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Well Ziyi, as we learnt in Lesson 2, the state of water depends not only on temperature, but also pressure.So, despite all the evidence of there once having been liquid water on Mars’, it no longer stably exists for long periods of time because the red planet’s surface pressure is too low 1 & 2.As seen in the diagram below, the pressure of the Martian surface is typically only about 0.6% of Earth’s 1 & 3. At this pressure, H2O can only exist as a liquid at the very specific temperature of 0.01°C (Known as the triple point). Anything higher or lower will lead to water being solid or vapor respectively. Image 1 (Reference 1)This means that the liquid H2O that once existed on the surface is now typically ice, water vapor in the atmosphere or has escaped into space 1, 4So why could liquid water exist on Mars billions of years ago?Well, the red planet’s atmosphere used to contain a lot more air molecules and therefore used to be much heavier than it is today 5 & 6. In fact, Mars’ atmosphere used to be around the same Earth’s around 4.2 billion years ago 7. According to the equation,8, the planet’s surface pressure used to be a lot higher than it is now because the weight used to be much greater.This higher pressure meant that liquid water could exist over a much broader range of temperatures and wouldn’t almost immediately freeze or evaporate like it does today.Nowadays, Mars’ atmosphere is only about 1% of our planet’s 7.This – as revealed by NASA’s Mars Atmosphere and Volatile Evolution Mission – has been the result of solar winds from the sun. These winds carry charged particles that strip the atmospheric particles of Mars away by kicking its air molecules into space. 7. This reduction of air molecules in Mars’ atmosphere has thus led to a reduction in the weight and atmospheric pressure.  If anyone wants to watch a quick simulation of these winds peeling away Mars’ atmosphere, here’s a link: https://www.youtube.com/watch?v=gX5JCYBZpcg 9So Ziyi, liquid water no longer exists on Mars essentially because: Solar winds eroded Mars’ atmosphere –> Thinner and lighter atmosphere has led to a lower surface pressure –> Pressure is now too low for water to exist as a liquid for prolonged periods of timePart 2 of b) Does Mars have water today?Well like I mentioned in part a, the liquid water that once existed on Mars is now: ice, vapor in its atmosphere or has evaporated out into space. In fact, scientists estimate that Mars has lost 87% of its water to space over the span of 4.5 billion years 10!Now some of you may be wondering, “Hey, didn’t they discover liquid water on Mars relatively recently? Doesn’t water on Mars exist as a liquid as well?”Well yes and no. Although there has been evidence of liquid water on the surface of Mars, it only exists as a liquid for a very short period of time before it becomes ice or vapor 11.If anyone would like to read more about why liquid water may exists temporarily on Mars, here’s a link: https://www.nasa.gov/press-release/nasa-confirms-evidence-that-liquid-water-flows-on-today-s-mars/ 11Ultimately though, any liquid water on Mars will very quickly become vapor in the atmosphere, ice or will escape into space.Now, seeing that almost all the H2O that Mars has left today is ice, let’s look at where that ice is located in abit more detail.1) Polar ice capsIn 2002 the Odyssey’s neutron instrument reported evidence of water in the subsurface at high latitudes. This evidence was strong enough for NASA to send the Phoenix – which confirmed the presence of ice in 2008 at the poles of the planet 12. Like Earth, Mars has a North and South Pole. But while Earth’s polar ice caps consist solely of water ice, Mars’ polar caps are 15% water ice and 85% carbon dioxide ice 13.  Image 2 (Reference 14) 2) Ice patchesOn the 28th of July 2005, the European Space Agency announced the existence of a crater partially filled with frozen water in Vastitas Borealis (a broad plain that covers much of Mars’ far northern latitudes) 15. An image of the crater, taken by the High-Resolution Stereo Camera on board the European Space Agency’s Mars Express orbiter, is shown below.Furthermore, as more and more of Mars has been imaged, it has become quite obvious that there are many more patches of ice scattered across the Martian surface 16. Image 3 (Reference 15)3) Underground iceIn 2001, the Mars Odyssey used its spectrometer to prove that ice not only existed on the surface of Mars, but also deep underground 17. As it has been shown certain areas of Mars such as the Arabia Terra 18, there is strong evidence that there are large amounts of ice several meters deep underground.One such example of evidence is the patterned ground that is often seen on the surface of Mars 19. On Earth, these shapes are caused by the freezing and thawing of ice in/underneath soil. As seen in the image below, the patterned ground exhibited in the Antarctic summer look discernibly similar to the ground during the Martian winter; hence suggesting that ice exists underneath the Martian soil. Image 4 (Reference 20)So Ziyi, the only H2O that exists on Mars stably for extended periods of time is predominantly ice (in the form of polar ice caps, ice patches and underground ice). The remaining water on the red planet is found as vapor in the atmosphere. 

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