very frustrating experience.
SF.
CC BY-SA 4.0
2020-07-16T09:22:41.813Z
If you're looking for a good map of Mars, have you checked out http://jmars.mars.asu.edu/maps/ ? There are a bunch of maps from many different missions.
usernumber
<mars><topographic-map>
Why are Mars northern/southern hemisphere altitudes so different?
SF.
# Why are Mars northern/southern hemisphere altitudes so different?

(Asked by: username_0 on 2018-05-21)

I'm looking at the Mars topography map:
[![enter image description here](https://i.sstatic.net/LP81v.jpg)](https://i.sstatic.net/LP81v.jpg)
[original link](https://commons.wikimedia.org/wiki/File:Mars_topography_(MOLA_dataset)_with_poles_HiRes.jpg)

...and I can't shake the impression that someone just messed up the placement of the center point of the planet, shifting it about 8km north from where it should be. If the center point was ~8km further south, the north and south altitudes would be roughly the same. Moreover, it's even hard to argue about placement of equator, as the northern pole is about 16km closer to the equatorial plane than the southern - due to the altitude difference. Move the equator and it all evens out.

It's fairly hard to believe such a mistake to have persisted so long, so... is there any deeper reason behind this strong asymmetry of the planet, other than just a topographical snafu?

**Tags:** mars, topographic-map
> **Comments:**
> **username_0**: @Kozaky: No, I haven't. I was quite excited to find one - I did look for a good detailed map of Mars in the past, and it was a very frustrating experience.  
> **username_1**: If you're looking for a good map of Mars, have you checked out http://jmars.mars.asu.edu/maps/ ? There are a bunch of maps from many different missions.  

---

## 2 answers

### Answer by username_1 on 2020-07-16. Score: 11 | Accepted answer

The placement of the center of the map is messed up, but it's nobody's mistake. From chapter 4 of The Surface of Mars:

> The elevation difference between the two hemispheres
> offsets the planet’s center of figure from its center of
> mass by 2.986 km as measured along the polar axis

As explained in another answer, the altitude is defined based on Mars' center of mass.

Regarding the question that is in the title ("Why are the hemispheres so different?"), this sharp contrast between the Northern and Southern hemispheres is known as the Martian dichotomy.

The elevation on Mars follows a bimodal distribution. That is most of the land is either 4 km under the mean altitude or 1.5 km above the mean altitude.
But not only is the altitude different between the two hemispheres, but the crustal thickness and the number of impact craters are also very different.

There are several competing hypotheses to explain the dichotomy. Here are a few.

# Geological phenomena

## Part of the crust was overturned

Very early in the history of Mars, there was a magma ocean that crystallized. [Solomon et al. (2005)](https://science.sciencemag.org/content/307/5713/1214) suggest that the top the mantle ended up denser than what was underneath, and it flipped.

## There used to be active margins

By analogy with the dichotomy seen on Earth between oceans and continents, [Sleep (1994)](https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/94JE00216) suggests that the northern hemisphere may be the result of new crust being generated and spreading from the center. The limit between the two areas would have been an active margin, similar to the Andes where the oceanic crust drops under the continental crust.

[![Figure 2.15, Mysteries of Mars](https://i.sstatic.net/qulKr.png)](https://i.sstatic.net/qulKr.png)

However, this would result in the formation of magnetic stripes that are observed in the southern rather than in the northern hemisphere.

## Thermal convection in the mantle

As suggested by [Zhong and Zuber (2001)](https://www.sciencedirect.com/science/article/pii/S0012821X01003454), there might have been strong convection in the mantle with a single convection cell.

These models succeed in reproducing some differences between the two hemispheres.

# Impacts

## A giant impact in the northern hemisphere

[Wilhelms and Squyres (1984)](https://www.nature.com/articles/309138a0.pdf?origin=ppub) suggest that a giant impact very early in the history of Mars might have caused the northern depression. They also indicate the location of the impact.

However, no thinning of the crust has been found there, like what is observed for younger and smaller craters. The ring deposits that such an impact would have created are missing. The edge of the northern plains are very irregular, when impact craters tend to be somewhat circular.

## A giant impact in the southern hemisphere

[Leone et al. (2016)](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014GL062261) propose that the dichotomy is the result of an impact in the southern hemisphere and that the result of the impact in more crust, rather than missing crust.

## A few not quite as giant impacts

Explaining the irregular boundary between the two areas is rather difficult with a single impact, so [Frey and Schultz (1988)](https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/GL015i003p00229) suggest that a few big impacts might have caused the peculiar geography of Mars.

However, this hypothesis doesn't completely explain why the impact basin isn't circular.

---

So the question of the martian dichotomy is still open and doesn't have a definitive answer yet.

### Further reading:

* [*Mysteries of Mars* by De Blasio](https://link.springer.com/book/10.1007%2F978-3-319-74784-2), chapter 2
* [*The Surface of Mars* by Carr](https://www.cambridge.org/us/academic/subjects/earth-and-environmental-science/planetary-science-and-astrobiology/surface-mars?format=HB), chapter 4
* [Origin of the Martian crustal dichotomy, George E.McGill and Steven W.Squyres](https://www.sciencedirect.com/science/article/abs/pii/001910359190221E)


---

### Answer by username_2 on 2018-05-21. Score: 10

One part of your question could be paraphrased "why do we measure altitude on Mars the way we do". As you observe we could redefine the ellipsoid of zero altitude to much more closely fit the planet by moving it South a bit. The [answer](https://www.quora.com/How-do-we-calculate-altitude-on-Mars-since-theres-no-large-body-of-water-to-use-as-reference) is based on the gravity. 0 altitude is on a surface of constant gravitational potential energy, which was originally determined in terms of atmospheric pressure, but more recently by direct gravity measurement.

The other part might ask why this is in terms of the history of Mars. I don't think there is a definitive accepted answer, but I'll leave that for someone else.

> **Comments:**
> **username_3**: I believe it's fairly well accepted that Mars "lopsidedness" is a result of a very large impact some time ago.   http://discovermagazine.com/2009/jan/060  If nobody else covers this I can make it into an answer later.  
> **username_4**: Does "...by moving it South a bit" mean moving the center of the ellipsoid away from Mars' center of mass? That doesn't sound like a good idea, does it?  
> **username_2**: @uhoh Yes. and No.  

---

